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Homework Help: Relativity - time in different reference frames

  1. May 10, 2014 #1
    1. The problem statement, all variables and given/known data

    An astronaut is traveling in a space vehicle moving at
    0.500c relative to the Earth. The astronaut measures her
    pulse rate at 75.0 beats per minute. Signals generated by
    the astronaut’s pulse are radioed to the Earth when the
    vehicle is moving in a direction perpendicular to the line
    that connects the vehicle with an observer on the Earth.
    (a) What pulse rate does the Earth-based observer measure?

    2. Relevant equations

    3. The attempt at a solution
    [itex]t_{astronaut}=\gamma t_{earth}[/itex]

    [itex]\gamma = 1.154701[/itex]

    Then [itex]\Delta t_{earth} = 51.962 s[/itex]

    [itex]\frac{75beats}{51.962s} = 1.443beats/s \frac{60s}{min} = 86.66 bpm[/itex]

    should be 64.9 bpm.

  2. jcsd
  3. May 10, 2014 #2
  4. May 10, 2014 #3
    But they are saying beats PER minute not minutes per beat. It's totally counter intuitive the way that they are doing it. What a joke question.
  5. May 10, 2014 #4
    The beats are like a clock, with a period 0.8 s. This is the proper time on the spaceship. An observer outside the ship notices, due to time dilation, that the period is longer. This translates into less beats per minute.
  6. May 10, 2014 #5
    I thought the proper time was always the stationary one?
  7. May 10, 2014 #6
    The idea of "stationary" doesn't work well with relativity. It could be easily argued that the ship is stationary and the Earth is moving with respect to it. Therefore, the definition of proper time is that the clock in question has not moved in a particular reference frame. For the astronaut, her "clock" is her pulse. Her pulse travels with her and is therefore "stationary" with respect to the ship. This is proper time. For the observers on Earth, her "clock" is moving. It was to the left in one instance and now it is moved to the right a little. This, is measured time.
  8. May 10, 2014 #7
    Gah this is so confusing. Everything was great until Einstein came along and ruined it.

    Alas, my struggle continues. Thank you for the help Sir!
  9. May 10, 2014 #8
    You're welcome.
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