Two astronauts stand close together on the Moon and shout.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Two astronauts stand close together on the Moon and shout. They cannot hear each other without a radio. But if they touch their helmets together, one can faintly hear the other. Explain both.
Need a reminder? The key ideaShow
Every sound is made by a vibration. Sound needs a medium and cannot cross a vacuum. It is a longitudinal wave: the particles vibrate along the direction of travel, making compressions (high pressure) and rarefactions (low pressure).
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The Moon has no air: there is no medium between the astronauts, so sound cannot travel across.
Step 2. Touching helmets gives a solid path. The vibration passes through the helmets (and the air inside each one) to the ear.
Answer: sound needs a medium; the helmets in contact become one.
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