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Homework Help: A rocket ship leaves earth at a speed 0.6c. When a clock...

  1. Feb 11, 2016 #1
    1. The problem statement, all variables and given/known data
    A rocket ship leaves earth at 0.6c. The clock of the rocket says 1hr has elapsed.
    (a.) According to the earth clock, when was the signal SENT?
    (b.) According to the earth clock, how long after the rocket left did the signal arrive BACK on earth?
    (c.) According to the rocket clock how long after it left did the signal arrive back on earth?

    2. Relevant equations
    Lorentz factor: gamma = 1/sqrt(1-(v/c)^2)
    Velocity = distance / time

    3. The attempt at a solution
    I calculated the gamma factor using 0.6c for the velocity and got 1.25. T=T(0)(gamma) = 1hr (1.25) = 1.25 hr. I don't know if this calculation is for part (a.) or (b.).
    I'm not sure where to go from here to find the solution for part (c.)
  2. jcsd
  3. Feb 12, 2016 #2

    Buzz Bloom

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    Gold Member

    Sorry about duplicate messsage
    Last edited: Feb 12, 2016
  4. Feb 12, 2016 #3

    Buzz Bloom

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    Gold Member

    Hi cosmos:

    As I read the problem statement, there are some ambiguities.
    1. References are made to a "signal", but the statement does not specify when the signal is sent I am guessing that the signal is sent from the ship when the ship's clock reads one hour has elapsed since take off.
    2. Re (a), The only information that Earth has about when the signal was sent is when it was received. Is the question what would someone om Earth calculate as the time on the ship's clock when the signal was sent? If the Earth person understands SR, then presumable they can calculate correctly that the ships clock said the the signal was sent one hour after takeoff, and they can make the adjustment for SR to get the corresponding reading of the Earth clock. I am guessing that this is desired answer.
    2. Re (c), a similar guess to (2) is needed about what the ship person has to calculate.

    I suggest that it would be useful to first establish clear statements about all these assumptions. It should be straight forward to develop an equation that shows a relationship between a time on the Earth clock and the time that a corresponding time an Earth person would calculate for the ship's clock, and vice versa. Because of symmetry, how would you conclude these two equations would relate to each other?

    Hope this helps.

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