Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed ?

  1. Jan 12, 2010 #1
    ok I know that nothing with mass can travel at c but i have to ask this question.
    if I am in a rocket ship traveling at .5c and another rocket ship is traveling at .5c in the opposite direction then am I traveling at c relative to him . Probably Galilean transformations don’t work in this situation and I am probably missing something can you guys help me out.
  2. jcsd
  3. Jan 12, 2010 #2
    Now I don't have its answer but I will tell you as I get an adequate amount of time. o:)
  4. Jan 12, 2010 #3
    Well, I was reading in another thread that nothing actually has an absolute mass. It depends on which force you're considering (that is applied on that mass). for example, a nuetron's gravitational mass and nuclear mass may have slightly different values.

    I think the same thing goes here. The guy in the other rocket ship will percieve you as a massless entity, but, somebody who is stationary wrt both of you, will not.

    Just a fleeting thought...... I haven't verified this from anywhere.
  5. Jan 12, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    You are correct that Galilean transformations don't work with such high speeds. You need to use relativistic velocity addition (derived from the Lorentz transformations):

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

    The speed of the other rocket with respect to you will be 0.8c, not c.
  6. Jan 12, 2010 #5
    thanks for the answer .
  7. Jan 13, 2010 #6
    Would a stationary observer see them coming together at a rate of c?
  8. Jan 13, 2010 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes. This is called the "closing velocity".
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook