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Relative motion - disagreement at very low speeds

  1. Oct 8, 2011 #1
    I understand the concepts behind SR and Lorentz transformations and was explaining the basics to my son by illustrating the classic laser-beam-in-a-train-car explanation. But then I started thinking... If I was in a boxcar w/ a clear wall and was throwing a ball straight up, measuring the time from toss to catch, and then computing the average speed... and an observer was standing outside and performed the same measurement s/he would see the ball take a much longer route than I would (assuming the train was moving at, say, 100 kph). That means we would have a significant disagreement about the average speed we compute for the ball.

    Am I correct on this? It seems bizarre that two observers would see such different path lengths at everyday (non-relativistic) speeds. ?
  2. jcsd
  3. Oct 8, 2011 #2


    Staff: Mentor

    The ball is not light; more precisely, it is not a zero rest mass object, so it doesn't move at the speed of light. For objects moving at less than the speed of light, their speed does vary with the state of motion of the observer.
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