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Homework Help: Relativity - Mass/Energy/Speed

  1. Oct 15, 2011 #1
    1. The problem statement, all variables and given/known data

    A spaceship of rest mass 7.5 x 10^3 Kg has a kinetic energy that is 0.12 times it's rest energy.

    a) Calculate it's total energy
    b) Calculate it's speed


    2. Relevant equations



    3. The attempt at a solution
    a)
    E = E0 + K
    E = mc^2 + K
    E = 7.5 x 10^3 x (3 x10^8)^2 + 0.12(7.5 x 10^3)
    E = 6.75 x 10^20 J


    b)
    E = γmc^2
    substituting above values in and solving for v, I get v = 0
     
  2. jcsd
  3. Oct 15, 2011 #2

    ehild

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    Correct the red part. It is the rest mass now, not the rest energy.


    ehild
     
  4. Oct 15, 2011 #3
    So should it be...

    E = 7.5 x 10^3 x (3 x10^8)^2 + 0.12(7.5 x 10^3 x (3 x10^8)^2)
    E = 7.56 x 10^20 J
     
  5. Oct 15, 2011 #4

    ehild

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    Yes.

    ehild
     
  6. Oct 15, 2011 #5
    Thanks very much ehild.:smile:
     
  7. Oct 15, 2011 #6

    ehild

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    You can save calculations if you not multiply with c^2 at the beginning:

    E= γmc^2 = mc^2+0.12mc^2

    γmc^2=mc^2(1.12)

    y=1.12


    ehild
     
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