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What is V of the small rocket as observed from inside it?

  1. Dec 7, 2016 #1
    1. The problem statement, all variables and given/known data

    If the total relativistic velocity of small rocket is 0.75c after being launched from a larger rocket traveling at 0.60c, what is the velocity of the smaller rocket as observed from inside it?

    Can someone put this in layman's for me please. I solved many relativistic velocity question with no problem then i get thrown stuff like this.

    2. Relevant equations

    V= (V1+V2) / {1+(V1V2)/ c^2}


    3. The attempt at a solution

    .75c -.60c = .15c
     
  2. jcsd
  3. Dec 7, 2016 #2

    phinds

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    Zero. Always zero, regardless of how it looks to anyone not at rest in the frame of reference of that rocket.

    If you are asking what is the V of the small rocket as observed by the larger rocket, then it's .6c

    If you are asking what is the V of the small rocket from the FOR in which the larger rocket is moving at .75c, THEN you need to bring in the equation for relativistic addition and the answer will be some 90%+ of c

    EDIT: As orodruin points out below, I reversed the two Vs (the .6 and the .75)
     
    Last edited: Dec 8, 2016
  4. Dec 7, 2016 #3
    Thanks can you link me to information on this Zero answer?
     
  5. Dec 7, 2016 #4

    phinds

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    Google "Special Relativity"
     
  6. Dec 7, 2016 #5
    Thanks but i think i will google "Physics"
     
  7. Dec 7, 2016 #6

    phinds

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    Don't know if you are being sarcastic or just silly.
     
  8. Dec 7, 2016 #7
    VTOTAL= V1 + V2/ (1 + V1 x V2/c2)

    0.75 = 0.60 + V / (1 + 0.60V)

    0.75 + 0.45V = 0.60 + V

    0.15 = 0.55 V

    V = 0.27c

    The correct answer is: 0.27
     
  9. Dec 8, 2016 #8

    Orodruin

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    Actually, "physics" is a better search term. It would be true in classical mechanics as well.
     
  10. Dec 8, 2016 #9

    Orodruin

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    No it isn't. The large rocket is moving at 0.6c and the small at 0.75c in the initial frame.
     
  11. Dec 8, 2016 #10

    Orodruin

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    Depending on the frame where you want to compute the velocity of the small rocket, you will get different results (naturally). You need to specify this much clearer.

    If it is the speed in the rest frame of the small rocket you want, it is always zero by definition as phinds has pointed out.

    If it is the speed in the rest frame of the large rocket you can do what you did, but this does not seem to be what the question is asking.
     
  12. Dec 8, 2016 #11
    The question should be alot clearer but this was not of my exact making. Its lack of clearity is why i posted it here. I always seem to get questions like these on my tests to make sure i cannot recieve 100%. Non of my lessons have practice problems as such. I believe you should be trained for any question and if you get it wrong you should be sole to blame. I recieved the solution. Thanks for the help
     
  13. Dec 8, 2016 #12

    phinds

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    ARRRGGGHHH. I need to pay closer attention. Obviously you're right, I got them backwards. Thanks for the correction.
     
  14. Dec 8, 2016 #13

    phinds

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    Well, yeah, but "physics" includes just a whole TON of stuff that has nothing to do with his question. He could spend months studying electricity, optics, thermodynamics, and on and on, and what would that have to do with his question?

    Pursuant to your point however, it would have been better if I had pointed him to Galilean relativity rather than Special Relativity as a starting point.
     
    Last edited: Dec 8, 2016
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