# Energy absorption

haynewp
If you had an infinitely rigid object and threw it against an infinitely rigid wall, where would the energy go?

Jonathan
I think it would go into that noise that just blew a hole in your ear drums. But this reminds me of something that I should post a thread about...

Mentor
Heat.

Jonathan
Is not sound kind of a form of organized heat? Oh what the heck, you're right, I should have said that...I'm getting forgetful.

haynewp
So I guess less heat be produced with a ductile material than an extremely rigid one since some of the energy is absorbed in the deformation. But isn't it molecular interaction that is causing the heat anyway?

Homework Helper
Why would the energy "go" anywhere?

"infinitely rigid" means "infinitely elastic" so that the infinitely rigid object would bounce of the infinitely rigid wall with its original speed and mass.

Originally posted by haynewp
If you had an infinitely rigid object and threw it against an infinitely rigid wall, where would the energy go?

What exactly is meant by "infinitely rigid"? If you press against a solid object, it deforms just the right amount to press back. If it is very rigid, it deforms very little to give the force (effectively it has a high spring constant dF/dx). If it is infinitely rigid, it deforms not at all. So how does it "know" how much force to react with? It doesn't. I conclude that the "infinitely rigid" case is not a useful model of anything.

It has been said that an infinitely rigid rod could be used for faster than light signaling. Of course there is no such thing. Infinitely rigid implies infinite density and sound traveling through the object at an infinite speed.

Mentor
Originally posted by HallsofIvy
"infinitely rigid" means "infinitely elastic"
Yeah, you're right. Tricky one.