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Physics · Forces and motion

Lever and the Principle of Moments

A see-saw is a lever. Whether it balances depends on both the weights and how far they are from the pivot.

1 m0.5 m0.5 m1 m2 kg4 kgTips to the left
Left moment (m × g × d)
19.6 N·m
Right moment
11.8 N·m
Result
Tips left

Try this

  1. Put 2 kg at 1 m on the left. Where must 4 kg go on the right to balance?
  2. Can a light child balance a heavy adult?
  3. Find three different ways to balance the see-saw.

Key ideas

  • Moment = force × perpendicular distance from the pivot.
  • A lever balances when clockwise moment = anticlockwise moment.
  • A small force far from the pivot can balance a big force close to it.