### Lunar Leaper

Gravity on the Moon is about 1/6th that on the Earth. A pole-vaulter 2 metres tall can clear a 5 metres pole on the Earth. How high a pole could he clear on the Moon?

### Whirlyball

Whirl a conker around in a horizontal circle on a piece of string. What is the smallest angular speed with which it can whirl?

### Bridge Builder

In this short problem we investigate the tensions and compressions in a framework made from springs and ropes.

# Cannon Balls

### Why do this problem?

This short problem is a reasonably routine application of kinematics; the interest lies in the numbers obtained and the questions concerning the validity of a physical model of constant gravitational force.

### Possible approach

Students could be asked to make an estimate of the speeds and heights before starting the calculation. Developing a skill and habit for estimation is very useful in more advanced applications of mathematics.

### Key questions

• What has the radius of the earth got to do with this problem?

### Possible extension

Try the follow up problem Escape from planet earth .

You could also extend this to suppose that the balls are fired upwards on a trampoline with coefficient of restituion 0.5. How many bounces would it take for each ball to bounce less than 1m high?

### Possible support

Provide students with the equation for motion under a constant force.