The inertia balance or 'wig-wag'

Class practical

This activity shows that resistance to motion depends on the mass of the body being accelerated, rather than on its weight.

Apparatus and Materials

For each student group

• Inertia balance
• Masses
• G-clamps
• Elastic band

Health & Safety and Technical Notes

If using hack-saw blades, take care to avoid cuts or scratches.

Do not overload the system, or the blades might buckle or snap. Test this safely before the lesson, and make sure that students cannot add an excessive load.

Take care when masses fall to the floor. Use a box or tray lined with bubble wrap (or similar) under heavy objects being lifted. This will prevent toes or fingers from being in the danger zone.

Read our standard health & safety guidance

You can purchase wig-wags, but it is cheaper and not too difficult to make them. Use two hack-saw blades or equivalent lengths of metal strip, and place a block of wood between them at one end. This serves as anchorage to be fixed to the table. Place another block of wood between them at the other end, to act as the platform which carries the loads.

It is essential to clamp each blade very firmly on both sides of the blocks of wood. Place another small block of wood or metal outside the blade, flush with the main block, so that the blade emerges as if from the well-matched jaws of a vice. Then drive screws through the small blocks, through holes in the saw blades, and into the big block.

For a temporary balance, use a large G-clamp to clamp small blocks, both blades, and the large block together, in a multiple sandwich with a similar clamp at the other end.

Cylindrical masses held in holes in the vibrating platform work well with this experiment, but are not essential (see illustrations below).

Procedure

1. Clamp one end of the wig-wag rigidly to a lab bench with G-clamps so that the blades stick out horizontally. The other end acts as a platform that can vibrate to and fro.
2. Pull the platform to one side, release it, and watch it vibrating.
3. Increase the load by adding masses to the platform. Use an elastic band to secure the loads. Note the change in resistance to motion (inertia) and in the vibration time (period of the motion).

4. Support the load vertically with a thread holding most of its weight, leaving only a little of it supported by the moving platform. The moving platform will still carry the full mass of the load in horizontal motion. With a bit of practice, you can do this by pulling upward on a taut thread and moving your hand to and fro in time with the motion of the platform. Or you can fix the thread to a support as far as possible above the platform. You have changed the vertical force.
5. Find out whether this makes a difference to the wig-wag motion.

Teaching Notes

• The oscillation of the system does not depend on the pull of the Earth. The period of oscillation, T , depends only on the mass, m , of the oscillating body, not its weight. T ∝√ m .
• If the system is suitably calibrated, the period of oscillation can be used to determine an unknown mass.
• This is the principle employed in machines which measure mass in Space.

This experiment was safety-tested in August 2007