velocity, change in displacement with respect to time. Displacement is the vector counterpart of distance, having both magnitude and direction. Velocity is therefore also a vector quantity. The magnitude of velocity is known as the speed of a body. The average velocity or average speed of a moving body during a time period t may be computed by dividing the total displacement or total distance by t. Computation of the instantaneous velocity at a particular moment, however, usually requires the methods of the calculus.

radial velocity, in astronomy, the speed with which a star moves toward or away from the sun. It is determined from the red or blue shift in the star's spectrum.

escape velocity, the velocity a body must be given in order to escape the gravitational hold of some other larger body, e.g., the earth, moon, or sun. A body given less than the escape velocity will fall back toward the surface of the larger body; a body given a velocity equal to or greater than the escape velocity will still be attracted by the larger body, but this force will not be sufficient to cause it to return. Escape velocity depends on the mass of the larger body and the distance of the smaller body from its center, being proportional to the square root of the ratio of these two quantities. The velocity of escape from the earth at its surface is about 7 mi (11.3 km) per sec, or 25,000 mi per hr; from the moon's surface it is 1.5 mi (2.4 km) per sec; and for a body at the earth's distance from the sun to escape from the sun's gravitation, the velocity must be 26 mi (41 km) per sec.

Quantity that designates the speed and direction in which a body moves. It can be represented graphically by an arrow (pointing in the direction of the motion), the length of which is proportional to the magnitude, or speed. For an object in circular motion, the direction at any instant is tangential to the circle at that point, and so is perpendicular to the radius at that point. The instantaneous speed of a vehicle, such as an automobile, can be determined by a speedometer, or mathematically by differential calculus. The average speed is the ratio of the distance traveled in any given time interval divided by the time taken.

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Speed sufficient for a body to escape from a gravitational centre of attraction without accelerating further. It decreases with altitude and equals the square root of 2 (about 1.414) times the speed needed to maintain a circular orbit at the same altitude. At the surface of Earth, disregarding atmospheric resistance, escape velocity is about 6.96 mi/second (11.2 km/second). Escape velocity from the surface of the Moon is about one-third of this.

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