Newton's First Law
Forces and Motion

Inertia with pendulums

Practical Activity for 14-16 PRACTICAL PHYISCS


The tendency of a body to resist acceleration is called its inertia. This experiment provides direct experience of applying force and experiencing inertia.

Apparatus and Materials

  • Tin cans, large, 2
  • Sand
  • String

Health & Safety and Technical Notes

A ladder or step-ladder must be used when attaching strings to a ceiling beam. A ladder must be steadied by a second adult. If it is necessary to stand more than two metres from the floor, the worker must have been trained for work at height.

Ensure that students use this apparatus in a manner that creates no hazard for themselves or for others.

Read our standard health & safety guidance

The string should be strong enough to support the load safely.


  1. Fill one can with sand and leave the other one empty. Hang them both by long strings.
  2. Give each of them a short, sharp push. Compare how easy it is to accelerate them.
  3. Allow the pendulums to swing and push them in a direction roughly perpendicular to their motions. This produces acceleration in a different direction from the motion, and the cans change direction. Again, the two cans do not experience the same accelerations.
  4. Try to stop swinging pendulums. Compare the changes in motion of the cans when you exert similar forces on them.

Teaching Notes

  • Do this as a participative demonstration. Ask your students to move the masses.
  • There is no question of friction causing a difference in this case because the cans are the same size and shape.
  • This simple activity makes fundamental points about force and related quantities. The points are relevant both at introductory and at advanced levels, and can be summarized as:
    • Force, when it is not ‘balanced’ by other forces, is what produces change in motion (acceleration)
    • Mass is what resists change in motion (acceleration).
  • Thus, all in one go, you have working definitions of both force and inertial mass. (There is an additional ‘parallel’ definition of mass: Gravitational mass is what allows a body to exert and experience gravitational force. To see how the two definitions of mass relate to each other you would have to consider Einstein’s theory of general relativity.)
  • Emphasize that it is not just the start of motion that is resisted by masses, but any change in motion, including slowing down and stopping, or changing direction.
  • The activity provides a suitable introduction to Newton’s first law.
  • You could fire ping-pong balls or peas at the cans as a way of comparing the responses of cans with different mass.
  • You could discuss seat belts in cars. Some are called inertia reel belts.

This experiment was safety-tested in March 2005

Newton's First Law
formalises Inertia
includes the quantity Force
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