Can sound waves travel in a material at a speed faster than the speed of sound?

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Discussion Overview

The discussion revolves around the behavior of sound waves and shock waves in a cylindrical rod when subjected to different pushing velocities. Participants explore the implications of pushing the rod at velocities both below and above the speed of sound in the material, considering the physical constraints and material properties involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the front of a rod will move only after a time equal to the length of the rod divided by the speed of sound, c, when pushed at a velocity less than c.
  • Another participant asserts that it is physically impossible to push the rod faster than the speed of sound, suggesting that the question is meaningless under that premise.
  • There is a discussion about the definition of c, with some participants clarifying that it refers to the speed of sound in the material, while others note that c is commonly used to denote sound speed.
  • A participant mentions that pushing the rod at a constant speed greater than the speed of sound would require an "infinite" amount of force, due to the increasing elastic modulus of materials under compression.
  • Another participant proposes that if a much harder object impacts the back of the rod at a speed greater than the rod's sound speed, the front of the rod may not immediately respond, leading to a potential scenario where part of the rod moves faster than its sound speed upon impact.
  • One participant shares an anecdote about observing a tensile stress wave traveling through a rod even after it had broken, indicating that the behavior of stress waves can be complex and not immediately intuitive.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of pushing the rod faster than the speed of sound, with some asserting it is impossible while others suggest scenarios where it might occur. The discussion remains unresolved, with multiple competing perspectives on the mechanics involved.

Contextual Notes

Participants highlight limitations in understanding related to the definitions of speed, the nature of material deformation, and the physical implications of pushing or pulling the rod. There is also mention of the complexities involved in stress wave propagation and material failure.

JustWonderingx
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I'm just trying to get a better grasp on sound waves and shock waves.

Let's say I have a cylindrical rod of length L with no forces acting on it, and I push on the back of it with some constant velocity less than the speed of sound, c, of the material the rod is made of. Will the front of the rod not move until time=L/c?

Now let's say I push on the same rod with a velocity, V, greater than c, and the material cannot fracture but can deform transversely. Would the length of the rod approach 0 at time=L/V as the back of the rod approaches the front? In this situation is the shockwave moving at velocity V?

Thanks
 
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c has nothing to do with propagation in a rod. It happens at the speed of sound in the material.

You cannot push on the rod faster than c. This is physically impossible, so that part of your question is meaningless.
 
phinds said:
c has nothing to do with propagation in a rod. It happens at the speed of sound in the material.

You cannot push on the rod faster than c. This is physically impossible, so that part of your question is meaningless.

I specified in my post that I am defining c as the speed of sound in the material.
 
JustWonderingx said:
I specified in my post that I am defining c as the speed of sound in the material.

Ah ... I missed that. BAD choice of symbols.
 
phinds said:
Ah ... I missed that. BAD choice of symbols.

I disagree, c is frequently used as a symbol for sound speed. But we digress.
 
JustWonderingx said:
I disagree, c is frequently used as a symbol for sound speed. But we digress.

Fair enough. I didn't know that and I now notice that this is posted in general physics, not cosmology, which is where my mind was. Sorry to have temporarily derailed your thread. :redface:
 
It's not obvious you can push the rod at a constant speed greater than the speed of sound, without using an "infinite" amount of force. The elastic modulos of mateirals in compression tends to increase, at least until the matieral fails in crushing. A compressive strain of more than 100% is impossible, if you think about what it would mean physically.

The question makes better sense "in real life" if you think about pulling the end of the rod rather than pushing it. In that case, you are right that the far end will not move until the stress wave has traveled the lengtth of the rod, indepedent of how fast or hard you pull.

FWIW I've seen this happen, in a "fail safe" device that pulled a rod violently to stop part of a machine working. In a test (using high speed video etc to see what happened) the rod broke near the end that was pulled, but the tensile stress wave continued along the rod and moved the other end after the rod had broken, even though there was no pulling force acting on the rod after it broke.

BTW "c" is a standard symbol for the speed of sound, if that is a more interesting quantity for modelling the situation than the speed of light.
 
AlephZero said:
It's not obvious you can push the rod at a constant speed greater than the speed of sound, without using an "infinite" amount of force. The elastic modulos of mateirals in compression tends to increase, at least until the matieral fails in crushing. A compressive strain of more than 100% is impossible, if you think about what it would mean physically.

I don't understand why you would need an "infinite" amount of force to push something faster than its speed of sound. If I have a rod with a low sound speed material, and I impact the back of it with a much harder, much more massive object moving at a speed, V, much greater than the rod's sound speed, c, wouldn't part of the rod be moving at a speed greater than it's sound speed as this object pushes it, while the other end remains undisturbed?

I feel like upon initial impact the atoms in the back plane of the rod begin moving with speed ~V. Since c<V, the next plane of atoms, and all subsequent planes, don't know the impact has occurred and cannot have increased their rigidity, and thus cannot have increased their sound speed.
 

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