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Levitation

Levitation

Levitation is the process by which an object is suspended against gravity, in a stable position, by a force without physical contact. This can be achieved through jets of gas pushing upwards against the object (as in air hockey), or pushing downwards from the object (as in helicopters, VTOL aircraft, and hovercraft). A sphere can be stably levitated in a stream of air without any type of control system, if conditions are right. These effects are due to Bernoulli's principle. Objects with the right properties can be levitated without even indirect contact through the use of magnetic or electric forces. Some claim there are other ways - using methods not yet known to modern science - to levitate objects. The term is also used in reference to an apparent levitation.

Gravity

Gravity is the force of attraction between massive particles. Weight is determined by the mass of an object and its location in a gravitational field. While a great deal is known about the properties of gravity, the ultimate cause of the gravitational force remains an open question. General relativity is the most successful theory of gravitation to date. It postulates that mass and energy curve space-time, resulting in the phenomenon known as gravity. The effect of the bending of spacetime is often misunderstood as most people seem to prefer to think of a falling object as accelerating when the facts do not support that assumption. Skydivers do not feel any acceleration (other than from wind resistance). Gravity is acceleration.

F=\dot=m\dot+\dotv\,\!

means (if the mass is unvarying) that there must be a force that causes a mass to accelerate. For a rocket ship, that is the rocket engine. For the earth, it is the compression of the mass between something on the surface of the earth and the earth's center of mass. The acceleration is in relation to spacetime in that the weight one feels is one's resistance to deviating from one's path in spacetime. The same holds true in the rocket ship except that a rocket engine supplies the force to accelerate an occupant from his spacetime path. There is no difference between the weight he feels because of gravity or the rocket.

Newton's law of universal gravitation

Newton's law of universal gravitation states the following: :Every object in the Universe attracts every other object with a force directed along the line of centers of mass for the two objects. This force is proportional to the product of their masses and inversely proportional to the square of the separation between the centers of mass of the two objects. Given that the force is along the line through the two masses, the law can be stated symbolically as follows. :

F = - G \frac

where: :F is the magnitude of the (repulsive) gravitational force between two objects :G is the gravitational constant, that is approximately : G = 6.67 × 10⁻¹¹ N m² kg^-2 :m₁ is the mass of first object :m₂ is the mass of second object :r is the distance between the objects It can be seen that this repulsive force F is always negative, and this means that the net attractive force is positive. The minus sign is used to hold the same value meaning as in the Coulomb's Law, where a positive force as result means repulsion between two charges. Thus gravity is proportional to the mass of each object, but has an inverse square relationship with the distance between the centres of each mass. Strictly speaking, this law applies only to point-like objects. If the objects have spatial extent, the force has to be calculated by integrating the force (in vector form, see below) over the extents of the two bodies. It can be shown that for an object with a spherically-symmetric distribution of mass, the integral gives the same gravitational attraction on masses outside it as if the object were a point mass.¹ This law of universal gravitation was originally formulated by Isaac Newton in his work, the Principia Mathematica (1687). Professor William Whewell of Cambridge University, author of History of the Inductive Sciences (1837) stated: ::The law of gravitation is indisputably and incomparably the greatest scientific discovery ever made, whether we look at the advance which it involved, the extent of the truth disclosed, or the fundamental and satisfactory nature of this truth. [In A Treasury of Science ed. Harlow Shapley et al, Harper & Bros. NY: 1946] The history of gravitation as a physical concept is considered in more detail below.

Vector form

below Newton's law of universal gravitation can be written as a vector equation to account for the direction of the gravitational force as well as its magnitude. In this formula, quantities in bold represent vectors. :

\mathbf_ =
 G 
 \, \mathbf_

\mathbf_ =
 - G 
 \, \mathbf_

where :F₁₂ is the force on object 1 due to object 2 :G is the gravitational constant :m₁ and m₂ are the masses of the objects 1 and 2 :r₂₁ = | r₂ − r₁ | is the distance between objects 2 and 1 :

\mathbf_ \equiv \frac

is the unit vector from object 2 to 1 It can be seen, that the vector form of the equation is the same as the scalar form, except for the vector value of F and the unit vector. Also, it can be seen that F₁₂ = − F₂₁. Gravitational acceleration is given by the same formula except for one of the factors m: :

\mathbf =
  G 
  \, \mathbf

Gravitational field

The gravitational field is a vector field that describes the gravitational force an object of given mass experiences in any given place in space. It is a generalization of the vector form, which becomes particularly useful if more than 2 objects are involved (such as a rocket between the Earth and the Moon). For 2 objects (e.g. object 1 is a rocket, object 2 the Earth), we simply write

\mathbf r

instead of

\mathbf r_

and

m

instead of

m_1

and define the gravitational field

\mathbf g(\mathbf r)

as: :

\mathbf g(\mathbf r) =
 G 
 \, \mathbf

so that we can write: :

\mathbf( \mathbf r) = m \mathbf g(\mathbf r)

This formulation is independent of the objects causing the field. The field has units of force divided by mass; in SI, this is N·kg⁻¹.

Problems with Newton's theory

Although Newton's formulation of gravitation is quite accurate for most practical purposes, it has a few problems:

Theoretical concerns

- There is no prospect of identifying the mediator of gravity. Newton himself felt the inexplicable action at a distance to be unsatisfactory (see "Newton's reservations" below).
- Newton's theory requires that gravitational force is transmitted instantaneously. Given classical assumptions of the nature of space and time, this is necessary to preserve the conservation of angular momentum observed by Johannes Kepler. However, it is in direct conflict with Einstein's theory of special relativity which places an upper limit—the speed of light in vacuum—on the velocity at which signals can be transmitted.

Disagreement with observation

- Newton's theory does not fully explain the precession of the perihelion of the orbit of the planet Mercury. There is a 43 arcsecond per century discrepancy between the Newtonian prediction (resulting from the gravitational tugs of the other planets) and the observed precession.
- The predicted deflection of light by gravity is only half as much as observations of this deflection, which were made after General Relativity was developed in 1915.
- The observed fact that gravitational and inertial masses are the same for all bodies is unexplained within Newton's system. General relativity takes this as a postulate. See equivalence principle.

Newton's reservations

It's important to understand that while Newton was able to formulate his law of gravity in his monumental work, he was deeply uncomfortable with the notion of "action at a distance" which his equations implied. He never, in his words, "assigned the cause of this power". In all other cases, he used the phenomenon of motion to explain the origin of various forces acting on bodies, but in the case of gravity, he was unable to experimentally identify the motion that produces the force of gravity. Moreover, he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was contrary to sound science. He lamented the fact that "philosophers have hitherto attempted the search of nature in vain" for the source of the gravitational force, as he was convinced "by many reasons" that there were "causes hitherto unknown" that were fundamental to all the "phenomena of nature". These fundamental phenomena are still under investigation and, though hypotheses abound, the definitive answer is yet to be found. While it is true that Einstein's hypotheses are successful in explaining the effects of gravitational forces more precisely than Newton's in certain cases, he too never assigned the cause of this power, in his theories. It is said that in Einstein's equations, "matter tells space how to curve, and space tells matter how to move", but this new idea, completely foreign to the world of Newton, does not enable Einstein to assign the "cause of this power" to curve space any more than the Law of Universal Gravitation enabled Newton to assign its cause. In Newton's own words: :I wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles; for I am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled towards each other, and cohere in regular figures, or are repelled and recede from each other; which forces being unknown, philosophers have hitherto attempted the search of nature in vain. If science is eventually able to discover the cause of the gravitational force, Newton's wish could eventually be fulfilled as well. It should be noted that here, the word "cause" is not being used in the same sense as "cause and effect" or "the defendant caused the victim to die". Rather, when Newton uses the word "cause," he (apparently) is referring to an "explanation". In other words, a phrase like "Newtonian gravity is the cause of planetary motion" means simply that Newtonian gravity explains the motion of the planets. See Causality and Causality (physics).

Einstein's theory of gravitation

Einstein's theory of gravitation answered the problems with Newton's theory noted above. In a revolutionary move, his theory of general relativity (1915) stated that the presence of mass, energy, and momentum causes spacetime to become curved. Because of this curvature, the paths that objects in inertial motion follow can "deviate" or change direction over time. This deviation appears to us as an acceleration towards massive objects, which Newton characterized as being gravity. In general relativity however, this acceleration or free fall is actually inertial motion. So objects in a gravitational field appear to fall at the same rate due to their being in inertial motion while the observer is the one being accelerated. (This identification of free fall and inertia is known as the Equivalence principle.) The relationship between the presence of mass/energy/momentum and the curvature of spacetime is given by the Einstein field equations. The actual shapes of spacetime are described by solutions of the Einstein field equations. In particular, the Schwarzschild solution (1916) describes the gravitational field around a spherically symmetric massive object. The geodesics of the Schwarzschild solution describe the observed behavior of objects being acted on gravitationally, including the anomalous perihelion precession of Mercury and the bending of light as it passes the Sun. Arthur Eddington found observational evidence for the bending of light passing the Sun as predicted by general relativity in 1919. Subsequent observations have confirmed Eddington's results, and observations of a pulsar which is occulted by the Sun every year have permitted this confirmation to be done to a high degree of accuracy. There have also in the years since 1919 been numerous other tests of general relativity, all of which have confirmed Einstein's theory.

Units of measurement and variations in gravity

tests of general relativity. (ESA image)]] Gravitational phenomena are measured in various units, depending on the purpose. The gravitational constant is measured in newtons times metre squared per kilogram squared. Gravitational acceleration, and acceleration in general, is measured in metres per second squared or in non-SI units such as galileos, gees, or feet per second squared. The acceleration due to gravity at the Earth's surface is approximately 9.81 m/s², more precise values depending on the location. A standard value of the Earth's gravitational acceleration has been adopted, called g_n. When the typical range of interesting values is from zero to tens of metres per second squared, as in aircraft, acceleration is often stated in multiples of g_n. When used as a measurement unit, the standard acceleration is often called "gee", as g can be mistaken for g, the gram symbol. For other purposes, measurements in millimetres or micrometres per second squared (mm/s² or µm/s²) or in multiples of milligals or milligalileos (1 mGal = 1/1000 Gal), a non-SI unit still common in some fields such as geophysics. A related unit is the eotvos, which is a cgs unit of the gravitational gradient. Mountains and other geological features cause subtle variations in the Earth's gravitational field; the magnitude of the variation per unit distance is measured in inverse seconds squared or in eotvoses. Typical variations with time are 2 µm/s² (0.2 mGal) during a day, due to the tides, i.e. the gravity due to the Moon and the Sun. A larger variation in the effect of gravity occurs when we move from the equator to the poles. The effective force of gravity decreases as the distance from the equator decreases, due to the rotation of the Earth, and the resulting centrifugal force and flattening of the Earth. The centrifugal force causes an effective force 'up' which effectively counteracts gravity, while the flattening of the Earth causes the poles to be closer to the center of mass of the Earth. It is also related to the fact that the Earth's density changes from the surface of the planet to its centre. The sea-level gravitational acceleration is 9.780 m/s² at the equator and 9.832 m/s² at the poles, so an object will exert about 0.5% more force due to gravity at sea level at the poles than at sea level at the equator [http://curious.astro.cornell.edu/question.php?number=310].

Comparison with electromagnetic force

The gravitational interaction of protons is approximately a factor 10³⁶ weaker than the electromagnetic repulsion. This factor is independent of distance, because both interactions are inversely proportional to the square of the distance. Therefore on an atomic scale mutual gravity is negligible. However, the main interaction between common objects and the Earth and between celestial bodies is gravity, because at this scale matter is electrically neutral: even if in both bodies there were a surplus or deficit of only one electron for every 10¹⁸ protons and neutrons this would already be enough to cancel gravity (or in the case of a surplus in one and a deficit in the other: double the interaction). However, the main interactions between the charged particles in cosmic plasma (that makes up over 99% of the universe by volume), are electromagnetic forces. In terms of Planck units: the charge of a proton is 0.085, while the mass is only . From that point of view, the gravitational force is not small as such, but because masses are small. The relative weakness of gravity can be demonstrated with a small magnet picking up pieces of iron. The small magnet is able to overwhelm the gravitational interaction of the entire Earth. Similarly, when doing a chin-up, the electromagnetic interaction within your muscle cells is able to overcome the force induced by Earth on your entire body. Gravity is small unless at least one of the two bodies is large or one body is very dense and the other is close by, but the small gravitational interaction exerted by bodies of ordinary size can fairly easily be detected through experiments such as the Cavendish torsion bar experiment. Cavendish torsion bar experiment Further reading
- Jefimenko, Oleg D., "Causality, electromagnetic induction, and gravitation : a different approach to the theory of electromagnetic and gravitational fields". Star City [West Virginia] : Electret Scientific Co., c1992. ISBN 0917406095
- Heaviside, Oliver, "[http://www.as.wvu.edu/coll03/phys/www/Heavisid.htm A gravitational and electromagnetic analogy]". The Electrician, 1893.

Gravity and quantum mechanics

It is strongly believed that three of the four fundamental forces (the strong nuclear force, the weak nuclear force, and the electromagnetic force) are manifestations of a single, more fundamental force. Combining gravity with these forces of quantum mechanics to create a theory of quantum gravity is currently an important topic of research amongst physicists. General relativity is essentially a geometric theory of gravity. Quantum mechanics relies on interactions between particles, but general relativity requires no exchange of particles in its explanation of gravity. Scientists have theorized about the graviton (a messenger particle that transmits the force of gravity) for years, but have been frustrated in their attempts to find a consistent quantum theory for it. Many believe that string theory holds a great deal of promise to unify general relativity and quantum mechanics, but this promise has yet to be realized. It is notable that in general relativity gravitational radiation (which under the rules of quantum mechanics must be composed of gravitons) is only created in situations where the curvature of spacetime is oscillating, such as for co-orbiting objects. The amount of gravitational radiation emitted by the solar system and its planetary systems is far too small to measure. However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as PSR1913+16). It is believed that neutron star mergers and black hole formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as LIGO have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.

Experimental tests of theories

Today General Relativity is accepted as the standard description of gravitational phenomena. (Alternative theories of gravitation exist but are more complicated than General Relativity.) General Relativity is consistent with all currently available measurements of large-scale phenomena. For weak gravitational fields and bodies moving at slow speeds at small distances, Einstein's General Relativity gives almost exactly the same predictions as Newton's law of gravitation. Crucial experiments that justified the adoption of General Relativity over Newtonian gravity were the classical tests: the gravitational redshift, the deflection of light rays by the Sun, and the precession of the orbit of Mercury. More recent experimental confirmations of General Relativity were the (indirect) deduction of gravitational waves being emitted from orbiting binary stars, the existence of neutron stars and black holes, gravitational lensing, and the convergence of measurements in observational cosmology to an approximately flat model of the observable Universe, with a matter density parameter of approximately 30% of the critical density and a cosmological constant of approximately 70% of the critical density. The equivalence principle, the postulate of general relativity that presumes that inertial mass and gravitational mass are the same, is also under test. Past, present, and future tests are discussed in the equivalence principle section. Even to this day, scientists try to challenge General Relativity with more and more precise direct experiments. The goal of these tests is to shed light on the yet unknown relationship between Gravity and Quantum Mechanics. Space probes are used to either make very sensitive measurements over large distances, or to bring the instruments into an environment that is much more controlled than it could be on Earth. For example, in 2004 a dedicated satellite for gravity experiments, called Gravity Probe B, was launched to test general relativity's predicted frame-dragging effect, among others. Also, land-based experiments like LIGO and a host of "bar detectors" are trying to detect gravitational waves directly. A space-based hunt for gravitational waves, LISA, is in its early stages. It should be sensitive to low frequency gravitational waves from many sources, perhaps including the Big Bang. Speed of gravity: Einstein's theory of relativity predicts that the speed of gravity (defined as the speed at which changes in location of a mass are propagated to other masses) should be consistent with the speed of light. In 2002, the Fomalont-Kopeikin experiment produced measurements of the speed of gravity which matched this prediction. However, this experiment has not yet been widely peer-reviewed, and is facing criticism from those who claim that Fomalont-Kopeikin did nothing more than measure the speed of light in a convoluted manner. The Pioneer anomaly is an empirical observation that the positions of the Pioneer 10 and Pioneer 11 space probes differ very slightly from what would be expected according to known effects (gravitational or otherwise). The possibility of new physics has not been ruled out, despite very thorough investigation in search of a more prosaic explanation.

Recent Alternative theories

- Brans-Dicke theory of gravity
- Rosen bi-metric theory of gravity
- In the modified Newtonian dynamics (MOND), Mordehai Milgrom proposes a modification of Newton's Second Law of motion for small accelerations.

Historical Alternative theories

- Nikola Tesla challenged Albert Einstein's theory of relativity, announcing he was working on a Dynamic theory of gravity (which began between 1892 and 1894) and argued that a "field of force" was a better concept and focused on media with electromagnetic energy that fill all of space.
- In 1967 Andrei Sakharov proposed something similar, if not essentially identical. His theory has been adopted and promoted by Messrs. Haisch, Rueda and Puthoff who, among other things, explain that gravitational and inertial mass are identical and that high speed rotation can reduce (relative) mass. Combining these notions with those of T. T. Brown, it is relatively easy to conceive how field propulsion vehicles such as "flying saucers" could be engineered given a suitable source of power.
- Georges-Louis LeSage proposed a gravity mechanism, now commonly called LeSage gravity, based on a fluid-based explanation where a light gas fills the entire universe.

Self-gravitating system

A self-gravitating system is a system of masses kept together by mutual gravity. An example is a binary star.

Special applications of gravity

A height difference can provide a useful pressure in a liquid, as in the case of an intravenous drip or a water tower, and can even supply enough power for hydroelectricity. A weight hanging from a cable over a pulley provides a constant tension in the cable, also in the part on the other side of the pulley. pulley Dubuque, Iowa]] Molten lead, when poured into the top of a shot tower, will coalesce into a rain of spherical lead shot, first separating into droplets, forming molten spheres, and finally freezing solid, undergoing many of the same effects as meteoritic tektites, which will cool into spherical, or near-spherical shapes in free-fall. A fractionation tower can be used to manufacture some materials by separating out the material components based on their specific gravity.

Comparative gravities of different planets and Earth's moon

The standard acceleration due to gravity at the Earth's surface is, by convention, equal to 9.80665 metres per second squared. (The local acceleration of gravity varies slightly over the surface of the Earth; see gee for details.) This quantity is known variously as g_n, g_e (sometimes this is the normal equatorial value on Earth, 9.78033 m/s²), g₀, gee, or simply g (which is also used for the variable local value). The following is a list of the gravitational accelerations (in multiples of g) at the Sun, the surfaces of each of the planets in the solar system, and the Earth's moon : Note: The "surface" is taken to mean the cloud tops of the gas giants (Jupiter, Saturn, Uranus and Neptune) in the above table. It is usually specified as the location where the pressure is equal to a certain value (normally 75 kPa?). For the Sun, the "surface" is taken to mean the photosphere. Within the Earth, the gravitational field peaks at the core-mantle boundary, where it has a value of 10.7 m/s². For spherical bodies surface gravity in m/s² is 2.8 × 10⁻¹⁰ times the radius in m times the average density in kg/m³. When flying from Earth to Mars, climbing against the field of the Earth at the start is 100 000 times heavier than climbing against the force of the sun for the rest of the flight.

Mathematical equations for a falling body

These equations describe the motion of a falling body under acceleration g near the surface of the Earth. mantle Here, the acceleration of gravity is a constant, g, because in the vector equation above,

_

would be a constant vector, pointing straight down. In this case, Newton's law of gravitation simplifies to the law :F = mg The following equations ignore air resistance and the rotation of the Earth, but are usually accurate enough for heights not exceeding the tallest man-made structures. They fail to describe the Coriolis effect, for example. They are extremely accurate on the surface of the Moon, where the atmosphere is almost nil. Astronaut David Scott demonstrated this with a hammer and a feather. Galileo was the first to demonstrate and then formulate these equations. He used a ramp to study rolling balls, effectively slowing down the acceleration enough so that he could measure the time as the ball rolled down a known distance down the ramp. He used a water clock to measure the time; by using an "extremely accurate balance" to measure the amount of water, he could measure the time elapsed. ² :For Earth

\ g=9.8\, \mbox/\mbox^2 \quad

For other planets, multiply

\ g

by the ratio of the gravitational accelerations shown above. Note: "Average" means average in time. Note: Distance traveled, d, and time taken, t, must be in the same system of units as acceleration g. See dimensional analysis. To convert metres per second to kilometres per hour (km/h) multiply by 3.6, and to convert feet per second to miles per hour (mph) multiply by 0.68 (or, precisely, 15/22).

Gravitational potential

For any mass distribution there is a scalar field, the gravitational potential (a scalar potential), which is the gravitational potential energy per unit mass of a point mass, as function of position. It is

- G \int dm

where the integral is taken over all mass. Minus its gradient is the gravity field itself, and minus its Laplacian is the divergence of the gravity field, which is everywhere equal to -4πG times the local density. Thus when outside masses the potential satisfies Laplace's equation (i.e., the potential is a harmonic function), and when inside masses the potential satisfies Poisson's equation with, as right-hand side, 4πG times the local density.

Acceleration relative to the rotating Earth

The acceleration measured on the rotating surface of the Earth is not quite the same as the acceleration that is measured for a free-falling body because of the centrifugal force. In other words, the apparent acceleration in the rotating frame of reference is the total gravity vector minus a small vector toward the north-south axis of the Earth, corresponding to staying stationary in that frame reference.

History of gravitational theory

The first mathematical formulation of gravity was published in 1687 by Sir Isaac Newton. His law of universal gravitation was the standard theory of gravity until work by Albert Einstein and others on general relativity. Since calculations in general relativity are complicated, and Newtonian gravity is sufficiently accurate for calculations involving weak gravitational fields (e.g., launching rockets, projectiles, pendulums, etc.), Newton's formulae are generally preferred. Although the law of universal gravitation was first clearly and rigorously formulated by Isaac Newton, the phenomenon was observed and recorded by others. Even Ptolemy had a vague conception of a force tending toward the center of the Earth which not only kept bodies upon its surface, but in some way upheld the order of the universe. Johannes Kepler inferred that the planets move in their orbits under some influence or force exerted by the Sun; but the laws of motion were not then sufficiently developed, nor were Kepler's ideas of force sufficiently clear, to make a precise statement of the nature of the force. Christiaan Huygens and Robert Hooke, contemporaries of Newton, saw that Kepler's third law implied a force which varied inversely as the square of the distance. Newton's conceptual advance was to understand that the same force that causes a thrown rock to fall back to the Earth keeps the planets in orbit around the Sun, and the Moon in orbit around the Earth. Newton was not alone in making significant contributions to the understanding of gravity. Before Newton, Galileo Galilei corrected a common misconception, started by Aristotle, that objects with different mass fall at different rates. To Aristotle, it simply made sense that objects of different mass would fall at different rates, and that was enough for him. Galileo, however, actually tried dropping objects of different mass at the same time. Aside from differences due to friction from the air, Galileo observed that all masses accelerate the same. Using Newton's equation,

F = m a

, it is plain to us why: :

F = - = m_1a_1

The above equation says that mass

m_1

will accelerate at acceleration

a_1

under the force of gravity, but divide both sides of the equation by

m_1

and: :

a_1 =

Nowhere in the above equation does the mass of the falling body appear. When dealing with objects near the surface of a planet, the change in r divided by the initial r is so small that the acceleration due to gravity appears to be perfectly constant. The acceleration due to gravity on Earth is usually called g, and its value is about 9.82 m/s². Galileo didn't have Newton's equations, though, so his insight into gravity's proportionality to mass was invaluable, and possibly even affected Newton's formulation on how gravity works. However, across a large body, variations in

r

can create a significant tidal force.

Notes

- Note 1: Proposition 75, Theorem 35: p.956 - I.Bernard Cohen and Anne Whitman, translators: Isaac Newton, The Principia: Mathematical Principles of Natural Philosophy. Preceded by A Guide to Newton's Principia, by I.Bernard Cohen. University of California Press 1999 ISBN 0-520-08816-6 ISBN 0-520-08817-4
- Note 2: See the works of Stillman Drake, for a comprehensive study of Galileo and his times, the Scientific Revolution.
- Max Born (1924), Einstein's Theory of Relativity (The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.)

References

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-
-

External links

- [http://einstein.stanford.edu/ Gravity Probe B Experiment]
- [http://www.hkshum.net/whatisgravity/ What Is Gravity? - Aimed for Kids 8+ ]
- [http://www.intelligent-forces.com Intelligent Forces Theory] Satirical "Anti-Gravitationalism" website Category:Introductory physics Category:Celestial mechanics ko:중력 ja:重力 ms:Graviti

Physical contact

:This article concerns physical and human touch. For other meanings, see Touch (disambiguation). Touching is having or getting a zero distance; in geometry it refers especially to a tangent line or curve (cf. collision). This term can be used to describe interaction between any physical objects. In medicine, the colloquial term "touch" is usually replaced with somatic senses, to better reflect the variety of mechanisms involved. Holding or moving something is usually done by touching (exceptions include blowing or using a magnet or engine), but this is sometimes done indirectly (e.g., with pliers). Touching another person is a form of physical intimacy and plays an important role in human sexual behavior. Similarly, touching oneself can be autoerotic; special is the dual feeling of a single skin contact. Touching is also integral in physical abuse (striking, pushing, pulling, pinching, kicking, strangling, etc.) and hand-to-hand fighting. In a sentence like "I never touched him/her" and "Don't you dare to touch him/her" the term touch may be meant as euphemism for either physical abuse or sexual touching. Touching is a form of nonverbal communication. Human babies have been observed to have enormous difficulty surviving if they do not possess a sense of touch, even if they retain sight and hearing. Babies who can perceive through touch, even without sight and hearing, fare much better. Touch can be considered a basic sense in that nearly all life forms have a response to being touched, while only a subset have sight and hearing. One can also be emotionally touched. In this metaphorical sense it refers to some action or object that has evoked a sad or joyful emotion. For example, to say "I was touched by your letter" would not imply the reader were angered by it, but that he or she felt joy or sadness when reading it. See also: body contact (dance), frotteurism, groping, grappling, haptic, massage, mosh, sexual harassment, tickling.

Quote

I needed so much/ To have nothing to touch/ I've always been greedy that way.. -Leonard Cohen, from The Night Comes On (1984)

External links

- [http://members.aol.com/doder1/touch1.htm Touch as nonverbal communication]
- [http://www2.rz.hu-berlin.de/sexology/GESUND/ARCHIV/SEN/CH23.HTM#b3-TOUCH%20AND%20SEXUALITY Touch and sexuality]
- [http://www.extendicare.com/consumer/article35.htm skin hunger]
- [http://www.touchmusic.org.uk UK audio-visual organisation] Category:Perception

Helicopter

]] , a four seat development of the R22]] A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors (propellers). Helicopters are classified as rotary-wing aircraft to distinguish them from conventional fixed-wing aircraft. The word helicopter is derived from the Greek words helix (spiral) and pteron (wing). The engine-driven helicopter was invented by the Slovak inventor Jan Bahyl. The first stable, fully-controllable helicopter placed in production was invented by Igor Sikorsky. Compared to conventional fixed-wing aircraft, helicopters are much more complex, more expensive to buy and operate, relatively slow, have shorter range and restricted payload. The compensating advantage is maneuverability: helicopters can hover in place, reverse, and above all take off and land vertically. Subject only to refuelling facilities and load/altitude limitations, a helicopter can travel to any location, and land anywhere with enough space (a diameter of length 1.5 times the rotor disk).

Applications

Helicopters have many uses, both military and civil, including troop transportation, infantry support, firefighting, [http://www.tropicaled.com/helicopter2.htm shipboard operations], business transportation, casualty evacuation (including MEDEVAC, and air/sea/mountain rescue), police and civilian surveillance, carrying goods (some helicopters can carry slung loads, accommodating awkwardly shaped items), or as a mount for still, film or television cameras. Helicopters suffer from significantly higher operating and maintenance costs compared with fixed wing aircraft. The costs are due to inherent mechanical complexity and greater power requirements for a given gross weight. For these reasons, helicopters are not economically viable for commercial transportation. Speed and range limitations also constrain commercial applications.

History

police] Since around 400 BC the Chinese had a flying top that was used as a children's toy. This toy eventually made its way to Europe via trade and has been depicted in a 1463 European painting. Incidentally, the Wright brothers as children were given a rubber-band-powered version of this toy invented by Alphonse Penaud and were very much fascinated by it and built their own copies. "Pao Phu Tau" was a 4th century book in China that described some of the ideas in a rotary wing aircraft. The first somewhat practical idea of a human carrying helicopter was first conceived by Leonardo da Vinci around 1490, but it was not until after the invention of the powered aeroplane in the 20th century that actual models were produced. Developers such as Jan Bahyl, Oszkár Asbóth, Louis Breguet, Paul Cornu, Emile Berliner, Ogneslav Kostovic Stepanovic and Igor Sikorsky pioneered this type of aircraft, with Juan de la Cierva introducing the first practical autogiro in 1923 that was to be the basis for the modern helicopter. A flight of the first fully controllable helicopter was demonstrated by Raúl Pateras de Pescara 1916 in Buenos Aires, Argentina. The German Focke-Wulf Fw 61 was the first practical helicopter. It first flew in 1934. The Bell 47 designed by Arthur Young was the first helicopter to be licensed (in March 1946) for use in the United States. Reliable helicopters capable of stable hover flight were developed decades after fixed wing aircraft. This is largely due to higher engine power density requirements when compared with fixed wing aircraft. Igor Sikorsky is reported to have delayed his own helicopter research until suitable engines were commercially available. Improvements in fuels and engines during the first half of the 20th century were a critical factor in helicopter development. The availability of lightweight turboshaft engines in the second half of the 20th century led to the development of larger, faster, and higher performance helicopters. Turboshaft engines are the preferred powerplant for all but the smallest and least expensive helicopters today.

Generating lift

A conventional aircraft is able to fly because the forward motion of its angled wings forces air downwards, creating an opposite reaction called lift that forces the wings upwards. A helicopter uses exactly the same method, except that instead of moving the entire aircraft, only the wings themselves are moved, in a circular motion. The helicopter's rotor can simply be regarded as rotating wings (hence the military appellation of "rotary wing aircraft"). lift

Conventional layout

There are several possible design layouts for arranging a helicopter's rotors. The most common design is the Sikorsky-layout, which is used by approximately 95% of all helicopters manufactured to date. It is as follows: turning the rotor generates lift but it also applies a reverse torque to the vehicle, which would spin the helicopter fuselage in the opposite direction to the rotor. At low speeds, the most common way to counteract this torque is to have a smaller vertical propeller mounted at the rear of the aircraft called a tail rotor. This rotor creates thrust which is in the opposite direction from the torque generated by the main rotor. When the thrust from the tail rotor is sufficient to cancel out the torque from the main rotor, the helicopter will not rotate around the main rotor shaft. The world's largest and smallest series-produced helicopters follow this principle. The Mil Mi-26 can lift 27 metric tons, the Robinson R22 has a crew of two and a gross weight of 1300 lbs (590 kg). Almost all civilian helicopters have the main rotor and tail rotor system. The world's fastest helicopter, the Westland Lynx can perform aerobatic loops and rolls with this conventional rotor system. aerobatic (Poland)]] Sometimes the blades of a tail rotor are not separated by the same angle, but laid out in an X-shape, which is supposed to reduce the noise levels for military use (e.g. AH-64 Apache). If the tail rotor is shrouded (i.e., a fan embedded in the vertical tail) it is called a fenestron. The fenestron rotor system on the model EC120 helicopter uses a shaft driven system and gearbox to turn the fan. It is less efficient but the advantages are that less noise is generated, it's safer for people that may walk near it and there is less chance of the blades being damaged by objects because it's shrouded, unlike the traditional tail rotor. Other helicopters use a Notar (an acronym meaning no tail rotor) design: they blow air through a long slot along the tail boom, utilizing the Coanda effect to produce forces to counter the torque. Notars adjust thrust by opening and closing a sliding circular cover near the end of the tail boom. The amount of power required to prevent a helicopter from spinning is significant. A tail rotor can use up to 30% of the engine's power, and this power does not help the helicopter produce lift or forward motion. To reduce this waste during cruise, the vertical stabilizer is often angled to produce a force which helps counter the main rotor torque. At high speeds, it is possible for the vertical stabilizer to counteract the entire torque, leaving more power available for forward flight. This is commonly known as slip-streaming and can make hovering turns difficult on windy days. Another reason for the angled vertical stabilizer is to make it possible to stage a successful high-speed, run-on landing, in case of the tail rotor failure or damage. Many military helicopters, especially attack types, have short wings called stub wings to add lift during forward motion. They are also used as external mounts for weapons. In extreme cases, such as that of the Mil Mi-24, the wings are large enough to obstruct airflow down from the rotors, making the helicopter all but unable to hover.

Alternative layouts

Mil Mi-24]] There are alternatives to Sikorsky's layout, which save the weight of a tail boom and rotor. Such designs use two rotors which turn in opposite directions, or contra-rotate. All of these systems are designed for the same purpose: to produce a net rotational speed of zero. These methods introduce even more mechanical complexity to the design and are usually relegated to specialized helicopter types. The co-axial design, where rotors are mounted on top of each other at the top of the fuselage and share a common main axle complex, was first built by Theodore von Karman and Asbóth Oszkár in 1918 and later became the hallmark of soviet Kamov design bureau (see for example the Kamov Ka-50 "Hokum"). Co-axial helicopters in flight are highly resistant to side-winds, which makes them suitable for shipboard use, even without a rope-pulley landing system. Another example is the Kamov Ka-26, a successful crop duster aircraft. The Kaman system of intermeshing rotors, which was developed in Nazi Germany for a small anti-submarine warfare helicopter, features two main rotors on separate, obliquely mounted axles. The contra-rotating rotors are located on top of the fuselage, close to each other. During the Cold War the American Kaman company started to produce similar helicopters for USAF firefighting purposes. Kamans have high stability and powerful lifting capability, thus the latest Kaman V-Max model is a dedicated sky crane design, used for construction works. In the flying-waggon or tandem rotor system (sometimes called "flying banana" for the peculiar shape of early U.S. examples), the two main rotors are located at the front and rear extremity of a long, boxy fuselage that resembles a railway wagon. A prime example is the Boeing CH-47 Chinook, that can carry 14 tons of payload. Waggon helicopters are practical for military logistical purposes, because entry and unloading is easily facilitated via the unobstructed front and rear ramps. The rotors and turbines are located very high on top of the fuselage, making them less sensitive to damage and dirt. The main drawback of a waggon is limited agility in air and the need for a highly trained crew, as the large main rotors have long outreach beyond the fuselage and may easily hit nearby obstacles (in 2001, a South Korean army CH-47 Chinook crashed onto a bridge for that reason while being shown live on TV). A helicopter built by Juan de la Cierva had three main rotors. These were placed at the corners of an equilateral triangle and all turned the same direction. equilateral triangle In the cross system, the rotary wing aircraft resembles a traditional fixed-wing airplane, with the two main rotors mounted at the extremities of its wings. Such helicopters are rare, because structural integrity of the wings is difficult to maintain against the amplified resonance of far off-board rotor-turbine units. The 1930s German FW-61 helicopter was built to such design. The world's largest ever helicopter, the Soviet Mil-V-12 prototype, was a cross of two Mil Mi-6 turbine-rotor units built onto a modified Antonov cargo plane. The U.S. V-22 Osprey tilting rotorcraft is similar, although its nacelles can be rotated, and shares some of the inherent technical problems of a cross system. nacelleA recent development in helicopter technology is the NOTAR system, which stands for NO TAil Rotor. The NOTAR eliminates the tail rotor by conducting high-velocity air through the tail boom. The NOTAR system was developed in the United States and is used exclusively by McDonnel Douglas Helicopters, or MD Helicopters. The most unusual design is the roto-rocket principle, where the single main rotor draws power not from the shaft, but from its own wingtip jet nozzles, which are either pressurized from a fuselage-mounted gas turbine or have their own pulsejet combustion chambers. Although this method is simple and eliminates precession, development of such helicopters ceased soon, because their extreme noise levels preclude both military and civilian use.

Controlling flight

Useful flight requires that an aircraft be controlled in all three dimensions (see flight dynamics). In a fixed-wing aircraft, this is easy: small movable surfaces are adjusted to change the aircraft's shape so that the air rushing past pushes it in the desired direction. In a helicopter, however, there often isn't enough airspeed for this method to be practical. flight dynamics, an aerodynamically restyled F28 for the corporate market.]] For rotation about the vertical axis (yaw) the anti-torque system is used. Varying the pitch of the tail rotor alters the sideways thrust produced. Dual-rotor helicopters have a differential between the two rotor transmissions that can be adjusted by an electric or hydraulic motor to transmit differential torque and thus turn the helicopter. Yaw controls are usually operated with anti-torque pedals, on the floor in the same place as a fixed-wing aircraft's rudder pedals. For pitch (tilting forward and back) or roll (tilting sideways) the angle of attack of the main rotor blades is altered or cycled during the rotation creating a differential of lift at different points of the rotary wing. More lift at the rear of the rotary wing will cause the aircraft to pitch forward, a increase on the left will cause a roll to the right and so on. Helicopters maneuver with three flight controls besides the pedals. The collective pitch control lever controls the collective pitch, or angle of attack, of the helicopter blades altogether, that is, equally throughout the 360 degree plane-of-rotation of the main rotor system. When the angle of attack is increased, the blade produces more lift. The collective control is usually a lever at the pilot's left side, near his leg. Simultanously increasing the collective and adding power with the throttle causes a helicopter to rise. angle of attack] The throttle controls the absolute power produced by the engine that is connected to the rotor by a transmission. The throttle control is a twist grip on the collective control. RPM control is critical to proper operation for several reasons. Helicopter rotors are designed to operate at a specific RPM. If the RPM is too low, rapid descent with power, known as settling with power could result. If the RPM is too high, damage to the main rotor hub from excessive forces could result. In general, RPM must be maintained within a tight tolerance, usually a few percent. In many piston-powered helicopters, the pilot must manage the engine and rotor RPM. The pilot manipulates the throttle to maintain rotor RPM and therefore regulates the effect of drag on the rotor system. Turbine engined helicopters, and some piston helicopters, use servo-feedback loop in their engine controls to maintain rotor RPM and relieves the pilot of routine responsibility for that task. The cyclic changes the pitch of the blades cyclically, causing the lift to vary across the plane of the rotor disk. This variation in lift causes the rotor disk to tilt, and the helicopter to move during hover flight or change attitude in forward flight. The cyclic is similar to a joystick and is usually positioned in front of the pilot. The cyclic controls the angle of the stationary section of the swashplate, which in turn controls the angle of the rotating section of the swashplate. The rotating section rotates with the rotor and is connected to blade pitch horns through pitch links, one link for each blade. When the swashplate is not tilted, the blades are all at the collective angle. When it is tilted, the links give a pitch-up at some azimuthal angle and a pitch-down at the opposite angle, hence creating a sinusoidal variation in blade angle of attack. This causes the helicopter to tilt in the same direction as the cyclic. If the pilot pushes the cyclic forward, then the helicopter tilts forward, and the rotor produces a thrust in the forward direction. angle of attack] As a helicopter moves forward, the rotor blades on one side move at rotor tip speed plus the aircraft speed and is called the advancing blade. As the blade swings to the other side of the helicopter, it moves at rotor tip speed minus aircraft speed and is called the retreating blade. To compensate for the added lift on the advancing blade and the decreased lift on the retreating blade, the angle of attack of the blades is regulated as the blade spins around the helicopter. The angle of attack is increased on the retreating blade to produce more lift, compensating for the slower airspeed over the blade. And the angle of attack is decreased on the advancing blade to produce less lift, compensating for the faster airspeed over the blade. If the angle of attack of any wing, including rotor blades, is too high, the airflow above the wing separates causing instant loss of lift and increase in drag. This condition is called aerodynamic stall. On a helicopter, this can happen in any of three ways. #As helicopter speed increases, the advancing blades approach the speed of sound and generate shock waves that disrupt the airflow over the blade causing loss of lift. #As helicopter speeds increase, the retreating blade experiences lower relative airspeeds and the controls compensate with higher angle of attack. With a low enough relative airspeed and a high enough angle of attack, aerodynamic stall is inevitable. This is called retreating blade stall. #Any low rotor RPM flight condition accompanied by increasing collective pitch application will cause aerodynamic stall. stall AH.1 (XV134), now on the UK Civil Register.]] Helicopters are powered aircraft, but they can still fly without power by using the momentum in the rotors and using downward motion to force air through the rotors. The main rotor acts like a "windmill" and turns. This technique is known as autorotation. A transmission connects the main rotor to the tail rotor so that all flight controls are available after engine failure. Autorotation can allow a pilot to make an emergency landing if the engine failure occurs while the helicopter is traveling high enough or fast enough. (see Height-velocity diagram). A very peculiar feature of the cyclic is that the lift is made to occur 90 degrees of rotation before the direction of tilt. This is because when one tries to tilt a spinning object (like a rotor), it moves at right angles to the direction of the force. This is called "gyroscopic precession". So control forces on the rotor are rotated 90 degrees before the desired motion. For example, forward motion requires less lift at the front of the disk and more lift at the rear of the disk, so the pilot pushes the cyclic forward. The helicopter's control linkages rotate the pitching forces 90 degrees backwards against the rotor spin, to push on the sides of the rotor rather than its front and back. It took inventors many years to recognize precession, and to learn how to arrange the cyclic's control system to overcome it.

Stability

Fixed wing aircraft are designed to be inherently stable. If a gust of wind or a nudge to one of the controls causes a fixed wing aircraft to pitch, roll, or yaw, the aerodynamic design of the aircraft will tend to correct the motion, and the aircraft will return to its original attitude. A small, fixed wing aircraft can be stable enough that a pilot can let go of the controls while looking at a map or dealing with a radio, and the plane will generally stay on course. precession In contrast, helicopters are very unstable. Simply hovering requires continuous, active corrections from the pilot. When a hovering helicopter is nudged in one direction by a gust of wind, it will tend to continue in that direction, and the pilot must adjust the cyclic to correct the motion. Hovering a helicopter has been compared to balancing yourself while standing on a large beach ball. Adjusting one flight control on a helicopter almost always has an effect that requires an adjustment of the other controls. Moving the cyclic forward causes the helicopter to move forward, but will also cause a reduction in lift, which will require extra collective for more lift. Increasing collective will reduce rotor RPM, requiring an increase in throttle to maintain constant rotor RPM. Changing collective will also cause a change in torque, which will require the pilot to adjust the foot pedals. Small helicopters can be so unstable that it may be impossible for the pilot to ever let go of the cyclic while in flight. While fixed-wing aircraft are generally designed so pilots sit on the left side of the aircraft, freeing up their right hand for dealing with radios, engine controls, and the like, helicopters are generally designed so pilots sit on the right side of the aircraft so they can keep their right hand (usually the strong hand) on the cyclic at all times, leaving the radios and engine controls for their left hand (usually the weaker hand).

Limitations

precession The single most obvious limitation of the helicopter is its slow speed. The current record is around 400 km/h set by the Westland Lynx. There are several reasons why a helicopter cannot fly as fast as a fixed wing aircraft.
- When the helicopter is at rest, the outer tips of the rotor travel at a speed determined by the length of the blade and the RPM. In a moving helicopter, however, the speed of the blades relative to the air depends on the speed of the helicopter as well as on their rotational velocity. The airspeed of the forward-going rotor blade is much higher than that of the helicopter itself. It is possible for this blade to exceed the speed of sound, and thus produce vastly increased drag and vibration. It is theoretically possible to have spiralling rotors, similar in principle to variable-pitch swept wings, which could exceed the speed of sound, but no presently known materials are light enough, strong enough, and flexible enough to construct them.
- Most rotors are not rigid. Because the advancing blade has higher airspeed than the retreating blade, a perfectly rigid blade would generate more lift on that side and tip the aircraft over. In consequence, rotor blades are designed to "flap" - lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack, thus producing less lift than a rigid blade would. Conversely, the retreating blade flaps down, develops a higher angle of attack, and generates more lift. At high speeds, the force on the rotors is such that they "flap" excessively and the retreating blade can reach too high an angle and stall. In some designs the hub is rigid. The blades are made from composites which can bend without breaking. Fully rigid rotors exist and create very responsive helicopters. In most such designs, the lift is varied cyclically and according to the speed of the helicopter. The adjustment is either by adjusting the angle of attack of the blades, or by engine-powered vacuum devices that suck air into the blades, adjusting the lift. speed of sound) twin rotor helicopter had a large cargo door and external hoist, and was used as personnel/paratroop transport, casualty evacuation, and for lifting large loads. The Belvedere had a production run of only 26 and went into RAF service in 1961.]]
- Rotorhead design is a limiting factor on many helicopters. Low or negative-G situations encountered in a semi-rigid system will result in blade flapping down until it hits the tail boom or other airframe structure, followed by rotor separation, causing a crash.
- Helicopters are susceptible to potentially disastrous vortex ring effects. In these, the downward wind from the rotor causes a circular vortex to form around the rotor. If this ring is augmented by terrain, wind, rain, or sea spray, the helicopter can lose enough lift to experience settling with power and hit the ground. During the closing years of the 20th century designers began working on helicopter noise reduction. Urban communities have often expressed great dislike of noisy aircraft, and police and passenger helicopters can be unpopular. The redesigns followed the closure of some city heliports and government action to constrain flight paths in national parks and other places of natural beauty. Helicopters vibrate. An unadjusted helicopter can easily vibrate so much that it will shake itself apart. To reduce vibration, all helicopters have rotor adjustments for height and pitch. Most also have vibration dampers for height and pitch. Some also use mechanical feedback systems to sense and counter vibration. Usually the feedback system uses a mass as a "stable reference" and a linkage from the mass operates a flap to adjust the rotor's angle of attack to counter the vibration. Adjustment is difficult in part because measurement of the vibration is hard. The most common adjustment measurement system is to use a stroboscopic flash lamp, and observe painted markings or coloured reflectors on the underside of the rotor blades. The traditional low-tech system is to mount coloured chalk on the rotor tips, and see how they mark a linen sheet.

Landing

On a ship

angle of attack] A helo deck is a helicopter pad on the deck of a ship, usually located on the stern and always clear of obstacles that would prove hazardous to a helicopter landing. In the U.S. Navy it is commonly and properly referred to as the flight deck. In the Royal Navy, landing on is usually achieved by lining up slightly astern and on the port quarter, as the ship steams into the wind and the aircraft captain slides across and over the deck. Shipboard landing for some helicopters is assisted though use of a haul-down device that involves attachment of a cable to a probe on the bottom of the aircraft prior to landing. Tension is maintained on the cable as the helicopter descends which assists the pilot with accurate positioning of the aircraft on the deck; once on deck locking beams close on the probe, locking the aircraft to the flight deck. This device was pioneered by the Royal Canadian Navy and was called "Beartrap". The U.S. Navy implementation of this device, based on Beartrap, is called the "RAST" system (for Recovery Assist, Secure and Traverse) and is an integral part of the LAMPS MK III (SH-60B) weapons system.

Hazards of helicopter flight

As with any moving vehicle, operation outside of safe regimes could result in loss of control, structural damage, or fatality. For helicopters the hazards are particularly acute since they are flying at relatively low altitude, with little time to react to a sudden event. The following is a list of some of the potential hazards:
- Retreating blade stall
- Settling with power
- Ground resonance
- Low-G condition
- Operating within the shaded area of the height-velocity diagram
- Vortex ring state, a problem the V-22 Osprey was associated with Each of these conditions is potentially fatal and recovery might not be possible. For this reason, good pilotage demands operation within safe flight regimes and avoiding hazardous conditions at all costs.

Helicopter models and identification

V-22 Osprey In identifying conventional helicopters during flight it is helpful to know that when viewed from below, the rotor of a French, Russian, Soviet or Ukrainian designed helicopter rotates counter-clockwise, whilst that of a helicopter built in Italy, the UK or the USA rotates clockwise (see list of helicopter models). Some companies, notably Schweizer in the USA, are developing remotely-controlled variants of light helicopters for use in future battlefields. [http://rotomotion.com/ Rotomotion] is currently selling a line of small (less than 50 kg) rotorcraft UAVs, including an all electric helicopter. Hybrid types that combine features of helicopters and fixed wing designs include the experimental Fairey Rotodyne of the 1950s and the Bell Boeing Osprey, which is on order by the U.S. Marine Corps and will be the first mass produced tilt-rotor aircraft to enter service. A helicopter should not be mistaken for an autogyro, which is a historical predecessor of the helicopter that gains lift from an unpowered rotor. Some common nicknames for helicopters are "copter", "chopper", "whirlybird", "windmill", "helo" (common U.S. Navy usage) or "paraffin budgie" (the latter term being mostly used in the UK offshore oil industry).

External links

- : "Aircraft, especially aircraft of the direct lift amphibian type and means of construction and operating the same"
- [http://www.helis.com/ Helicopter history]
- [http://centennialofflight.com/history/helicopter.html Helicopter history]
- [http://www.aerospaceweb.org/design/helicopter/history.shtml Image of a Chinese flying top]
- [http://www.centennialofflight.gov/essay/Rotary/early_20th_century/HE2.htm Helicopter development in the early 20th century]
- [http://www.centennialofflight.gov/essay/Dictionary/helicopter/DI27.htm Description of a helicopter]
- [http://www.heli-szene.de/ Helicopter pictures and videos (in German)]
- [http://www.mh-53pavelow.com/ Sikorsky MH-53J/M PAVE LOW helicopter]

References

- Thicknesse P, Jones A et al, Military Rotorcraft, 2nd edition, 2000, Brassey's World Military Technology series, Shirvenham UK, xvi + 160pp, ISBN 1857533259
- Wragg D, Helicopters at War: A pictorial history, 1983, Robert Hale Ltd, London UK, 283pp, ISBN 0709008589
- ko:헬리콥터 ja:ヘリコプター nb:Helikopter

Hovercraft

A hovercraft, or air-cushion vehicle (ACV), is a vehicle or craft that can be supported by a cushion of air ejected downwards against a surface close below it, and can in principle travel over any relatively smooth surface, such as gently sloping land, water, or marshland, while having no substantial contact with it. The first recorded design for a vehicle which could be termed a Hovercraft was in 1716 by Emanuel Swedenborg, a Swedish designer, philosopher and theologian. His man-powered air cushion platform resembled an upside-down boat with a cockpit in the center and manually operated oar-like scoops to push air under the vehicle on each downward stroke. No vehicle was ever built, no doubt because it was realised that human effort could not have generated enough lift. In the mid-1870s, the British engineer Sir John Thornycroft built a number of ground effect machine test models based on his idea of using air between the hull of a boat and the water to reduce drag. Although he filed a number of patents involving air-lubricated hulls in 1877, no practical applications were found. Over the years, various other people had tried various methods of using air to reduce the drag on ships. Col. Melville W. Beardsley (1913-1998), an American inventor and aeronautical engineer, along with Dr. W. Bertelsen worked on developing early ACV's in the USA. It was not until 1952 that the British inventor Christopher Cockerell designed a vehicle based on his 'hovercraft principle'. This was the missing link everyone else had not seen and made a commercial craft possible. He was knighted for his services to engineering in 1969 for his work on the Hovercraft. Sir Christopher even invented the word 'Hovercraft' to describe his invention. Cockerell used simple experiments involving a vacuum cleaner motor and two cylindrical cans he proved the workable principle of a vehicle suspended on a cushion of air blown out under pressure, making the vehicle easily mobile over most surfaces. His significant advance was developing a peripheral jet system to retain the air cushion under the vehicle. The supporting air cushion would enable it to operate over soft mud, water, and marshes and swamps as well as on firm ground. The British aircraft manufacturer Saunders Roe which had aeronautical expertise developed the first practical man-carrying hovercraft, the SR-N1, which carried out several test programmes in 1959 to 1961 (the first public demonstration in 1959), including a cross-channel run. The SR-N1 was powered by one (piston) engine, driven by expelled air, and could carry little more than its own weight and two men,and did not have any skirt at first trials. It was found that the craft's lift was improved by the addition of a 'skirt' of flexible fabric or rubber around the hovering surface, to contain the air. The skirt was a independant invention made by a Royal Navy officer who worked with Sir Christopher to develop the idea further. The first true passenger-carrying hovercraft was the Vickers VA-3, which in the summer of 1961 carried passengers regularly along the North Wales Coast from Wallasey to Rhyl. It was powered by two turboprop aero-engines and driven by propellers. During the 1960s Saunders Roe developed several larger designs which could carry passengers, including the SR-N2, which operated across the Solent in 1962 and later the SR-N6, which operated across the Solent from Southsea to Ryde on the Isle of Wight for many years. Operations commenced on 24th July 1965 using the SR-N6 which carried just 38 passengers. Two modern 98 seat AP1-88 hovercraft now ply this route, and over 20 million passengers have used the service as of 2004. As well as Saunders Roe and Vickers (which combined in 1966 to form the British Hovercraft Corporation), other commercial craft were developed during the 1960s in the UK by Cushioncraft (part of the Britten-Norman Group) and Hovermarine (the latter being 'sidewall' type hovercraft, where the sides of the hull projected down into the water to trap the cushion of air). In the late 1960s and early 1970s, Jean Bertin developed a hovercraft train dubbed the Aérotrain in France. His I-80 prototype established the world speed record for overland air cushion vehicles with a mean speed of 417.6 km/h (260 mp/h) and a top speed of 430 km/h (267 mp/h). By 1970 the largest British hovercraft were in service, the 'Mountbatten class' SR-N4 model, each powered by four Rolls-Royce Proteus engines, regularly carrying cars and passengers across the English Channel from Dover or Ramsgate to Calais. This service ceased in 2000 after years of competition with traditional ferries, catamarans, and the opening of the Channel tunnel. In 1998, the US Postal Service began using the British built Hoverwork AP.1-88 to haul mail, freight, and passengers From Bethel, Alaska to and from eight small villages along the Kuskokwim River. Bethel is far removed from the Alaska road system, thus making the hovercraft an attractive alternative to the air based delivery methods used prior to introduction of the hovercraft service. Hovercraft service is suspended for several weeks each year while the river is beginning to freeze to minimize damage to the river ice surface. The hovercraft is perfectly able to operate during the freeze-up period, however, it could potentially break the ice creating hazards for the villagers using their snowmobiles for transportation along the river during the early winter. The commercial success of hovercraft suffered from rapid rises in fuel prices during the late 1960s and 1970s following conflict in the Middle East. Alternative over-water vehicles such as wave-piercing catamarans (marketed as the Seacat in Britain) use less fuel and can perform most of the hovercraft's marine tasks. Although developed elsewhere in the world for both civil and military purposes, except for the Solent crossing, hovercraft disappeared from the coastline of Britain until a range of Griffon Hovercraft were bought by the Royal National Lifeboat Institution. There are an increasing number of small homebuilt and kit-built hovercraft used for fun and racing purposes, mainly on inland lakes and rivers but also in marshy areas and in some estuaries. Hovercraft typically have two (or more) separate engines (some craft, such as the SR-N6, have one engine with a drive split through a gearbox). One engine drives the fan (aka the impeller) which is responsible for lifting the vehicle by forcing air under the craft. One or more additional engines are used to provide thrust in order to propel the craft in the desired direction.

External links

- The Hovercraft Museum: http://www.hovercraft-museum.org/
- Hovercrafters - A Hovercraft Construction Journal : http://www.hovercrafters.com
- English hovercraft of the 1960s http://www.bartiesworld.co.uk/hovercraft/
- Hovercraft Club of Great Britain: http://www.hovercraft.org.uk
- Find a personal hovercraft for sale: http://www.hovercrafthomepage.com
- ABS Hovercraft, a major hovercraft designer/manufacturer: http://www.abs-hovercraft.com
- Hovercraft books, models and plans: http://www.hovercraftmodels.com Category:Amphibious vehicles ja:ホバークラフト

Magnetic force

A magnet is an object that has a magnetic field. The word magnet comes from the Greek "magnítis líthos" (μαγνήτης λίθος), which means "magnesian stone". Magnesia is an area in Greece (Now Manisa, Turkey) where deposits of magnetite have been discovered since antiquity.

Introduction

In the modern sense, a magnet is any material that has a magnetic field. It can be in the form of a permanent magnet or an electromagnet. Permanent magnets do not rely upon outside influences to generate their field. Electromagnets rely upon electric current to generate a magnetic field - when the current increases, so does the field. Magnets are attracted to or repelled by other things. If a magnet is strongly attracted to something, then that something is said to have a high permeability. Iron and steel are two examples of materials with very high permeability, and they are strongly attracted to magnets. Liquid oxygen is an example of something with a low permeability, and it is only weakly attracted to a magnetic field. Water has such a low permeability that it is actually repelled by magnetic fields. Everything has a measurable permeability: people, air and even the vacuum of space.

Physical origin of magnetism

Permanent Magnets

All normal matter is composed of particles (protons, neutrons, and electrons), and all of these particles have the fundamental property of quantum mechanical spin. Spin gives each one of these particles an associated magnetic field. Because of this, and the fact that the average macroscopic piece of matter contains huge numbers of these particles, it would be expected that all matter would be magnetic. Everyday experience shows that this is not the case. Within each atom and molecule, the spin of each of these particles is highly ordered as a result of the Pauli Exclusion Principle. However, there is no long range ordering of these spins between atoms and molecules. Without long range ordering, there is no net magnetic field because the magnetic moment of each one of the particles is cancelled by the magnetic moment of other particles. Permanent magnets are special in that long range ordering does exist. The highest degree of ordering exists within magnetic domains. These domains can be likened to microscopic neighbourhoods in which there is a strong reinforcing interaction between particles, and as a result, a great deal of order. The greater the degree of ordering within and between domains, the greater the resulting field will be. Long range ordering (and the resulting strong net magnetic field) is one of the hallmarks of a ferromagnetic material.

More detail

Electrons play the primary role in generating a magnetic field. Within an atom, electrons can exist either individually or in pairs within any given orbital. When they are paired, the individuals in that pair always have opposite spin (one up, one down). The fact that the spins have opposite orientation means that the two cancel one another. If all electrons are paired, no net magnetic field will be generated. In some atoms, there are electrons that are unpaired. All magnets have unpaired electrons, but not all atoms with unpaired electrons are ferromagnetic. In order for the material to become ferromagnetic, not only must there be unpaired electrons present, but those unpaired electrons must interact with one another over long ranges such that they are all oriented in the same way. The specific electron configuration of the atoms (as well as the distance between atoms) is what leads to this long range ordering. The electrons find that they can exist in a lower energy state if they all have the same orientation.

Electromagnets

electron configuration. There are four steel pole tips, two opposing magnetic north poles and two opposing magnetic south poles. The steel is magnetized by a large electric current that flows in the coils of tubing wrapped around the poles.]] An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops. This coil is known as a solenoid. When electric current flows along the coil, a magnetic field is generated around the coil. The orientation of this field can be determined via the right hand rule. The strength of the field is influenced by several factors, including:
- the number of loops
- the amount of current
- the material in the core The more loops of wire and the greater the current, the stronger the field will be. If the coil of wire is empty in the center, it will tend to generate a very weak field. Different ferromagnetic or paramagnetic items can be placed in the center of the core with the effect of magnifying the magnetic field, for example an iron nail (soft iron is commonly used for this purpose). The addition of these types of materials can result in a several hundred- to thousand-fold increase of field strength. At long distances, magnetic fields obey an inverse square law. This means that the field strength is inversely proportional to the distance from the magnet. If the face of an electromagnet is machined to a high degree of precision, it will be able to get much closer to the surface it is trying to attract. Take the case of an electromagnet trying to attract an extremely smooth, flat metal plate. If the electromagnet's face is extremely smooth and flat as well, there will be many more points of contact with the plate, and so the magnetic circuit will have less resistance to the magnetic field. Electromagnets find uses in many places, ranging from particle accelerators, to junkyard cranes, to MRI machines. If an electromagnet is strong enough, the magnetic force between neighbouring loops of wire can cause the electromagnet to be crushed by its own magnetic field.

Characteristics of magnetic materials

Permanent magnets and dipoles

All magnets are dipoles: that is, all magnets have a north and a south pole. The poles are not a pair of things on or inside the magnet. They are a concept used to discuss and describe magnets. In the image at the top of this page, the poles look like specific locations (because the highest surface intensity of the field occurs at the poles), but this does not mean that they are specific locations. To understand the concept of pole, imagine a row of people who are all facing the same direction and standing in line. While there is a "face" end of the line and a "back" end of the line, there is no one place where all of the faces are and all of the backs are. The person at the front of the face end has a back; and the person at the back end has a face. If you divide the line into two shorter lines, each one of the shorter lines still has a face end and a back end. Even if you pull the line completely apart so that there are just individuals standing around, each one of the individuals still has a face and a back. This can continue without end. The same holds true with magnets. There is not one place where all of the north or south poles are. If a magnet is divided in two, two magnets will result--and both magnets will have a north and a south pole. Those smaller magnets can then be divided, and all of the resulting pieces will have both a north and south pole. In most instances, if the material continues to be broken into smaller and smaller pieces there will be a point where the pieces are too small to retain a net magnetic field. They won't become individual north or south poles though; instead, they will just lose the ability to maintain a net field. Some materials, however, can be divided down to the molecular level and still maintain a net field with both a north and a south pole. There are theories involving the possibility of north and south magnetic monopoles, but no magnetic monopole (single pole) has ever been found.

North/south pole designation and the Earth's magnetic field

A standard naming system for the poles of magnets is important. Historically, the terms north and south reflect awareness of the relationship between magnets and the earth's magnetic field. A freely suspended magnet will eventually orient itself north-to-south, because of its attraction to the north and south magnetic poles of the earth. The end of a magnet that points toward the Earth's geographic North Pole is labeled as the north pole of the magnet; correspondingly, the end that points south is the south pole of the magnet. The Earth's current geographic north is thus actually its magnetic south. Confounding the situation further, it is known that the Earth's magnetic field has reversed itself in the past, so this system of naming is likely to be backward at some time in the future (see Earth's magnetic field). Fortunately, by using an electromagnet and the right hand rule, the orientation of the field of a magnet can be defined without reference to the Earth's geomagnetic field. To avoid the confusion between geographic and magnetic north and south poles, the terms positive and negative are sometimes used for the poles of a magnet. The positive pole is that which seeks geographical north.

Common uses for magnets

- Magnetic recording media: Common VHS tapes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. This is why magnets will destroy the information in these types of tapes. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.
- Credit, debit, and ATM cards: All of these cards have a magnetic strip on one of their sides. This strip contains the necessary information to contact an individuals financial institution and connect with their account(s).
- Common televisions and computer monitors: The vast majority of TV's and computer screens rely in part on an electromagnet to generate an image--see the article on cathode ray tubes for more information. Plasma screens and LCDs rely on different technology entirely.
- Loudspeakers and microphones: Loudspeakers actually rely on a combination of a permanent magnet and an electromagnet. A speaker is fundamentally a device to convert electric energy (the signal) into mechanical energy (the sound). The electromagnet carries the signal, which generates a changing magnetic field that pushes and pulls on the field generated by the permanent magnet. This pushing and pulling moves the cone, which creates sound. Not all speakers rely on this technology, but the vast majority do. Standard microphones are based upon the same concept, but run in reverse. A microphone has a cone or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is generated in the coil (see Lenz's Law). This voltage in the wire is now an electric signal that is representative of the original sound.
- Electric motors and generators: Some electric motors (much like loudspeakers) rely upon a combination of an electromagnet and a permanent magnet, and much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy.
- Transformers: Transformers are devices that transfer electric energy between two devices that are electrically disconnected via magnetic coupling.
- Chucks: Chucks are used in the metalworking field to hold objects. If these objects can be held securely with a magnet then a permanent or electromagnetic chuck may be used. Magnets are also used in other types of fastening devices, such as the magnetic base, the magnetic clamp and the refrigerator magnet.

How to magnetize materials

Ferromagnetic materials can be magnetised in the following ways:
- Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the earth's magnetic field and which are subject to vibration (eg frame of a conveyor) have been shown to aquire significant residual magnetism.
- Placing the item in a solenoid with a direct current passing through it.
- Stroking - An existing magnet is moved from one end of the item to the other repeatedly in the same direction.

How to demagnetize materials

Permanent magnets can be demagnetized in the following ways:
- Heat. Heating a magnet past its Curie point will destroy the long range ordering.
- Contact. Stroking one magnet with another in random fashion will demagnetize the magnet being stroked, in some cases; some materials have a very high coercive field and cannot be demagnetized with other permanent magnets.
- Hammering or jarring. Such activity will destroy the long range ordering within the magnet.
- Being placed in a solenoid which has an alternating current being passed through it. The alternating current will disrupt the long range ordering, in much the same way that direct current can cause ordering. In an electromagnet, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.

Types of permanent magnets

- Rare Earth or Neodymium Magnets, which are some of the most powerful permanent magnets
- Samarium-Cobalt Magnets
- Ceramic Magnets
- Plastic Magnets
- Alnico Magnets

Magnetic forces

Magnetized items interact with other items in very specific ways.

Magnets and other magnets

If a magnet is brought close enough to another magnet, their fields will begin to interact in the following ways:
- If each magnets north pole is brought together, the magnets will repel one another (like poles repel)
- If the north pole of one magnet is brought to the south pole of the other magnet (or vice versa), the magnets will attract one another (opposite poles attract)

Magnets and ferromagnetic materials

If a magnet is brought close enough to a ferromagnetic material (that is not magnetized itself), the magnet will strongly attract the ferromagnetic material regardless of orientation. Both the north and south pole of the magnet will attract the other item with equal strength.

Magnets and diamagnetic materials

By definition, diamagnetic materials weakly repel a magnetic field. This occurs regardless of the north/south orientation field.

Magnets and paramagnetic materials

By definition, paramagnetic materials are weakly attracted to a magnetic field. This occurs regardless of the north/south orientation of the field.

Calculating the magnetic force

Calculating the attractive or repulsive force between two magnets is, in the general case, an extremely complex operation, as it depends on the shape, magnetization, orientation and separation of the magnets. However, a formula exists for the simple case of the force between two magnetic poles: :

F=

[http://geophysics.ou.edu/gravmag/mag_basic/mag_basic.html] where :F is force (SI unit: newton) :m is pole strength (SI unit: weber) :μ is the permeability of the intervening medium (SI unit: tesla meter per ampere) :r is the separation (SI unit: meter).

Online references

- [http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html HyperPhysics E/M], good complete tree diagram of electromagnetic relationships with magnets
- [http://www.arnoldmagnetics.com/mtc/index.htm Magnet Education] and [http://www.magnetsales.com/Design/DesignG.htm Understanding Commercial Magnets] useful companies who have many helpful magnet equations
- [http://en.wikipedia.org/wiki/Maxwell%27s_equations Maxwell's Equations] and some history...
- [http://www.oz.net/~coilgun/theory/home.htm Detailed Theory on Designing a Solenoid] or a Coil Gun
- [http://www.makeitlouder.com/High%20Technology.html Help on designing magnet systems]

Printed references

- "positive pole n." The Concise Oxford English Dictionary. Ed. Catherine Soanes and Angus Stevenson. Oxford University Press, 2004. Oxford Reference Online. Oxford University Press.

External articles

- Joseph J. Stupak Jr., "[http://oersted.com/magn

Levitation

See also

Gravity

Newton's law of universal gravitation

Vector form

Gravitational field

Problems with Newton's theory

Theoretical concerns

Disagreement with observation

Newton's reservations

Einstein's theory of gravitation

Units of measurement and variations in gravity

Comparison with electromagnetic force

Gravity and quantum mechanics

Experimental tests of theories

Recent Alternative theories

Historical Alternative theories

Self-gravitating system

Special applications of gravity

Comparative gravities of different planets and Earth's moon

Mathematical equations for a falling body

Gravitational potential

Acceleration relative to the rotating Earth

History of gravitational theory

Notes

See also

References

External links

Physical contact

Quote

External links

Helicopter

Applications

History

Generating lift

Conventional layout

Alternative layouts

Controlling flight

Stability

Limitations

Landing

On a ship

Hazards of helicopter flight

Helicopter models and identification

See also

External links

References

Hovercraft

See also

Related topics

External links

Magnetic force

Introduction

Physical origin of magnetism

Permanent Magnets

More detail

Electromagnets

Characteristics of magnetic materials

Permanent magnets and dipoles

North/south pole designation and the Earth's magnetic field

Common uses for magnets

How to magnetize materials

How to demagnetize materials

Types of permanent magnets

Magnetic forces

Magnets and other magnets

Magnets and ferromagnetic materials

Magnets and diamagnetic materials

Magnets and paramagnetic materials

Calculating the magnetic force

See also

Online references

Printed references

External articles