# Physics KS4 / GCSE: Centripetal force - explained

Tim Peake introduces Yan Wong. Yan puts a glass of water on a tray hanging from a string.

He challenges various people in a park to swing the tray over their heads.

The water doesn’t fall out of the glass and the glass doesn’t fall off the try.

These people then try to explain what happened.

Two people swing each other around in a circle. Yan explains what happens if one person lets go in term of Newton’s first law of motion.

So, there must be a centripetal force pulling inwards on anything moving in a circle.

For the cup of water, it’s the tension in the string and for the Earth orbiting the Sun it’s gravity.

### Teacher Notes

Key Stage 4

Can be used to explain what a centripetal force does.

A similar practical activity could be carried out using a bucket of water in which students are challenged to swing the bucket over their heads.

Students could be challenged to label a diagram with the directions of the velocity and acceleration of the cup of water at four equally spaced positions around a circle.

Students could be challenged to explain how the cup can be travelling at a constant speed but is also accelerating.

Students could then explain why a centripetal force is needed using Newton’s Second Law of Motion.

Students could apply Newton’s First Law of Motion to explain what happens if the string breaks.

GCSE/Higher Levels

Can be used to explain what a centripetal force does.

Students could be challenged to explain why an object orbits Earth in terms of Newton’s Laws of Motion

A similar practical activity could be carried out using a bucket of water in which students are challenged to swing the bucket over their heads.

Students could be set the challenge of calculating the minimum speed at which they would need to rotate the bucket so that they don’t get wet.

Answer: the critical speed occurs when the acceleration of the water due to gravity equals the centripetal acceleration, that is when g equals velocity squared divided by radius.

So students just need to estimate the radius using the length of their arm and height of bucket.

### Curriculum Notes

This clip will be relevant for teaching Physics/Science at KS4/GCSE in England Wales and Northern Ireland. Also at National 4/National 5 and Higher in Scotland.