Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

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 inPrincipia 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, v2/R, is centripetal, and so the force required to cause it is also centripetal.


CO2 puck on a glass plate

The motion of the Moon around the Earth

Fine beam tube

Whirling a rubber bung on a string

Whirling a rubber bung and letting go

Introducing circular motion

Penny on a turntable

Looping the loop

Sketching a satellite orbit and predicting its period

Experimental test of F = mv²/R

Further test of mv²/R

Test of mv²/R on a turntable

Related Guidance

Estimating the Moon’s orbit time

Orbits of satellites and moons

Proof of F = mv2/R

Centrifugal motion at a fun fair

What pushes planets along?