# When you push a stick will the end of it respond later than the beginning? Is so what is the fastest material and what forces are involved?

Normally when two people hold a long stick the receiving person would feel at the same time the stick moving when the first person gives the stick a push. But is that in reality exactly the case? Because the atoms of a stick are actionally pushing each other from the beginning toward the end of the stick and that takes time. But is that true or is immediate reaction still possible?

Perhaps material with a crystal structure can pass through the moving signal the fastests. So a stick of diamand could fe be moved very quickly. If so are the electric fields of the atoms just pushing each other or is there also photons involved which forbid to push the stick faster then light?

It is also said that the speed of sound is involved, but what is the difference in such a case? Isn't elektro-magnetic moving not always the fastest

• Related: physics.stackexchange.com/q/2175/2451 and links therein. – Qmechanic Mar 25 '16 at 9:09
• Presumably the fastest would be that for which sound propagates the fastest (which I think is beryllium). – lemon Mar 25 '16 at 9:11

## 1 Answer

The speed you are looking for is the speed of sound, which is the speed at which forces are transmitted within a material. So materials with the highest sound speed will transmit this information the fastest, and indeed nonporous solids have the highest sound speed.

• Can you explain why it is the sound and not the em force that pushes the stick? – Marijn Mar 25 '16 at 9:30
• @Marijn When you compress one end of the stick, you create a pressure wave that must then propagate through the material (which is precisely what a sound wave is). The reason EM is irrelevant is because atoms within a material 'look' neutral when you're as little as a few nanometres away. – lemon Mar 25 '16 at 10:23
• Ok, but I don't know if compressing is the same as pushing. I just hold the stick in my hand and push it forward. Is that also causing a pressure wave? – Marijn Mar 25 '16 at 10:47
• @Marijn It is. When you push a rod from one end you create a shock wave that moves through the material; you can see a simulation of it here. It's difficult to comprehend because it happens so quickly (that simulation will be ~100 picoseconds long). – lemon Mar 25 '16 at 11:08
• Beryllium soundwaves travels with about 40.000km/u (12km/s). So that means that wiht a stick of berrylium of 12km long it takes 1 second before the other person notice it?! – Marijn Mar 25 '16 at 11:51