# Circular motion

Objects move in a straight tine at a constant speed unless a force acts on them. This is Newton's First Law. However, many things move in curved paths, especially circles, and so there must be a force acting on them to pull them out of their straight line paths and make them turn corners.

The tighter the curve that the object is made to move in, the bigger the change of direction and so the bigger the force.

Examples of objects moving in curves are:

The hammer swung by a hammer thrower
Clothes being dried in a spin drier
Chemicals being separated in a centrifuge
Cornering in a car or on a bike
A stone being whirled round on a string
A plane looping the loop
A DVD or CD spinning on its turntable
Satellites moving in orbits around the Earth
Many fairground rides

From now on we will just think of the simple case of things moving in a circular path.

We call the force that makes objects move in a circle the CENTRIPETAL FORCE
(the name comes from Latin and means centre-seeking)

The centripetal force always acts towards the centre of the circle to pull the object out of its straight-line path. Although an object may travel round the circle at a constant speed its direction of motion is always changing and so its velocity must be changing. Since a change of velocity is an acceleration there must be a force acting on the object - the centripetal force.

### What is centrifugal force?

If a force acts on an object being whirled round in a circle, like a stone on the end of a string, there must be an equal and opposite force acting somewhere else. This is the centrifugal force – it acts on the ‘pivot’ at the centre of the circle. It is sometimes called a fictitious force because it does not act on the object being whirled round.

### What produces the centripetal force?

The actual way the force is produced depends on the particular example:
In a spin drier it is the wall of the drum pressing on the clothes. When a car, motorbike or bicycle corners it is the friction between the wheels and the road. (You know how difficult it is to corner on ice where there is hardly any friction.)
When the Earth orbits the Sun it is the pull of gravity.
When a railway train corners it is the force of the rails on the flanged wheels.
When a stone is whirled round on a string it is the tension in the string.

What happens if we remove the centripetal force? If you are whirling a stone round your head on a piece of string and the string suddenly breaks, which way does the stone fly off?

### Circular motion experiments

You could try this with a soft ball instead of a stone and just let go. The ball will carry on in the direction in which it was moving when you let go, as you ought to expect from Newton's First Law. It DOES NOT go off along a radius of the circle.

If the string wraps itself round your finger then the stone will be moving in a circle with a smaller radius and the force will be bigger.

If you whirl a bucket of water around in a vertical circle you will not get wet if the speed of the bucket is great enough. At every point in the circle the water tries to fall vertically out of the bucket due to the force of gravity but also tries to move in a straight line due to its circular motion.

If the velocity is large enough the water will not drop out of the bucket far enough before it is moved round the circle. However, if you spin it slowly……!

### The coat hanger experiment

A very good demonstration of centripetal force is the coat hanger experiment.
Bend a wire coat hanger into a square shape, file the end flat and bend it so that the end points towards the opposite corner of the square (see diagram).

Put one finger in the top corner and then balance a penny on the end of the hook. Now swing the hanger in a vertical circle. The penny should stay in place. (It may need a bit of practice).

The centripetal force acts towards the centre (the top corner) and so keeps the penny balanced on the hook while the hanger is swung in a vertical circle. No water and no mess!

### schoolphysics: Circular motion animation

To see an animation of circular motion click on the animation link.

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD

© Keith Gibbs 2020