# Speed of push

## Main Question or Discussion Point

When you push on an object and accelerate it to some relativistically (so we can really, really ignore lorentz contraction) small velocity (say a nice 1 m/s), does the impluse from your push travel through that object as a longitudinal pressure wave at the speed of sound (what ever that may be for that object)? It seems like that should be true, but that also implies that the object should contract slightly and that one end of the object would attain the 1 m/s velocity before the other end does, (assuming that the object is long enough that one can reach this velocity before the pressure wave meets the other end) and for some reason I don't like that conclusion, even though it makes sense.

## Answers and Replies

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HallsofIvy
Homework Helper
Perhaps your post would make sense if you explained WHY you don't like the idea of pressure waves in solids.

Doc
Learn to like that conclusion. This falls in line with something I just posted in the thread entitled 'energy absorption'. Consider a steel rod that is about 5 to 10 feet long. Hit it with a hammer from one end. The 'shock wave' will arrive at the other end a short time later. The 'shock wave' is basically the far end being moved by the hammer only at a later time. So yes, there is a speed of push so to speak.

russ_watters
Mentor
There isn't anything about solids that special and makes them not compressible. Metals are actually very close to perfectly elastic for small displacements.

Your conclusion is correct.

That's what I thought; HallsofIvy: What I don't like is that one end will move before the other. It feels to me like macroscopic quantum superpostion, ie two places, one time, one object. (I knows that that's unrelated, it just feels as weird as that would.) It's like one of Zeno's paradoxes or something.

Doc
Originally posted by Jonathan
That's what I thought; HallsofIvy: What I don't like is that one end will move before the other. It feels to me like macroscopic quantum superpostion, ie two places, one time, one object. (I knows that that's unrelated, it just feels as weird as that would.) It's like one of Zeno's paradoxes or something.
Read my post in 'energy absorption'. If both ends moved at the same time then an object could be used for faster than light signaling. It's not permitted.

russ_watters
Mentor
Another way to consider your objection, what would happen to sound in a solid if there weren't pressure waves moving around in there? Heck, how would an earthquake work (would the whole earth shake?)?

Some things in science don't make sense when you look at them from one direction, but if you look at them from another direction, they make a lot more sense.