# What "propagates" a force through the rest of a solid?

So, in typing the title of this question I was recommended this awesome one, which confirmed my guess that this effect "propagates" at the speed of sound (though I just had a feeling, I don't really understand why). I also only have high-school level physics experience.

I feel like I don't know physics well enough to really know what to ask correctly, but I'll try to explain.

So, I rotate my arm, and, even though tendons are just pulling it at a certain location, the rest of it follows suit. Why is that? What is happening at an atomic/molecular scale that ends up "conveying forces" over a distance?

And, something else I don't understand- what is so special about the speed of sound that makes it this "fundamental unit of translation", or something? Maybe the better question is "what processes at an atomic/molecular scale lead to the speed of sound being associated with all of these behaviors", which ties both questions together.

• Speed of sound is just a name for your "fundamental unit of transition" its properties have nothing to do with sound itself, sound just happens to be a particular case of deformation propagation. Commented May 26, 2014 at 5:50
• That's what I was referring to with "what processes at an atomic/molecular scale lead to the speed of sound being associated with all of these behaviors?"
– TND
Commented May 26, 2014 at 15:29

You see that it isn't an instantaneous process. You correctly identify the speed of the propagation of this effect as the speed of sound, $c$. However, this is just how the speed of sound is defined: the propagation speed of a small deformation in the material. This depends on the stiffness (measured by the bulk modulus $K$) and the density $\rho$ of the material: $$c = \sqrt{\frac{K}{\rho}}$$ This can vary a lot. The speed of sound in air at SATP is about 343 m/s, whereas in steel it is closer to 6000 m/s.