Moments – Turning effects, levers and stability.

Moments – Turning effects, levers and stability.

Turning effects

Physics GCSE science revision- momentsIf you want to unscrew a nut then you need to use a spanner. The longer the spanner is the easier it is for you to unscrew the nut. This is due to the fact that you don’t need to use as much force.

The process of unscrewing a nut is known as a turning effect. The moment is the turning effect of the force and can be increased by:

  • – increasing the length of the instrument being used
  • – increasing the amount of force being used

Moment can be calculated using the following equation:

moment = force x perpendicular distance from pivot(the line of action of the force to the axis of rotation)

  • – the moment is measured in newton metres (Nm)
  • – the force is measured in newtons (N)
  • – perpendicular distance from pivot is measured in metres (m)



Physics GCSE science revision- momentsImagine you wanted to lever up a heavy box using a crowbar. You place the curved end of the crowbar under the box so that the other end sticks up into the air. This is the end that you push down on. When you push down you apply force which is known as effort. The object you’re trying to lift, the box, is the load. The pivot is the point at which the crowbar can turn, the curved end underneath the box.



All objects have a certain stability. Take a brick for example: you can tilt the brick for a certain amount but after a point it will topple over.

  • Physics GCSE science revision- momentsIf you only tip the brick a little and then release it the turning effect created by the weight of the brick will return it to its original, upright position.
  • If you tip it a little further you’ll find that you reach a point at which the brick is able to balance on one edge. In this position there isn’t any turning effect. This is because the centre of mass is directly above the balancing edge.
  • If you were to continue tilting the brick past this point then, when you released it, it would topple over. This is because the weight’s line of action is now outside the base of the brick.

Many objects topple over much more easily than bricks. This is why some are created so that they can’t as easily. Tractors, for example. Their engines are purposefully built in a low position to keep the centre of mass low. This means that for the line of action of the tractor’s weight to fall outside base, the tractor would have to tilt a lot. But, if it was to, the wheels, which are built very wide apart, would push the line of action back further.

Other large vehicles, like coaches and buses, have to be vigorously tilt-tested before they’re let out on the roads. The tilt-tests involve seeing how far they can tilt before they topple over. This is important in order for them to be able to deal with sharp corners and hills.