Forces and Motion

An inertia meter

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Increasing inertial mass reduces acceleration.

This apparatus gives students the chance to separate out the effects of mass from those of the pull of gravity, and that's rather hard to do on this planet. So we regard it as an essential experience. It represents a similar situation to that faced by astronauts in free fall for long periods (say in the international space station) who have to determine their mass in the absence of any perceptible weight.

What to Prepare

  • an inertia balance

What Happens During this Activity

Students add mass to the inertia balance, and note the increasing period. Then there is a chain of reasoning to be developed, in order to understand the connection between adding mass and the longer period of oscillation:

  • Same force and larger mass  →  lower average acceleration.
  • Lower average acceleration  →  lower average speed.
  • Lower average speed and same distance to cover  →  longer time to travel back and forth.

You might point out that calibrated versions of this device are used to measure the mass of astronauts and show a photograph of the chair.

Scaling down such a chair to work in the laboratory requires some serious engineering to reduce the frictional forces by an acceptable amount, tempting though it is to try.

is formalised by Newton's First Law
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