Per group of students:
various bouncy balls (ping pong, tennis, rubber balls, etc.)
play dough, rolled into a ball
silly putty, rolled into a ball
ball bearing or other hard ball
Did all of the balls bounce? Which one bounced the highest?
Which ball has the most elasticity?
For the ball of play dough, what happened to all of the kinetic energy when the ball hit the ground? Why didn’t it bounce?
What To Do
- Divide students into groups, giving each 4 or 5 balls (bouncy, play dough, marbles) to test.
- Students should make as many observations about each ball as possible. (How does it feel, can it be squeezed, can it be shaped).
- Students predict which balls will bounce the highest.
- Drop the balls from a specified height (e.g. 1 m) and measure the rebound height after the balls hit the ground and bounce back up.
- Measure each ball three times to ensure a good average value for each ball.
- Compare findings to predictions and observations.
- Hypothesise what properties objects should have in order to bounce.
When we drop a ball and it hits the floor, some of that kinetic energy changes (or deforms) the shape of the ball. Because bouncy balls are elastic, the molecules will want to return to their original shape. The energy of the downward motion of the ball will be converted into upward motion as the molecules spring back into their original shape. In elastic materials, like play dough, the molecules don’t return to their original shape at all. When we drop a ball of play dough, the kinetic energy is spent by squashing it. This flattens the ball and releases thermal energy. Instead of bouncing, the ball of play dough actually warms up!
- elasticity: The tendency of an object to return to its original shape after being stretched or compressed.
Put some of the bouncy balls in the freezer for a few hours. Take them out and bounce them again. Do they go as high when they are cold?
'Happy/Unhappy balls' look alike, but have different elastic properties. See Bounce No Bounce Balls or Boreal for sources of this demonstration.
Did any of the balls bounce higher than 1 metre? Why does this make sense?
Where there any unintended energy conversions? Why can't a ball ever bounce all the way back to where it started?
Science World Resources | Full Unit | Elastic Energy