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Transmission of energy

  1. Apr 30, 2009 #1
    If you have a stick and push it from one end, the whole stick moves instantly. You have given the other end of the stick kinetic energy. So doesn't this mean that you have transmitted energy at an infinite rate, rather than at or below the speed of light?
     
  2. jcsd
  3. Apr 30, 2009 #2

    Doc Al

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    No it doesn't. The impulse moves through the stick at about the speed of sound. So nothing is being transmitted anywhere near the speed of light, much less infinitely fast.
     
  4. Apr 30, 2009 #3
    Doc Al:
    What if you push it with a speed that's twice the speed of sound- wouldn't that mean that you could see a noticeable difference in the length of the stick, as the atoms were compressed together, if you filmed it?
     
  5. Apr 30, 2009 #4

    Doc Al

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    I'm talking about how long it takes for a smack given to one end of a stick to travel to the other end. If you push one end of the stick, it takes time for all parts of the stick to get moving. It's not instantaneous. So, yes, the stick will be compressed a bit when you push on one end. (Push too hard and you'll just break it, though. :frown:)
     
  6. Apr 30, 2009 #5
    What if it's in space and there's no friction- would it be transmitted faster?
     
  7. Apr 30, 2009 #6

    Doc Al

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    Not sure I understand what you're getting at with this question. I see no reason why being in space would have anything to do with it.
     
  8. Apr 30, 2009 #7
    I'm not talking about vibrations.
    Ok, how about if I put it like this:
    There is a long tube full of marbles which are all touching each other. I put another marble in at one end and another comes out of the other end. I have transferred kinetic energy through the marbles. How quickly was it transferred, or how long did it take between the original marble at the entry end starting to move, and the exiting marble starting to move.
     
  9. Apr 30, 2009 #8
    Actually, forget it- I've realised my error.
     
  10. Apr 30, 2009 #9

    russ_watters

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    No one answered this part:
    Material mechanics gets a little complicated and I'm not exactly sure how a stick works, but a steel rod has a certain range over which the deformation is perfectly elastic and perfectly reversible. If you try to move it faster than its speed of sound (with as much acceleration as it is willing to absorb), you'll start to move into the range of plastic deformation, whereby the impulse will travel through it faster than its speed of sound but only at the cost of permanent deformation. Much faster than that and the forces trying to accelerate it will exceed the material's strength and it will start to disintegrate.

    A stick likely has a much lower and smaller range of elastic deformation and essentially no range of plastic deformation: exceed its speed of sound by more than a little and it'll just shatter.
     
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