Forces and Motion

Starting out on calculations

Physics Narrative for 11-14 Supporting Physics Teaching

Lines of action and forces

Describing the world using forces can be tricky, since you cannot see them.

For situations where levers are used you need to be able to correctly identify the forces acting on the lever and lengths to the line of action of those forces from the pivot point.


Then you can calculate moments. Since lengths are measured in metre and forces in newton, be sure to give the moments units of metre newton (m N).

You should be able to show that the moments shown here are:

  • 1 metre newton.
  • 0.2 metre newton.
  • 16 metre newton.

Balanced levers

Once you have the idea that the clockwise moment is equal to the anti-clockwise moment, L1 × F1 = L2 × F2, you can tackle some simple calculations which allow you to find a missing force or length if you know values for the other three quantities.

Here are a few for you to try out, just to build confidence.

You should be able to show that the unlabelled forces are:

  • 5 newton.
  • 0.4 newton.
  • 9 newton.
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