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Homework Help: Applet about special relativity

  1. Oct 11, 2007 #1
    1. The problem statement, all variables and given/known data

    In the above applet, the clocks at the front and rear mirrors of the light clock do not show the same time when viewed from the moving frame.

    Why is that? The clocks move with the same speed (same system of inertia), so I believe the reason why is that the signal takes a longer time to reacha stationary observer - but this answer was wrong.
  2. jcsd
  3. Oct 11, 2007 #2
    The other two possible answers are

    1) The clock in the front is farther away from the clock in the back, so it must experience a greater time delation.

    - I believe not, since they are both moving with the same speed - same system of inertia.

    2) Since both clocks start at same time when in rest, they cannot start at same time when moving.

    - I believe not (although the correct answer is either 1 or 2), because again - same inertial system, so why would they have to differ?
  4. Oct 11, 2007 #3


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    If you're in the moving frame (e.g. if you're at the center of the bottom blue bar, moving along with the system, the clocks will be the same (for all purposes, it will be like you are in the top situation).

    But if you are in the top situation and you see the bottom clock pass by, then they will show different times. The reason is exactly the one depicted in the applet: the light beam has to travel farther to overtake the mirror moving away, and when it is reflected it will have to travel less than l because the other mirror is moving toward it.

    This all makes perfect sense to me (at least, presented this way): longer path = longer time.
    So what exactly is the question ?
  5. Oct 11, 2007 #4
    The exact question is, why the front clock is not starting at 0.
  6. Oct 11, 2007 #5
    We are vieweing them fron the moving frame.

    The correct answer is 2 - why is that?!
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