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Homework Help: Time Dilation spacecraft watches

  1. Jan 29, 2006 #1
    I have the attached equation
    but when I plug it, I dont get the same answer as in the back of the book...

    Here is the question

    Two observers, A on earth, B in a spacecraft whose speed is 2.0X10^8 m/s both set their watches to the same time, when the ship is abreast the earth.

    How much time must elapse by A's reckoning before the watches differ by 1.00 seconds?

    Attached Files:

  2. jcsd
  3. Jan 29, 2006 #2
    just a quick thought

    you do know that the speed of light is normalised to 1

    hence if you are travelling at 2x10^8 ms^-1 you are traveling 2/3 times the speed of light hence your speed is 2/3 relative to light

    does this help in anyway
  4. Jan 29, 2006 #3
    i understand it is 2/3 the speed of light but am I using the right equation?
    using this equation I found that one second passes for A as 1.34 seconds passes to person B. But not sure how the book gets 3.92s as the answer
  5. Jan 29, 2006 #4
    I can't see the equation yet because it hasn't been moderated however if you are using the following equation


    then I get the same answer as you 1.34s
    Last edited: Jan 29, 2006
  6. Jan 29, 2006 #5
    yes I am using the same equation as you, but the answer in the back of the book is 3.93, not 1.34
  7. Jan 29, 2006 #6
    For that question, my answer is 2.93 for A. If the question asks B's time, then I guess it's 3.93... I don't know... xD
  8. Jan 29, 2006 #7


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    Here's the point, you are looking for how much time has passed for A when the difference between A and B is one sec. In the example you just gave, A reads 1.34 sec and B reads 1 sec, for a difference of 0.34 sec.

    Hint: you are looking for the value of T0 when T1= T0-1
  9. Jan 29, 2006 #8
    or in a different notation,

    [tex]\Delta t_A - \Delta t_B = 1 sec [/tex]

    You are asked to find [tex]\Delta t_A [/tex]
  10. Jan 29, 2006 #9
    I get it now. Thanks guys I appreciate the help. This semster of modern physics is going to be tough.
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