1Turning forces
A force can make something turn about a pivot. Its turning effect is called a moment. Push a door far from its hinge and it swings open easily.
pivotmoment
Lesson 3.4 notes
Turning forces: moments
The turning effect of a force, balancing moments, levers and cranes.
2Moment = force × distance
moment = force × distance from the pivot. A 40 N push on a spanner, 0.25 m from the nut, gives a moment of 40 × 0.25 = 10 newton metres (N m).
newton metres
3Further is easier
Double the distance from the pivot and the same force gives twice the moment. A longer spanner undoes a tight nut with a smaller push.
4Clockwise and anticlockwise
On a seesaw, one person makes an anticlockwise moment and the other a clockwise moment. The bigger moment wins, and that side goes down.
anticlockwiseclockwise
5Balancing
A seesaw balances when the clockwise moment equals the anticlockwise moment: the principle of moments. 300 N × 2 m = 600 N × 1 m.
principle of moments
6Levers
A lever lets a small force move a big load. Push far from the pivot, and the load close to the pivot gets a much bigger force.
lever
7Cranes
A crane's counterweight balances the moment of its load, so the crane doesn't tip over. The further out the load moves, the bigger its moment.
counterweight
Explore: Seesaw balancer
Put two people on a seesaw and move them along it. Compare the clockwise and anticlockwise moments and make it balance.
Common mistake: Measure the distance from the pivot
A moment uses the distance from the pivot to where the force acts, not the length of the whole lever or the distance from its end.
Remember
- A moment is the turning effect of a force: moment = force × distance from the pivot, measured in newton metres (N m).
- The further from the pivot a force acts, the bigger its moment. That is why long spanners, and door handles far from the hinge, work well.
- An object balances when the total clockwise moment equals the total anticlockwise moment: the principle of moments.
Going further at GCSE