Moments, levers and turning forces
Physics · Key Stage 3 · The School
A force can turn as well as push
Push a door near its hinge and it barely moves; push at the handle and it swings easily. The force is the same, so the difference is the distance from the pivot. The turning effect is called the moment, and it is force multiplied by the perpendicular distance from the pivot, measured in newton metres.
Moment = force × distance from the pivot
A 20 N force pushing 0.8 m from the hinge gives a moment of 16 Nm. The same 20 N applied 0.1 m from the hinge gives 2 Nm — eight times less turning effect for exactly the same push. This is why door handles are fitted as far from the hinge as the door allows, and why nobody designs one in the middle.
Balanced means the moments are equal
On a seesaw in balance, clockwise moment equals anticlockwise moment. A 200 N child 1.5 m from the pivot gives 300 Nm; to balance, a 300 N adult must sit 1 m away, because 300 × 1 = 300. The lighter person sits further out — which is exactly what children work out for themselves in a playground, without the arithmetic.
Levers trade distance for force
A crowbar lets a small force at the long end produce a large force at the short end. Nothing is created: you move the long end a long way to move the short end a little. The energy in equals the energy out. A lever changes how a force is delivered, never how much energy is available — that is why perpetual motion machines built from levers do not work.