Forces - Moments

Forces can make objects turn if there is a pivot. Think of a playground see-saw. The pivot is the thing in the middle of it. When no-one is on the see-saw it is level, but it tips up if someone gets onto one end. Turning forces around a pivot are called moments.

It is possible to balance the see-saw again if someone else gets onto the other end and sits in the correct place. This is because the turning forces are balanced. We say the moments are equal and opposite.

Working out moments

To work out a moment, we need to know two things:

• the distance from the pivot that the force is applied.

• the size of the force applied

Example

Imagine that a force of 10 N acted on a see-saw 2 m from the pivot. This is how we would work out the moment:

force × distance = moment

10 × 2 = 20 Nm

Notice that the unit of moment is Nm (newton metre). Don't get confused with a 'newton meter', which is another name for a force meter.

Here is an example of balanced moments. 10N at 2m from the pivot is balancing 20N at 1m from the pivot. The objects create moments of 20Nm that are equal and opposite, so the see-saw is balanced.

Moments on a balanced see-saw

Using moments

• A see-saw will balance if the moments on each side of the pivot are equal. This is why you might have to adjust your position on a see-saw if you are a different weight from the person on the other end.

• If a nut is difficult to undo with a short spanner, a longer spanner will help. This is because there will be a bigger moment on the nut, when the same force is applied further from the pivot.

• Using the same principle you can increase the moment applied by a lever or a crowbar, and this can help you move heavy objects more easily.

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