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(adsbygoogle = window.adsbygoogle || []).push({}); significantdisagreement 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. ?

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# Relative motion - disagreement at very low speeds

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