Moving in a Circle (Also seen in GCSE Physics 3)

For an object to continue to move in a circle a force is needed that acts on the object towards the centre of the circle. This is called the centripetal force and is provided by a number of things:

For a satellite orbiting the Earth it is provided by gravitational attraction.

For a car driving around a roundabout it is provided by the friction between the wheels and the road.

For a ball on a string being swung in a circle it is provided by the tension in the string.

Centripetal force acts from the body to the centre of a circle

Since F=ma the object must accelerate in the same direction as the resultant force. The object is constantly changing its direction towards the centre of the circle.

Centripetal acceleration has direction from the body to the centre of the circle

Centrifugal Force

Some people thought that an object moving in a circle would experience the centripetal force acting from the object towards the centre of the circle and the centrifugal force acting from the object away from the centre of the circle.

They thought this because if you sit on a roundabout as it spins it feels like you are being thrown off backwards.

If someone was watching from the side they would see you try and move in a straight line but be pulled in a circle by the roundabout.

The centrifugal force does not exist in these situations.