How far away was the fireball?
The question
Can we measure the speed of sound, and use it to work out how far away a fireball's sonic boom came from?
Very bright fireballs sometimes make a boom heard minutes after the flash, because light arrives almost instantly but sound travels at only about 340 m/s.
Equipment
- Two wooden blocks to clap together
- A large wall in an open space, such as a school building, with at least 50 m of clear ground in front
- A trundle wheel or long tape measure
- A stopwatch
Method
- Stand at least 50 m from the wall and measure the distance.
- Clap the blocks in a steady rhythm so that each clap lines up with the echo of the one before.
- A partner times 20 claps. The sound travels to the wall and back 20 times in that time.
- Speed of sound = (2 × distance × 20) ÷ time. Repeat three times.
- What to measure
- The distance to the wall, the time for 20 claps, and your calculated speed of sound.
- How to present it
- Your method and results, then a worked problem: a fireball's boom arrives 3 minutes after the flash. How far away was the source? (About 61 km at 340 m/s.)
- Take it further
- Find out why many fireball booms are not heard at all, and why sound from very high up does not travel in a straight line.
Safety
Stay well away from roads and car parks, and keep the clapping blocks away from fingers.