Forces and Motion

Falling together

Classroom Activity for 5-11 Supporting Physics Teaching

What the Activity is for

The idea here is a bit subtle: we are working with the ideas that falling is simply a kind of motion, and that all motions are relative. So falling itself is relational. In particular if two things are both falling together, then there really is no relative motion. This is no different from being unsurprised to find that your passenger in the car is still beside you after several miles of journey: you are moving together – moving past the scenery, but not moving past each other.

What to Prepare

  • two soft toys
  • a chair, ladder or balcony to give you a free-fall distance of around 2 metre

What Happens During this Activity

Start by telling a story about the two soft toys going on a journey, just walking across the table top, or through the undergrowth, or across the lawn. If they walk together, hand-in-hand, or side-by-side, then they're travelling at the same speed: they don't move apart or separate.

Then switch the kind of journey to one where they are falling, perhaps a kind of controlled falling in a lift to start off with. Again emphasise that they fall together – there is no relative motion.

Develop this to put them in free fall. Talk through their journey again from the point of view of first one and then the other. What will each one see? If technology permits you might equip one with a small video camera to record a fall. Practice will be essential, so that you do get a clear video of the other toy falling beside the first one.

You might supplement this with videos taken by skydivers, as they fall together.

To take it further, look for such a video where one skydiver opens his parachute. This really drives home the idea that motion is relative – the camerawoman keeps on falling, the other appears to shoot upwards – but of course that's just the relative motion in action.

With practice and for a bit of fun, you might manage to reproduce this with your soft toys and video camera.

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