Newton's Third Law
Forces and Motion

Giving in to gravity

Practical Activity for 14-16 Supporting Physics Teaching

What the Activity is for

The idea to nurture here is that free-fall is a natural state. We experience forces only if we are prevented from going with the flow.

The floater in the image feels no force; it's anchored boats that experience a force.

Use the groaner tubes to hear the difference when one part of a system is constrained and the other is allowed its natural motion.

What to Prepare

  • a set of groaner tubes

What Happens During this Activity

Allow the students to find out, by gentle experimentation, the conditions under which the tubes make a noise, and not (when one part – the inside – is free to follow its natural motion, whilst the other – the outside – is not free to fall).

Make connections between this and free-fall, or weightless situations. Students will have found that there is no noise when the tubes are dropped or thrown, and that the noise depends only on one part being in free-fall and the other not. It doesn't matter whether the tube is on the way up or the way down.

Finally you might like to make the connection between this and Einstein's insight that gravity and free-fall acceleration are essentially the same thing – that is, gravity and space-time are connected. You experience weight only when you prevent natural motion.

Newton's Third Law
is used in analyses relating to Collisions
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