Two person, $A$ and $B$, each holding one end of a long solid rod.

Now person $A$ pushes the rod on one end.

Question: Is it correct that the information that the rod has been pushed will travel to the other end at the speed of light whereas the actual 'push' will travel at the speed of sound in the rod?

i.e. If the rod has length $ ct $ , then will person $B$ feel the push in time $t$ or $ct/v$? ($c$ is speed at which EM waves propagates in this experiment, and $v$ is the speed of sound in the rod.)

  • 2
    $\begingroup$ Related: physics.stackexchange.com/q/72598 physics.stackexchange.com/q/2175 $\endgroup$
    – tpg2114
    Aug 13 '13 at 1:38
  • $\begingroup$ @tpg: They're more of duplicates... $\endgroup$ Aug 13 '13 at 1:46
  • $\begingroup$ @Dimension10 I was going to suggest them as duplicates but they aren't exact duplicates. One is asking about faster than light, another is how things go faster than sound... Only tangentially duplicate, not exact. $\endgroup$
    – tpg2114
    Aug 13 '13 at 1:48
  • $\begingroup$ @Dimension10, sorry about that, I should've searched more carefully before posting. Thanks for pointing it out. $\endgroup$
    – Jin
    Aug 13 '13 at 1:57
  • $\begingroup$ Umm... I don't think a downvote is justified here, just because it' is a duplicaate . $\endgroup$ Aug 13 '13 at 11:39


Disturbances always travel at the speed of sound in the medium. The wave equation is usually solved with

$$ u(x,t) = U \sin \left( \omega t \pm \frac{\omega\, x}{c} \right) $$

where $c$ is the speed of sound and $f=\frac{\omega}{2\pi}$ is the frequency of the disturbance.


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