# Bounce efficiency

Classroom Activity for 11-14

In this activity, students investigate the relationship between drop-height and rebound-height for a tennis ball. You can use it to introduce the idea of efficiency.

## Equipment

Each students will need:

- Metre rule
- Tennis ball
- Hard floor
- Clamp stands to hold metre rule (optional)
- Mobile phone that can record in slow motion (optional)

## Procedure

Ask students to:

- Drop the ball onto a hard floor from a height of 1 m.
- Devise a method for determining the height that the ball bounces up to (by eye or using slow motion video).
- Take repeat measurements to find an average bounce-height for a 1 m drop.
- Repeat for at least three other drop-heights.
- Plot a graph of drop-height against (average) bounce-height.
- Draw a best-fit straight line through your results and use the graph to predict the bounce-height from an unknown drop height (eg 2 m).

## Teaching notes

Students should find that bounce-height and drop-height are proportionally related. Their gradient will depend on the ball and surface used, but will always be less than 1. They can calculate an average value for the bounce efficiency as a percentage by multiplying the gradient of their graph by 100%.

You could discuss how their ball's bounce effciency compares to that required for tennis tornaments. The International Tennis Federation stipulates that only balls that rebound to between 135 and 151 cm when dropped onto a concrete floor from a height of 254 cm can be used for professional play (this is known as the 100-inch drop test).

## Learning outcome

Students can calculate the efficiency of a single bounce of a ball.