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If one throws an object into the air, gravity will tend to slow it down, stop
it, and then pull it back down to the surface of the Earth. The faster
one throws the object, the further up it will go before stopping. If
one throws it fast enough it will never stop and will continue travelling
away from the Earth forever. The speed which an object has to travel
to escape the gravitational field of another object is called the
escape velocity.
The escape velocity is determined by two quantities for a given object, namely the mass and the size. More massive objects have higher escape velocities. For a given mass, smaller objects are denser, and thus also have a higher escape velocity. The (link pop window: derivation.html) *formula* for calculating the escape velocity is fairly straightforward. In the 18th Century an otherwise obscure English clergyman named John Michell was the first to consider the properties of an object whose escape velocity is equal to that of the speed of light. [REFERENCE PAPER - we may need to scan part of the paper and highlight the parts we want people to read]. Michell determined the relationship of size and mass for objects whose escape velocity was the speed of light. He then proceeded to consider what the properties of such an object might be. Michell concluded that, since light would be unable to escape the gravitational field of the object, such objects would be dark and, hence, unobservable. Michell's calculations represent the first consideration of BLACK HOLES. The most basic definition of a black hole is simply an object whose escape velocity is equal to or greater than the speed of light. It was not until Einstein developed the theory of Special Relativity, however, that the bizarre properties of black holes began to be appreciated. Special Relativity showed that the speed of light is a very special velocity. |
Click (link popup window: http://www3.ltu.edu/~s_schneider/physlets/main/gravity_launch.shtml) *here* for an animation on how escape velocity works.
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