Moments and levers

A moment is the turning effect of a force. Levers, spanners, door handles and see-saws all use moments: a small force far from the pivot can balance a big force close to it.

Key facts

  • Moment = force × perpendicular distance from the pivot (Nm).
  • Principle of moments: when balanced, clockwise moments = anticlockwise moments.
  • Longer spanners and handles need less force for the same moment.
  • Gears pass on moments; a bigger gear turns more slowly with a bigger moment.

Worked examples

Example 1

A 400 N child sits 1.5 m from the pivot of a see-saw. How far from the pivot must a 600 N adult sit to balance?

  1. 400 × 1.5 = 600 × d
  2. d = 600 ÷ 600 = 1 m

Answer: 1 m