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Will see the same speed for the ball always?

  1. Sep 17, 2013 #1
    We have two people. A person(A) that is stadning and another(B) one who drives his car,so he has some velocitty. Lets say that the person B throw a photon from his car. Lets suppose that photon will hit down and will go up again. So both guys see the same speed.
    But If instead of light,he throw a ball then both will see the same speed for the ball always?
  2. jcsd
  3. Sep 18, 2013 #2


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    No, there is a unique invariant speed, which we call the speed of light. According to relativity, there is one formula that covers all cases such as you describe:

    Suppose B is moving at v relative to A. Suppose A throws 'something' at velocity u as measured by A. Then B sees the 'something' moving at:

    (u-v)/ (1 - uv/c^2)

    If you put u=c, you get c also for what B sees. For any other value of u, you get a different speed as observed by B. for u much less than c, this formula becomes empirically indistinguishable from u-v. But it is one formula for all cases.
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