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Math Help - special relativity - relative velocity

  1. #1
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    special relativity - relative velocity

    A magnetic rail gun is used to shoot a packet of metal from the new asteroid mining facility towards a capture facility on the moon. The package is shot at a velocity of 0.82c. How fast does the package appear to be moving to a nearby spacecraft that is travelling in the same direction at a speed of 0.67c?


    since they are going in the same direction? is just simply a vector addition
    0.82 c + 0.67c?
    Last edited by mr fantastic; November 17th 2009 at 03:02 PM. Reason: Changed post title
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    Quote Originally Posted by jddery View Post
    A magnetic rail gun is used to shoot a packet of metal from the new asteroid mining facility towards a capture facility on the moon. The package is shot at a velocity of 0.82c. How fast does the package appear to be moving to a nearby spacecraft that is travelling in the same direction at a speed of 0.67c?


    since they are going in the same direction? is just simply a vector addition
    0.82 c + 0.67c?
    Except that in relativity, the sum formula is not "u+ v". It is \frac{u+ v}{1+ \frac{uv}{c^2}}.

    Notice that "0.82 c+ 0.67c" would give a speed of 1.49 c which is impossible.
    Last edited by mr fantastic; November 17th 2009 at 03:02 PM. Reason: Fixed latex
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    Quote Originally Posted by jddery View Post
    A magnetic rail gun is used to shoot a packet of metal from the new asteroid mining facility towards a capture facility on the moon. The package is shot at a velocity of 0.82c. How fast does the package appear to be moving to a nearby spacecraft that is travelling in the same direction at a speed of 0.67c?


    since they are going in the same direction? is just simply a vector addition
    0.82 c + 0.67c?
    Surely your notes tell you how to add velocity in Special Relativity. I'm certain you would have been told that a relative velocity greater than c is not possible. Are you attending classes and paying attention?
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