Physics 3 - Turning Forces - Moments

To undo a very tight wheel-nut on a bicycle, you need a spanner. The force you apply to the spanner has a turning affect on the nut.

You couldn't undo a tight nut with your fingers but with the spanner you can. This is because the spanner exerts a much larger turning effect on the nut than the force you apply with your fingers to the spanner.

If you had to choose between a long-handled spanner and a short-handled spanner, which would you choose? The long-handled spanner. Why? Because the longer the spanner handle, the less force you need to exert on it to untighten the nut.

In this example, the turning effect of the force, called the moment of the force, can be increased by: 

- Increasing the size of the force

- using a spanner with a longer handle

Levers:

A crowbar is a lever that can be used to shift a heavy weight.

The weight is called the load and the force the person applies to the crowbar is called the effort. Using the crowbar, the effort needed to lift something is only a small fraction of its weight. The point about which the crowbar turns is called the pivot.

You can work out the moment of a force using this equation:

Moment (Newton metres, Nm) = Force (Newtons, N) x perpendicular Distance from the pivot to the line of action of the force (Metres, M)

or

M = FD (F x D)