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Velocity is a vector expression of the displacement that an object or particle undergoes with respect to time . The standard unit of velocity magnitude (also known as speed ) is the meter per second (m/s). Alternatively, the centimeter per second (cm/s) can be used to express velocity magnitude. The direction of a velocity vector can be expressed in various ways, depending on the number of dimensions involved.

Velocity is relative. Consider a car moving at 20 m/s with respect to the surface of a highway, traveling northward. If you are driving the car, the velocity of the car relative to your body is zero. If you stand by the side of the road, the velocity of the car relative to you is 20 m/s northward. If you are driving a car at 15 m/s with respect to the road and are traveling northward, and another car moving 20 m/s with respect to the road passes you in the same direction, that other car's velocity relative to you is 5 m/s northward. But if that other car passes you going the opposite way on the road, its velocity relative to you is 35 m/s southward.

Circular motion illustrates the fundamental difference between speed and velocity. Think of yourself whirling a ball, tied to a string, around your body so the ball's tangential speed is 10 m/s. Even though the ball's speed is constant relative to your body, its velocity relative to your body constantly changes as the direction vector describes circles.

Velocity can be expressed either as an average over a period of time, or as an instantaneous value at a single moment in time. Suppose you are in a car that is not moving relative to the road surface, and then you hit the accelerator and increase velocity uniformly from zero to 30 m/s northward in a few seconds. Your average velocity over that time frame is 15 m/s northward. However, the instantaneous velocity depends on time, and might be anything between zero and 30 m/s northward, depending on the exact moment at which it is measured.

In agile software development , velocity (V) is the budget of story units available for planning the next iteration of a development project. Velocity is based on measurements taken during previous iteration cycles. Velocity is calculated by adding the original estimates of the stories that were successfully delivered in an iteration.

In biochemistry , velocity (V) is the number of reactions per second catalyzed per mole of an enzyme .

Also see meter per second , vector , International System of Units ( SI ), and the Table of Physical Units .

This was last updated in June 2010
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