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How would a long object move?

  1. Feb 17, 2010 #1
    How would a "long" object move?

    My question is seemingly simple.

    Let us suppose that we had an unbreakable/inflexible rod whose length was 11.8 million miles in length (one light-minute). Now assume that we applied enough force at one end of the rod to move the entire rod altogether. As nothing can travel faster than the speed of light, how long would it take for the opposite end of the rod to move? Does the motion of the rod become a wave?
  2. jcsd
  3. Feb 17, 2010 #2


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    Staff: Mentor

    Re: How would a "long" object move?

    An inflexible rod wouldn't move because it doesn't exist. But yes, a real rod begins to move by pasing the force through the rod in the form of a longitudinal wave: a sound wave.
  4. Feb 17, 2010 #3
    Re: How would a "long" object move?

    Sure. Lets say this rode is made from a chunk of a neutron star. Well, if this is the case, it will still take more than a second (several hours i guess) before the other end moves. But for a hypothetical stiff, rigid rode I think it can never be at rest neither can it be accelerated. This means at any given time both ends are moving at the same speed irrespective of any gravitavion fields. Think ether. NOTE: This is my own speculation. It should not be taken as fact. Thanks.
  5. Feb 18, 2010 #4
    Re: How would a "long" object move?

    This suggests that the molecular composition would affect the "speed of motion" of a rod.

    After doing a little reading (now since I know what to read), I saw some sources calling it "mechanical information" that travels in sound waves, as the guy said above. Is there any theoretical limit on the speed of sound?
  6. Feb 18, 2010 #5


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    Science Advisor

    Re: How would a "long" object move?

    Yes, a rubber rod beheves differently than a steel rod, when rotated.

    Yes, the speed of light in vacuum. The electromagnetic interactions between the atoms of the rod, cannot go faster than that.
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