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 ]

☞ 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 = 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 ]

n.; pl. Velocities [ L. velocitas, from velox, -ocis, swift, quick; perhaps akin to volare to fly (see Volatile): cf. F. vélocité. ] [ 1913 Webster ]

1. Quickness of motion; swiftness; speed; celerity; rapidity; as, the velocity of wind; the velocity of a planet or comet in its orbit or course; the velocity of a cannon ball; the velocity of light. [ 1913 Webster ]

☞ In such phrases, velocity is more generally used than celerity. We apply celerity to animals; as, a horse or an ostrich runs with celerity; but bodies moving in the air or in ethereal space move with greater or less velocity, not celerity. This usage is arbitrary, and perhaps not universal. [ 1913 Webster ]

2. (Mech.) Rate of motion; the relation of motion to time, measured by the number of units of space passed over by a moving body or point in a unit of time, usually the number of feet passed over in a second. See the Note under Speed. [ 1913 Webster ]

Angular velocity. See under Angular. --
Initial velocity, the velocity of a moving body at starting; especially, the velocity of a projectile as it leaves the mouth of a firearm from which it is discharged. --
Relative velocity, the velocity with which a body approaches or recedes from another body, whether both are moving or only one. --
Uniform velocity, velocity in which the same number of units of space are described in each successive unit of time. --
Variable velocity, velocity in which the space described varies from instant to instant, either increasing or decreasing; -- in the former case called accelerated velocity, in the latter, retarded velocity; the acceleration or retardation itself being also either uniform or variable. --
Virtual velocity. See under Virtual. [ 1913 Webster ]

☞ In variable velocity, the velocity, strictly, at any given instant, is the rate of motion at that instant, and is expressed by the units of space, which, if the velocity at that instant were continued uniform during a unit of time, would be described in the unit of time; thus, the velocity of a falling body at a given instant is the number of feet which, if the motion which the body has at that instant were continued uniformly for one second, it would pass through in the second. The scientific sense of velocity differs from the popular sense in being applied to all rates of motion, however slow, while the latter implies more or less rapidity or quickness of motion. [ 1913 Webster ]

Syn. -- Swiftness; celerity; rapidity; fleetness; speed. [ 1913 Webster ]