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Grades 6, 9, and 10 Intermediate About 20 min Version 1 Awaiting review Premium

Balancing a lever: the principle of moments

A lever is a rigid bar that turns about a fixed point called the fulcrum. A force on a lever has a turning effect called its moment: moment = force × perpendicular distance from the fulcrum. The load tries to turn the lever one way and your effort tries to turn it the other way. The lever balances when the two moments are equal. This is the principle of moments. Because distance counts as much as force, a small effort far from the fulcrum can balance a large load close to it.

What you will learn

  • Calculate the moment of a force about a pivot.
  • State and test the principle of moments.
  • Explain how a lever lets a small effort lift a large load, and calculate its mechanical advantage.

Before you start

  • Know that force is measured in newtons (N).

Materials

  • Loads of 20 N, 40 N and 60 N
  • Spring balance to apply the effort

Equipment

  • Metre rule used as the lever
  • Fulcrum (knife edge)

Procedure

  1. Look at the load hanging on the left. Change the effort and its distance from the fulcrum until the lever balances.
  2. Record the balanced measurement.
  3. Move the effort to at least two other distances, balance the lever again each time and record it.
  4. Find a way to balance the load with an effort that is half the load or less.

How this simulation is simplified

  • The metre rule's own weight acts at the fulcrum, so it has no moment.
  • The lever has no friction at the fulcrum.