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Long rods and the speed of light

My question is regarding the speed of light. When one end of a rod is struck, how long does it take before the other end moves? Presumably there is a delay, but what would happen if this rod was a light year in length – would it be a year (or more, or less) before the other end moved?
R Lindeck from West Sussex (Aged 55+)


3 Responses

  1. The physical movement within the rod travels at the speed of sound in the rod which is a lot slower than the speed of light. It would be a bit like the vibrations through the earth when there is an earthquake. These travel at the speed of the different types of shock waves through the earth’s rocks. Therefore to travel a light year’s distance would take Speed of light/Speed of sound x year.

  2. Perhaps my question would have been better phrased had I replaced the word “struck” with either “pushed” or “pulled”.

    It maybe that the answer given would still apply, but I am not concerned with the vibrations that pass through the rod, but am interested in the lengthways movement of the rod. If the opposite end of the rod does not respond instantly, then the rod itself would have to be squashed or stretched by a considerable amount. Is that what would happen?

  3. I think the answer is still the same. You are describing different types of impulse(force x time) to differentiate the two conditions but they are both still impulses. One is over a short period(being struck) and one is over a long period(ie being pushed).

    In short, for both conditions the rod is squashed slightly as you try to move one end.

    To answer your concern as to whether the rod would be squashed or stretched by a considerable amount with a slow movement. For a very long rod it would indeed be squashed or stretched with the rod’s length recovering to “normal” once the impulse has reached the other end.

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