Newton's Third Law
Forces and Motion

Knocking down a block

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Making it hard or easy by choice of type of collision.

Collisions in which the incoming projectile bounces apply more impulse to the target than those in which the incoming projectile does not.

Whilst a simple analysis of the momentum of the projectile suggests that this is necessary, many students find this hard to accept, so it's worth showing this simple demonstration, so long as it's sufficiently well built up.

What to Prepare

  • two balls of identical mass, but one bouncy and one not
  • a concave chute as a means of launching the balls at a repeatable and variable velocity
  • a small block that can be knocked over by the balls at some velocities, but not others

What Happens During this Activity

Arrange the chute so that the ball hits the standing block near the top, travelling horizontally.

Introduce the two balls as bouncy and not, by dropping them from the same height. Make a show of selecting the bouncy ball.

Explore the velocity that the bouncy ball needs to hit the chute at in order to knock over the block. Mark the release height on the chute. Now ask:

Teacher: How fast will the not-so-bouncy ball have to travel in order to knock the block over? Who thinks faster? Who thinks slower? Who thinks the same?

After a vote or extended discussion, or both, perform the experiment. Then build up to two diagrams of the momenta of the two balls before and after the collisions to help explain the effect.

This diagram could be useful

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