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The Physics Of A Subway Ride

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Posted by Aurino Souza on November 28, 2003 14:46:38 UTC

I wondered about this on the subway a few days ago:

A train is stopped at the station. Its position is well-determined and its velocity is zero. You check your wristwatch. At some point you realize the train is moving. Then you wonder: at what time did the train start moving? What was the time in your watch when the train went from "stopped" to "moving"?

This is a very interesting question (and somehow related to the "infinity" thing in a post below). The problem is, there is no point in time in which the train's velocity changes from zero to non-zero. At time t, v = 0; at time t', v > 0; there is absolutely no time between t and t'. Time is granular/quantum/not-infinitely-divisible by definition!

Three issues immediately come to my mind:

- Why do we consider time to be continuous when define it otherwise?
- How do we calculate the "minimum time" (t' - t)?
- When you think about the two issues above, you realize something amazing. But you have to see that for yourself.

Have fun

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