# Propagating signal in a rod

1. Mar 3, 2008

### Zhivago

Hello everyone, this is my first thread :)

It all started from the perfectly rigid rod connected from earth to the moon and how fast the signal would get there if I pushed it. I understand the signal can't get there instantaneously because the inner structure of the atoms of the rod has a propagating limit.

But what happens if I push a rod (can be a regular sized one) with an energy high enough to give the first atoms in one end of the rod a speed greater then the propagating speed in the rod?
My answer would be: the rod would want to deform itself, or the energy would be converted to heat / radiation.
But I'm not quite sure of any of these... What do you think?

2. Mar 3, 2008

### rcgldr

The limiting factor is the speed of sound in the rod. If you impart an impulse faster than the rods speed of sound, it's similar to the air. The result is a shock wave that travels at the speed of sound, which dissipates into a normal sound wave over distance and time, although I'm not sure of the rate of disspation, espeically in the case of a very long rod.

A bit off topic, but in the air, shock waves sould like a crack, not a boom. You have to be far enough away from the source of the shock wave for it to dissipate and turn into a normal sound wave in order to hear the boom.

Assuming the rod is conductive, the speed of electrical propagtion is much faster than the speed of sound in a rod.