# The Speed of Force

Hey I'm new here and I would like to propose a question about the speed of kinetic energy. If lets say we had a newton's cradle one light-year long, will the last ball fly up as soon as the first ball hits the second? Assume all the balls are in perfect contact with each other. Does this happen instantly or at light or sub-light speed? I do not think the speed is limited, but what do you people think?

-tdz

chroot
Staff Emeritus
Gold Member
Kinetic energy does have a "speed."

Futhermore, the balls communicate force via pressure. Pressure variations propagate at the speed of sound, which is different for different media, and is always less than the speed of light.

So no, the ball at the far end won't move until the pressure wave reaches it, which will take some time.

- Warren

I think the the Newtons cradle example, the shockwave will travel through the material at the speed of sound (about 5-10 km/s).

Originally posted by chroot
Kinetic energy does have a "speed."
Don't you mean 'does not'?