I had a discussion with someone. it started with the fact that if you spew water on earth it makes a ‘arc’. (as we used to be). and if you were to do that on the moon, you would probably see the water you spew start to float, or at least not go in a curve anymore. now I wondered if that was also the case with sound. on the month, the sound arrives faster or slower than on Earth. and how come?
Answer
Dear Jordan,
You are confusing two things here that have nothing to do with each other.
First there is the effect of gravity. On the moon, there is also gravity that causes things to fall (accelerate downwards). On the moon, that acceleration is 1/6 of that on Earth, but if you spew water, it will end up on the lunar surface in a nice parabolic arc. It will just take longer, and if you throw something away you will be able to throw it away further. Because the vertical downward movement due to gravity is six times slower, the object will therefore be able to “enjoy” its horizontal movement component longer before it hits the lunar floor.
For example, if you drop a stone from a resting position on Earth from a height of 10 meters with a horizontal speed of 10 meters per second (= 36 km/h), it will take that stone about 1.4 seconds to land on the ground. So he will fall 14 meters further to the ground. On the moon, that takes about 3.5 seconds. (this was calculated from memory, hence the “approximately”). There the stone will therefore fall 35 meters further.
But in any case, if you throw something away on the moon, it will end up with a nice arc on the lunar surface, just like on Earth.
Second, about the speed of sound. A sound wave such as here on earth works as follows. The source of the sound is, for example, a loudspeaker. The surface of the speaker vibrates, causing the air molecules that touch it to vibrate as well. They pass on that vibration to the next molecules and so on. So there is no global movement of molecules, they remain globally in place and transmit the vibration to each other. On Earth, this happens at a speed of approximately 340 meters per second.
And on the moon? Well, a sound wave needs a medium in which to propagate. This can be the air around us so that we can speak to each other. But on the moon there is no atmosphere. There is vacuum. A sound wave therefore has no medium in which to propagate. If we stood on the moon a few centimeters apart, we wouldn’t hear anything anyway. The concept of sound speed as we know it makes no sense on the moon because there is no atmosphere. However, shock waves can propagate through the lunar soil. This happens at speeds (depending on the composition of the soil) between 1200 and 2000 meters per second. Geologists will be able to inform you better about this. But sound as we know it on earth is not there on the moon because there is a vacuum.
Answered by
prof.dr. Paul Hellings
Department of Mathematics, Fac. IIW, KU Leuven
Old Market 13 3000 Leuven
https://www.kuleuven.be/
.