Forces and Motion

Just increase the force a bit on this side to make it work!

Teaching Guidance for 11-14 Supporting Physics Teaching

Balanced moments

Wrong Track: The force times distance on this side equals the force times distance on that, so I'll just need to push a bit harder on this side to raise the load.

Right Lines: Once the lever is turning at a steady rate, the clockwise moment (force times length) on the lever is exactly equal to the anti-clockwise moment.

Steady motion from balanced moments

Thinking about the learning

A subtle point here takes us back to the earlier work on balanced forces (see episode 05 of the SPT: Forces topic). In other words, if the forces acting on an object are balanced, then it keeps moving with steady motion. The same rule applies to the lever: When the clockwise turning effect (or moment) balances the anti-clockwise turning effect, the lever continues to move around at a steady rate.

Thinking about the teaching

By far the best way of dealing with this learning challenge is to simply assume from the start that the levers you deal with move at a steady rate and to focus on the balancing of moments.

Note (for your own interest but not the pupils) that we're ignoring the part of the action of the lever where it is accelerated into motion (for that fraction of a second) and focusing on the part where it is then turning at a steady rate.

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