fundamental physics question

[im a model]

my question is basically: how long does it take an object to completely move, given an impetus? this is very vague, so ill explain. i first thought about this thinking about a line of dominoes. say they are spaced a half-inch apart, i flick the first one over, and then it takes a measurable amount of time before the last in the line moves (not falls all the way, just moves any measurable increment, before it obviously will fall). now say they are spaced a tenth of an inch apart, i flick the first one over, and it should take less time before the last one is touched and moves. now the dominoes are set up so there is no space in between them--they are all touching each other in a stack--now i flick the first one, how long does it take the last one to move?

it cant move instantaneously--there must be some elapsed time for the force to travel through the dominoes (be they wood, plastic, whatever) and reach the domino at the end of the stack. does this information that i flicked the domino travel at the speed of light, the speed of sound through that material (this seems the most reasonable), or some other speed?

it seems harder for me to visualize if say we had a 2X4 of wood that was a mile long, or a steel beam that was a mile long (sitting on a frictionless surface). when i kick one end of the 2X4 or beam, wont there be a measurable amount of time between my kicking it and the end that is a mile away moving? so lets say sound (my guess) travels through steel at 5280 ft/s, will it take a second after i kick the beam for the end a mile away to move? that seems really weird and wrong. it seems like it would move instantly, but that would mean the "information" of my kick would have to travel instantly through the material.

alright, ive repeated myself enough so that the problem should be clear. so whats the answer--speed of sound, light, instantly, something else?

thanks.

[amplify]

You are correct that if movement across great distances was instantaneous, then information would travel faster than the speed of light. What I don't understand is why you are trying to shoehorn the speed of sound or something into the equation. Push a stick. Molecules in the stick push against other molecules, transferring the energy of your push at the speed of wood. Information passes through metal at the speed of metal. If you yell at a stick, that would move at the speed of sound.

[Sephus]

[ QUOTE ]
when i kick one end of the 2X4 or beam, wont there be a measurable amount of time between my kicking it and the end that is a mile away moving?

[/ QUOTE ]

yes. this seems wrong because you're assuming the 2x4 never changes shape. when you kick the end, you compress it.

[Phil153]

Interesting question. To put in physicist speak: If you tap on one end of a very long, thin, rigid, incompressible body with sufficient force to move it, what happens?

[Sephus]

[ QUOTE ]
Interesting question. To put in physicist speak: If you tap on one end of a very long, thin, rigid, incompressible body with sufficient force to move it, what happens?

[/ QUOTE ]

you're describing the faster-than-light communication device that i invented. i call it a sephus rod.

[im a model]

so is amplify right in that there are speeds of materials? i thought that this is what the speed of sound was--ie, the molecules bouncing off one another (conveying sound information), which is why sound travels so much faster through metal than water. is there an index somewhere on the web that shows the "speed of iron" or the "speed of gold"?

[amplify]

I totally made that up about the "speed of wood" but you can see what I'm getting at. It is the nature of solids to appear as if they are incompressable, rigid and unitary but we know that is not the case. I simply inferred that depending on the structure and density of the solid that movement is transferred throughout a body at slightly different rates. The speed of sound is the measurement of a wave traveling through different media, such as air or water, not the measurement of "information" travel. I have no idea what the speed at which information travels through wood is, but I am 100% certain that it is way faster than sound.

For example, standing down the street from my neighbor, he is bouncing a basketball. I can see the ball hit the ground quite a bit sooner than I hear it. Imagine I had a long stick and poked him with it as soon as I saw the ball hit the ground. Assuming for the moment that I have perfect reflexes, he gets poked before I hear the ball hit.

[amplify]

[ QUOTE ]
Interesting question. To put in physicist speak: If you tap on one end of a very long, thin, rigid, incompressible body with sufficient force to move it, what happens?

[/ QUOTE ]
Physicists generally don't spend a lot of time wondering what would happen to an incompressible body, since the existence of such a body would violate all the laws of physics. If it did though, yeah, it would move faster than light.

[TimM]

What you're looking for is the speed of sound in wood (not speed of wood), and it is much faster than the speed of sound in air:

http://www.uk-piano.org/sound.html

[Borodog]

What he said ^. When you strike a solid material you create a compression wave that travels through the material at the speed of sound in that material, which is basically the square root of the bulk modulus divided by the density.