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Pushing a stick into the moon

  1. Aug 17, 2011 #1
    It takes light roughly 1.3 seconds to travel from the earth to the moon. I have read that even if you push on one end of a stick, the effect of the push cannot reach the other side of the stick faster than c.

    So suppose I held a stick in my right hand and one end of this stick is only 1cm away from the surface of the moon. My right arm is bent at a right angle. I now push my end of the stick toward the moon.

    Intuitively, it seems that as soon as my arm pushed forward 1cm it would meet instant resistance that stopped it from moving more than 1cm. But if this really happened, then if I tapped Morse code on the surface of the moon I could send a signal faster than c.

    So I'm guessing I would be able to continue to push my end of the stick forward more than 1cm. Indeed, I suppose I could push until I locked out my bent right arm at the elbow. I would not be able to feel the effect until enough time has passed for light to travel to the moon and back---roughly 2.6 seconds.

    So suppose at time T=0 seconds I begin to push my end of the stick toward the moon. At T=1.0 seconds I have locked out my right arm---I have pushed my end of the stick forward about, say, 60cm. At T=1.3 seconds the other end of the stick hits the moon. At T=2.6 seconds I feel the effect of the other end of the stick hitting the surface of the moon.

    But what will this effect be? My arm cannot remain locked out unless the stick has shortened by 59cm. But the stick has not in fact shortened, right?

    So does my right hand experience a rebound at 2.6 seconds that forces it 59cm back from the moon? (I.e. a rebound with a force equivalent to the force it took me to accelerate the stick's mass 60cm toward the moon in 1 second?)
  2. jcsd
  3. Aug 17, 2011 #2


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    Staff: Mentor

    The motion of the stick or the propagation of the taps happens at the speed of sound in the stick, not at c.
  4. Aug 17, 2011 #3


    Staff: Mentor

    Maybe we should make this a FAQ. It comes up at least once or twice a month.
  5. Aug 17, 2011 #4

    Last edited by a moderator: Aug 17, 2011
  6. Aug 17, 2011 #5
    I did read the FAQ for this forum before posting. I also did a search for "stick" in the search box before posting and didn't see anything relevant.

    Though I should have tried Google before posting...as "speed of light stick" turned up this hit:

  7. Aug 17, 2011 #6


    Staff: Mentor

    No worries, there are a lot of variations on the theme that don't affect the idea but make it hard to search for. It is a question that deserves a FAQ I think.
  8. Aug 17, 2011 #7


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    Staff: Mentor

    I swear its more like 1-2 times a week!
  9. Aug 17, 2011 #8
    It falls under "rigid body doesn't square well with finite maximal speed of propagation" category. Perhaps the "Pole and the barn" problem should go into it too?
  10. Aug 18, 2011 #9
    Why is the propogation of taps at the speed of sound?
  11. Aug 18, 2011 #10


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    Gold Member

    Just a thought, is it worth having a specific thread for people to recommend FAQ's? For example, I've seen a number of things that could be potential candidates.

    I know we all should make the recommendation in the thread where we see the opportunity for the FAQ, but having one thread dedicated to FAQ requests might help eliminate duplicate requests and offer a way to manage all of the potential FAQ's needed.

    It would also give others the opportunity to offer a potential FAQ answer in the request. Might help save some Admin time.

    Anyway, as I said, it was just a thought. :smile:
  12. Aug 18, 2011 #11


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    Science Advisor

    Because that's what sound is- waves of vibrations. The speed of sound in a medium is the speed at which waves are transmitted through the medium.
  13. Aug 18, 2011 #12


    Staff: Mentor

    It is a good thought. I did exactly that, but got no traction:
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