Stacked ball drop

In this activity, students explore how high a ping pong ball bounces when dropped by itself and then with a golf ball. You can use it to show how an energy analysis allows us to put limits on possible outcomes.

Equipment

Each group of students will need:

• 30 cm ruler
• Golf ball
• Ping pong ball
• Bench/table to bounce off
• Sticky tape
• A4 sheet of clear plastic (eg document wallet)
• Access to a mass balance (capable of measuring to nearest g or better)

Procedure

Ask students to:

1. Roll the clear plastic A4 sheet into a tube with a diameter slightly wider than the golf ball. Measure the length of the tube (this should be 30 cm).
2. Use sticky tape to hold the tube in shape and stand it upright on a bench or table. Ask an assistant to gently grip the bottom of the tube (or use a clamp stand at the top to keep it upright).
3. Hold the ping pong ball so that the bottom of the ball is at the top of the tube. Let go. Measure the height the ping pong ball bounces to.
4. Repeat for the golf ball.
5. Measure masses of golf ball and ping pong ball.
6. Hold the ping pong ball directly above the golf ball and drop the two together so that the ping pong ball bounces straight up. Measure the height the golf ball reaches.

Discussion prompts

• What percentage of its original height did the ping pong ball reach when it bounces off the bench?
• How high would it bounce if the bounce efficiency was 100%?
• What is the maximum possible height the ping pong ball can reach in the two-ball drop?

Teaching notes

The start and end points of an energy analysis for the stacked ball drop are shown below.

The length of the tube is 𝐿 and, for the ping pong ball and golf ball respectively, the rebound heights are 𝐻 and ℎ and masses are 𝑀 and 𝑚. For a perfectly elastic collision, we can say that the energy stored gravitationally before the drop would be the same as the energy stored gravitationally afterwards. Therefore:

(𝑀+𝑚)𝑔𝐿 ≥ 𝑚𝑔ℎ + 𝑀𝑔𝐻

And so the height of the ping pong ball can be predicted using:

ℎ ≤ 𝐿 + 𝑀𝑚 (𝐿−𝐻)

Substituting in experimental values should give a maximum value for ℎ of up to a few metres. The actual height will be lower as the real bounce efficiency will be less than 100%.

Learning outcome

Students use an energy analysis to put an upper limit on the height an object can reach after a collision.

Home learning

For a version of this activity for younger pupils to try at home, see Do Try This at Home: episode 13

This experiment was safety-checked in March 2020.