  ##### Circular motion The force that pulls an object out of a straight line path into a circular orbit is called the centripetal force. (The word means centre seeking). It is the force on the orbiting object.

Angular velocity
When an object is travelling in a circle it has an instantaneous linear velocity but it also has an angular velocity (w). This is defined as the rate of change of angle with time and is usually expressed in radians per second.

 Period of the motion (T) = circumference/linear velocity =2pr/v = total angle /angular velocity = 2p/w

Since T is the period (the time to make one complete rotation) the number of rotations per second (n) is 1/T and is the frequency of the motion.

 Linear velocity and angular velocity: v = rw

When an object moves in a circle the linear velocity must be constantly changing as the direction of motion is changing - there must therefore be an acceleration - the centripetal acceleration, and therefore a force - the centripetal force. Both the centripetal acceleration and the centripetal force are directed towards the centre of the circle.