Problem with the iron staff

Jim,

Except for the speed of propagation (replace speed of sound with speed of light) there is no difference between using an iron beam or a light beam.

It doesn't matter whether you push it, pull it, bend it, twist it, rotate it about a fulcrum or move it sideways. Whatever change you make effects only the part of the beam you can touch. That change will propagate down the beam at some speed that is characeristic of whatever the beam is made from. The guy at the other end won't know anything has happened until the change reaches him.

(This is also true if you use a beam of gravitons. Just the speed of propagation changes.)

If you could view the process "from afar" as rousejohnny suggests, you would indeed see the change moving from one end of the beam to the other. Like watching someone "throw a loop" in a rope or a garden hose.

LB

Larry,

I thought about the seesaw example I used and have an conceptual issue that popped in my head.

The seesaw is on earth. If the speed of souund for the seesaw were X meters/sec, and the length of the seesaw were 4x with the fulcrum at 2x, when you push down on the up side of the seesaw, from the time you pushed down to the time you let go must be > or = 2 seconds or the seesaw would not stay down, is this accurate?

rousejohnny,

Not exactly. Sort of. Kinda, but not really.

===

If you move your end of the seesaw down, then back up (exerting many times the minimum effort needed to move the seesaw), in 1 millisecond, the other end will try to follow 4 seconds later.

If moving your end that fast doesn't cause any part of the seesaw to need to move faster than the speed of sound in the seesaw, then the far end will move up, then back down, in 1 millisecond (after the 4 second propagation delay).

===

If you push down on your end and let go before *4* seconds, there probably will be some "backlash". IOW, part of the energy of your push will reflect back to your end and cause your end of the seesaw to want to move back up. How much will depend partly on when you stop pushing.

But the rest of the energy continues to the far end and makes it move.

===

NOTE - backlash between 0 and 2 seconds (before the bend propagates past the fulcrum) will probably be different (larger) than backlash after 2 seconds.

NOTE - these are seat-of-the-pants estimates. No equations were harmed during their production.

LB

Larry,

So when I release my hand in the millisecond and thus removing the force before the wave reaches the fulcrum, the speed of gravity would not cancell out the impulse of my push and automatically pop my end of the seesaw back upwards, why?

Your end of the seesaw will pop back up. I didn't explain it very well, though, so I'll try again.

(And it has nothing to do with the speed of gravity.)

When you push down on your end, the seesaw begins to bend. That bend propagates down the length of the seesaw at the speed of sound in the seesaw. Until the bend reaches the other end, there is some tension in the seesaw. You can feel it.

If you let go before the bend has reached the other end, that tension is what makes your end of the seesaw pop back up. The amount of tension (and the amount of pop up) will depend on how far the bend has traveled, and I suspect that once it propagates past the fulcrum some things will change.


LB

Larry,

Does elasticity of a material have an effect on the speed of sound or any wave through that material. Does thickness of the material have an effect? i.e. thin strip of aluminum verses a thicker one? This is what is causing my confusion on this whole issue, the rigidity of the medium a wave propagates through and the characteristics of the wave that is produced.

Another thing is if you have a garden hose that is tangled and you flip a wave through it, it will move much slower than if you pulled one. What is the speed of a force with a flat amplitude vs. one with a high amplitude. This is why the speed of sound answer does not sit well. Is the light wave the flatest amplitude or is there something that can move faster such as a straight pull or push on a rigid object? I am sure this is physics 101 but something doesn't sit well.