Velocity

The velocity of an object is the rate of change of its displacement and direction of the change. In other words, it is a combination of both speed and direction. If the position of an object is some function of time, then the velocity is the derivative of that function:

<math>v(t)={\mathrm{d}x \over \mathrm{d}t}</math>

where x is the position and v is the velocity. The velocity is a vector, and its length or absolute value is called the speed. The velocity has dimensions of length / time, and may thus be expressed in meters per seconds (in scientific usage), or miles per hour in more common usage.

For an object moving along a straight trajectory, if acceleration and velocity have the same sign, the object is gaining speed. If acceleration and velocity have different signs, the object is losing speed.

If velocity is zero, acceleration is not necessarily zero. If acceleration is zero, velocity is constant, but not necessarily zero.