When an object moves in a circular path the object is accelerating because the velocity is constantly changing direction in order that it can keep moving in a circle. Since acceleration is cause by a force, there must be a net resultant force acting on the object. The acceleration isand is directed towards the centre of the circle, so the force must be directed towards the centre. The force,found by using is called the centripetal force. Without this force, an object will simply continue moving in a straight line. The force may have many origins – gravitational, the tension in a rope, electrical, and we can analyse the system be equation this force to the centripetal force.

Example: The force of gravity which keeps the Earth moving around the Sum, with an orbit very nearly a circle.

Example: The force of friction between the road and a car's tyres keeps the car on the road when the car rounds a bend.