/veuh los"i tee/, n., pl. velocities.
1. rapidity of motion or operation; swiftness; speed: a high wind velocity.
2. Mech. the time rate of change of position of a body in a specified direction.
3. the rate of speed with which something happens; rapidity of action or reaction.
[1540-50; < L velocitas speed. See VELOCIPEDE, -TY2]
Syn. 1. See speed.

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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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      quantity that designates how fast and in what direction a point is moving. A point always moves in a direction that is tangent to its path; for a circular path, for example, its direction at any instant is perpendicular to a line from the point to the centre of the circle (a radius). The magnitude of the velocity (i.e., the speed) is the time rate at which the point is moving along its path.

      If a point moves a certain distance along its path in a given time interval, its average speed during the interval is equal to the distance moved divided by the time taken. A train that travels 100 km in 2 hours, for example, has an average speed of 50 km per hour.

      During the two-hour interval, the speed of the train in the previous example may have varied considerably around the average. The speed of a point at any instant may be approximated by finding the average speed for a short time interval including the instant in question. The differential calculus (differentiation), which was invented by Isaac Newton for this specific purpose, provides means for determining exact values of the instantaneous velocity.

      Because it has direction as well as magnitude, velocity is known as a vector quantity and cannot be specified completely by a number, as can be done with time or length, which are scalar quantities. Like all vectors, velocity is represented graphically by a directed line segment (arrow) the length of which is proportional to its magnitude.

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Universalium. 2010.