from
The Collaborative International Dictionary of English v.0.48
escape velocity \es*cape" vel*o"ci*ty\, n. (Physics)
The minimum velocity at which an object must be moving in
order for it to overcome the gravitational attraction of a
massive celestial body, such as the earth or the sun, and
escape beyond its gravitational field into free space. The
velocity is calculated as though attained instantaneously at
the surface of the celestial body, and is pointed directly
away from its center, and neglecting effects of atmospheric
friction. Rockets, which accelerate gradually and are moving
rapidly at a high altitude when their fuel is exhausted or
their engines shut off, may escape even if moving slightly
slower at that point than the surface escape velocity.
Compare orbital velocity.
[PJC]
Note: The escape velocity at the surface of the earth is 11.2
km/sec (25,100 miles per hour), at the moon's surface
is 2.4 km/sec, and at the sun's surface is 617.7
km/sec. The escape velocity is calculated as:
Ve = root{2Rg}
where R is the radius of the celestial body and g is
the acceleration due to the gravitational field at its
surface. The peculiar chracteristic of a {black hole}
is that the escape velocity at its "surface" (called
its "event horizon") is greater than the speed of
light. Therefore nothing, not even light, may escape
from it. --Dict. Sci. Tech.
[PJC]