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Old riddle.

  1. Jan 30, 2006 #1
    OK this kind of has to do with physics, but more of what happnes when you squash something i suppose. Anyways my dad told me this riddle and when i gave up he said he forgot the answer and it has been bugging me for a while.

    One thing traveling on a straight path must come to a stop before traveling in the opposite direction.

    If a man is running on a train track at lwt's say 1mph, and a train is coming at him at 100mph, when they collide, does he stop the train?

    I am guessing that the air around him would kind of slow him down to zero as it gets pushed out to the sides, but what if it was in a vaccum (if he could survive)?
  2. jcsd
  3. Jan 30, 2006 #2
    It's a little meaningless to ask then if 'he' stops moving, since 'he' is not really defined. In reality, he (and the train) deform during the course of the collision.
  4. Jan 30, 2006 #3

    I have no idea what you just said lol
  5. Jan 31, 2006 #4


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    Of course he doesn't stop the train; the train would hardly notice!
    He will be stopped of course, before being splattered in the other direction all over the place. But in order to define strictly what it means "he will be stopped", it is best to say that THE CENTER OF GRAVITY OF ALL HIS BODY MATTER, will be, at a certain moment, with velocity 0 wrt the train track. This might be because his foot is still moving _towards_ the train, while his brains have already been moving _with_ the train, so that this averages out, or another disgusting scenario. Take your pick. At a certain moment in time, the velocity of his center of gravity will be zero.
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