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Relativity of Simultaneity

  1. Apr 23, 2016 #1
    1. The problem statement, all variables and given/known data
    Two rockets are each 1000m long in their rest frames. Rocket A, travelling at 0.800c relative to the earth, is overtaking Rocket B, which is moving in the same direction at 0.600c.

    (i) According to the crew on B, how long does A take to completely pass? I.e. how long is it from the instant the nose of A is at the tail of B until the tail of A is at the nose of B?
    (ii) Would the crew of A measure the same time interval according to their clocks? If so, explain why. If not, what is the time interval measured by the crew of A?

    2. Relevant equations
    l = l0/γ --- Eqn 1
    v = (v' + u)/(1 + uv'/c2) --- Eqn 2

    3. The attempt at a solution
    (i) Using Eqn 2, A is moving 0.384c relative to B.
    Using Eqn 1, length contracted A from B's perspective = 0.923l0
    From B's perspective, B is stationary, and A is moving past at 0.384c.
    Time taken = (1+0.923)l0/0.384c = 1.67 * 10-5 s
    Is this correct?

    (ii) I am not sure, but I think the crew would measure the same time interval because of symmetry...??

  2. jcsd
  3. Apr 23, 2016 #2


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    Your answers to both parts appear to me to be correct (including the symmetry argument). Since you are not confident of (ii), try working it out with the same type of reasoning as (i).
  4. Apr 23, 2016 #3
    Hmm I thought it would be symmetrical because according to the crew of A, A is stationary and B is moving at -0.384c relative to A. Thus the length of B is contracted to 0.923l0. So time taken for B to pass A is (1+0.923)l0/0.384c = 1.67 * 10-5, which is the same as (i). Is this correct?
  5. Apr 23, 2016 #4


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    Yes. Exactly.
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