Pushing Supersonic: Beyond Sound Speed

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Discussion Overview

The discussion revolves around the behavior of a pole when one end is pushed faster than the speed of sound, exploring the implications of supersonic deformation in solids. Participants examine the propagation of stress waves and the resulting physical phenomena in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant posits that when one end of a pole is pushed, the other end reacts after a time delay corresponding to the speed of sound in the pole.
  • Another participant clarifies that the speed of sound is relative to the pole itself, and that stress pulses propagate at this speed regardless of the motion of the pushed end.
  • A further contribution suggests that if the strain velocity exceeds the speed of sound in the medium, a shock wave might form within the solid, leading to complex wave interactions over time.
  • One participant speculates that, similar to supersonic fluids, energy from a collision in a solid could be transferred upstream, although this idea remains unverified in the context of solids.

Areas of Agreement / Disagreement

Participants express differing views on the implications of pushing the pole faster than sound speed, with no consensus reached on the exact outcomes or mechanisms involved.

Contextual Notes

The discussion includes assumptions about the nature of stress wave propagation and the definitions of strain velocity and sound speed in solids, which may not be fully resolved.

Who May Find This Useful

This discussion may be of interest to those studying wave propagation in materials, solid mechanics, and the behavior of materials under extreme conditions.

daniel_i_l
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When you push one end of a pole the other end reacts (starts moving to) after the amount of time that it would take sound to travel to that side. So what happens when one end is pushed faster than the speed of sound?
Thanks.
 
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The speed of sound is with respect to the pole.
The stress pulse always moves with that speed in the rest frame of the pole, independently of how the struck end moves.
 
daniel_i_l said:
When you push one end of a pole the other end reacts (starts moving to) after the amount of time that it would take sound to travel to that side. So what happens when one end is pushed faster than the speed of sound?
Thanks.

If with "pushing faster" you mean that the strain velocity applied is faster than the speed of the sound in the medium, I would expect to see "sort of" shock wave in the solid. If [tex]t_o[/tex] is the characteristic time of strain and [tex]t_1=L/c[/tex] is the time of a traveling stress wave, I would expect to see for times [tex]t/t_o\sim O(1)[/tex] a bunch of stress waves stored at the compressed pushed end, and for [tex]t/t_o>>1[/tex] such that [tex]t/t_1\sim O(1)[/tex] the stress waves would reach the other end and the deformation is propagated collaborating in the bulk motion of the pole. That's the case of a supersonic deformation.
 
I know that in the case of supersonic fluids energy from a collision is transferred back upstream by EM. Perhaps something similar would happen in a solid?
 

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