Why is the speed of sound used here? And is this correct?

[fascinated]

If two people are holding a stick going from the Earth to the moon and the one on Earth pulls the stick, would the person on the moon feel it immediately?

This question was answered on Quora and the answer given involves the speed of sound and claims that the person on the moon would not feel it for a little over a day.

"No. The impulse could not, by definition, propagate through the stick any faster than the speed of sound through the stick."
https://www.quora.com/If-there-was-...ld-the-person-on-the-moon-feel-it-immediately

Does this make sense? Is this the correct answer?

Thanks guys

 

[phinds]

Does this make sense? Is this the correct answer?

Yes and yes

 

[Bandersnatch]

Think of it in terms of interactions of molecules in the material of the stick.
Whether it's sound waves, or transferring displacement from one end of the stick to the other, it's all propagated by one molecule being displaced then 'hitting' the next one in line, which hits the next one, and so on. It's essentially the same process, so the speed of propagation is the same in both cases.

 

[fascinated]

Ok, so if I am the one on Earth and I push my end one foot towards the moon, the person on the moon would not know the stick has moved until the next day?

This is what seems off to me. Is there shrinkage involved where the stick takes another form?

 

[Bandersnatch]

Yes. A good way of thinking about this is to start with a spring. You press on one end of a very long spring, and the compression travels towards the other end. Then imagine increasingly more rigid springs. A stick is just a very rigid spring, in a way. Instead of macroscopic loops whose compression one can observe with naked eyes, it's the microscopic arrangement of molecules being compressed and propagating along the length of the stick.

 

[Ibix]

Ok, so if I am the one on Earth and I push my end one foot towards the moon, the person on the moon would not know the stick has moved until the next day?

This is what seems off to me. Is there shrinkage involved where the stick takes another form?

Yes. Sound waves are literally waves of alternating compression and extension. Your particular example is an impulsive compression which will propagate along the rod, leaving "moved rod" behind it.

 

[vela]

Ok, so if I am the one on Earth and I push my end one foot towards the moon, the person on the moon would not know the stick has moved until the next day?

This is what seems off to me. Is there shrinkage involved where the stick takes another form?

It seems off to you because it violates your expectations based on your everyday experience. Everyday objects are relatively small so the time it takes sound to go from one end to the other is unnoticeable. A stick so long that it goes from the Earth to the Moon, however, is not an everyday object, so your expectations are leading you astray.

It's similar to how lightning and thunder seem simultaneous when the lightning bolt is overhead, but if the bolt occurs a mile away, you don't hear the thunder until a few seconds after you see the lightning. Because of the larger distance and relatively slow speed of sound, you notice the delay in the second case.

 

[Dale]

A stick is just a very rigid spring, in a way.

Yes, both are described using Hooke’s law.

 

[Ibix]

See also this Insights article and this post (linked from the article) where an ex-forum member made a demo of bashing one end of a metal bar and seeing a delay before the other end moves.

 

[Baluncore]

If two people are holding a stick going from the Earth to the moon and the one on Earth pulls the stick, would the person on the moon feel it immediately?

What multiple choice alternatives are there?
A. The speed of sound.
B. The speed of light.
C. Instantly.
D. None of the above.

 

[fascinated]

Thanks, all of y'all! I see exactly what's happening... I think.?.

What if, hypothetically, the stick is some 'perfect' material that does not give? Let's say that at the absolute smallest, subatomic parts of reality within this stick that it is so dense that it simply cannot compress.

What happens then?

 

[Baluncore]

What happens then?

The speed of sound may be very high, but it cannot be faster than light.

 

[Dale]

What if, hypothetically, the stick is some 'perfect' material that does not give? Let's say that at the absolute smallest, subatomic parts of reality within this stick that it is so dense that it simply cannot compress

Such a material violates the laws of physics. So we cannot use the laws of physics to say “what if”. All we could do is try to write an interesting science fiction story around it.

 

[phinds]

Thanks, all of y'all! I see exactly what's happening... I think.?.

What if, hypothetically, the stick is some 'perfect' material that does not give? Let's say that at the absolute smallest, subatomic parts of reality within this stick that it is so dense that it simply cannot compress.

What happens then?

https://www.physicsforums.com/insights/how-to-avoid-breaking-physics-with-your-what-if-question/

 

[fascinated]

Got it. Thanks... that's actually what I expected, but for some reason, it just doesn't sit right. Ahhh... spacetime.

 

[phinds]

Got it. Thanks... that's actually what I expected, but for some reason, it just doesn't sit right. Ahhh... spacetime.

We are animals that evolved in what turns out to be a very narrow range of physical existence and our "logic" and "intuition" and "common sense" are all based within that narrow range. There are lots of things in cosmology (the very large) and quantum mechanics (the very small) that are totally counter-intuitive to what we think "natural" or "logical" or whatever and to assume otherwise often doesn't work.

 

[fascinated]

We are animals that evolved in what turns out to be a very narrow range of physical existence and our "logic" and "intuition" and "common sense" are all based within that narrow range. There are lots of things in cosmology (the very large) and quantum mechanics (the very small) that are totally counter-intuitive to what we think "natural" or "logical" or whatever and to assume otherwise often doesn't work.

Aint that the truth, brother. Thanks again, all.

 

[Baluncore]

The maximum speed of sound is limited by other things than the speed of light.
https://scitechdaily.com/fundamenta...discover-the-fastest-possible-speed-of-sound/