# Circular motion

Commonsense suggests that any object in circular motion strives constantly to recede from the centre. This observation troubled great thinkers like Descartes and Newton. It took Newton some 20 years to incorporate circular motion into his thinking about forces and motion, as published in*Principia Mathematica*.

Newton’s laws of motion are now commonly taught in school science. It should come as no surprise that students find the idea of a centripetal force difficult.

At the introductory stage, some teachers prefer to use the term 'centrifugal force' because it draws on students’ common belief. Others hold that this start will lead to difficulties. One thing is certain: a mixture of the two is very confusing.

In this collection, the idea of a centripetal force is consistently used. We begin with qualitative experiments that focus students’ attention on the direction of the force causing a circular motion; later experiments are quantitative. Acceleration, *a*, needs a force, *F*, in the same direction, generally expressed as *F* = *ma*. Acceleration in a circle, v^{2}/*R*, is centripetal, and so the force required to cause it is also centripetal.

**Experiments**

The motion of the Moon around the Earth

Whirling a rubber bung on a string

Whirling a rubber bung and letting go

Sketching a satellite orbit and predicting its period

Experimental test of F = mv²/R

**Related Guidance**

Estimating the Moon’s orbit time

Orbits of satellites and moons