Newton's Third Law
Forces and Motion

Balancing explosions

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Two vehicles interact on a single lever.

This demonstration connects the action of a lever with the conservation of momentum. The lever starts off balanced, with the vehicles stationary. You can exploit what students know about levers to explain that the vehicles must at all times be constrained in where they can be. So their masses fix their velocities.

What to Prepare

  • a large, long and easily-balanced lever, with a track to constrain the vehicles
  • two vehicles, that can be blown apart by a spring or some opposing magnets
  • possibly some other vehicles of different masses

What Happens During this Activity

Position the two equal-mass vehicles on the lever so that they balance. Ask where else you could position them so that balance is maintained. Repeat for several locations.

Point out that you need to exert a force to drive the situation from being in balance to being out of balance.

Then ask students to predict what will happen to the lever, and so to the positions of the vehicles, if the vehicles explode apart from the centre.

There are several arguments you could use here, and we'd recommend starting from recording that the centre of mass being stationary, and remaining stationary, as no resultant forces act on the whole edifice (vehicles and lever).

You could also start from considerations of symmetry, pointing out that there is a mirror line through the pivot.

Either of these approaches is ripe for extension to situations where the masses of the vehicles are not equal, or where the force between the two is attractive (use thin elastic cord) rather than repulsive.

We suggest that the focus is on the whole system – that is, both vehicles. This follows the approach in the Physics Narrative.

As a further extension you could ask what the whole would look like from another point of view: perhaps that of Alice, cruising past at constant velocity (perhaps with her web cam – you could do it for real).

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