Problem with the iron staff

I would suspect that the impulse would travel along the rod at the speed of sound in iron. It would take over 17 million years for your friend to feel the movement on the other side. Meanwhile, the rod stretches up to a meter.

It's not really a paradox, though. It's just that we're so accustomed to seeing this kind of thing happen on our everyday scale--in which iron rods are short enough that an impulse travels from one end of a rod to another in a fraction of a second, which we perceive as instantaneous.

--Astro

Not that he needs me to confirm it, but Astro is right. There is no such thing as a truly rigid body, and instantaneous action at a distance is a violation of logic.

Consider what happens in detail when you push one end of a rod. The push gets applied only to the surface atoms. They move a little, then run into their neighbors and start to push them. The second-layer atoms then move a bit and run into their third-layer neighbors. And so on. Only after the sequence of relayed pushes reaches the far end of the rod does that end being to move. The pressure wave travels at the speed of sound, which is well below the speed of light. -|Tom|-

I never even considered the speed of sound in iron. Hopefully I can get this out of my head now.
Thanks both of you []

You can verify these things for yourself with a slinky and a video camera.

* Tape a meter stick on the wall and hold the slinky next to it until it stops moving.

* Adjust the camera to be able to see the entire slinky.

* Release the top of the slinky.

* Study the recording one frame at a time.

You will see that the bottom of the slinky doesn't start falling when you let go of the top. It waits for almost all of the stretch to be used up.

The stretch in the slinky has the effect of slowing down the speed of "pressure wave movement" (AKA speed of sound) enough so that you can see what is happening. A steel rod also stretches when you hang it from one end, and when the top is released the bottom won't start falling until the stretch is used up. But the stiffness of the steel rod makes everything happen faster.

Regards,
LB

What if the steel rod was a beam of light since this is what seems to be the the meaning within the frame of reference in the original post?

Pheonix,

I also had the same question a while back with a seesaw and recieved the same answer. It makes since to me now, but from a far it would look as though the see saw were bending. You would actually see the wave move across the sceen.