Motion may be uniform or variable. When equal distances are traversed or covered in the same length of time the speed is constant. On the other hand, when the speed changes and equal distances are not traversed or covered in the same length of time, the motion is said to be variable. The rate of change of velocity is called the acceleration. It is said to be positive when it increases, and negative when it decreases.
A body moving from one place to another in a straight line is said to undergo translation or rectilinear motion. When a body moves around a fixed point or axis it is said to rotate. The particles of the body make concentric circles, as a pulley rotates on a shaft; such motion is said to be curvilinear.