[Coda]

Electrical resistance is very much like friction, as I mentioned before. It converts kinetic energy into heat.

As for objects knocking into each other making noise: it's a shockwave! The speed of sound in an object is actually measuring how fast force can propagate through that material.

You already know Newton's third law. Object hits other object; other object exerts force back on first object. Well, imagine that down at the atomic level. The atoms at the point of collision get knocked into the atoms behind them, which get knocked into the atoms behind them, etc... and how quickly this happens is what defines the speed of sound in that material.

Well, on the other side of the object, the atoms get knocked back, but there aren't any other atoms of the material there to bump into -- instead, they bump into the atoms of the air. And those get knocked back through space until they hit something, and so on and so forth.

Eventually, if one of those hits your eardrum, it stimulates some nerves, and you perceive it as sound.

And now you know what a sound wave is -- it's kinetic energy getting rapidly transferred from atom to atom through a series of collisions. (Well. Technically not collisions. At the subatomic scale, everything's a field force, so really it's the atoms repelling each other when they get too close together. But at the macro scale, that's a collision.)