Doppler Spin

What you need

• A buzzing egg timer
• A piece of string ~1 m
• Sticky tape
• A friend

Instructions

1. Tie the piece of string to the egg timer securely, using some sticky tape as well if needed
2. With the timer buzzing, hold the other end of the string very tightly and start to swing it in a circle around your head. Make sure your friend is standing well back and there’s nothing breakable near you

Results and Explanation

Your friend will hear the pitch of the egg timer change as it moves around your head - but you won’t hear any change at all!

As the egg timer circles towards your friend, the sound waves bunch up closer together and the pitch of the sound gets higher. As it circles away, the sound waves spread out and the pitch becomes lower.  Your position relative to the egg timer is always the same so you don’t hear any difference.

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.