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Speed of light spacecraft

  1. May 18, 2009 #1
    If you had a spaceship travleing at half the speed of light then shot a bulit at half the speed of light again, would the bulit appear to go the speed of light relative to an outside obverver?
    (All preposed in theory, of cource.)
     
  2. jcsd
  3. May 18, 2009 #2
    If not, why?
     
  4. May 18, 2009 #3

    Doc Al

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    No. To find the speed of the bullet with respect to that outside observer, you must use the relativistic addition of velocity formula:

    [tex]V_{a/c} = \frac{V_{a/b} + V_{b/c}}{1 + (V_{a/b} V_{b/c})/c^2}[/tex]

    Which gives: (0.5c + 0.5c)/(1 + (.5)^2) = 0.8 c
     
    Last edited: May 18, 2009
  5. May 18, 2009 #4

    HallsofIvy

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    The "outside observor" being assumed to be one relative to which the spaceship is moving at (1/2)c, of course. There can exist "outside observors" with other speeds relative to the spaceship!
     
  6. May 18, 2009 #5

    Doc Al

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    Good point. :wink:
     
  7. May 18, 2009 #6

    NWH

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    So this is like the idea that objects within a black hole can appear to be exceeding c, even though they aren't?
     
  8. May 18, 2009 #7
    Hi there,

    But no matter what, the bullet would never have c for any observer, in any relative frame.

    Cheers
     
  9. May 18, 2009 #8

    HallsofIvy

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    Since we cannot observe an object within a black hole, I have no idea what "appear" can mean here.
     
  10. May 18, 2009 #9

    NWH

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    Hypothetically speaking, of course...
     
  11. May 18, 2009 #10
    ccording to Einstein's special theory of relativity, objects gain mass as they accelerate to greater and greater speeds. Now, to get an object to move faster, you need to give it some sort of push. An object that has more mass needs a bigger push than an object with less mass. If an object reached the speed of light, it would have an infinite amount of mass and need an infinite amount of push, or acceleration, to keep it moving. No rocket engine, no matter how powerful, could do tthis
     
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