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Catching a Train

  1. Oct 30, 2009 #1

    First, a very basic physics question - if you could just step onto a speeding train (pretending that the force of such an act wouldn't break every bone in your body), are you accelerating to the speed of the train, or did your speed just suddenly increase?

    The underlying question - If you have a nearby object going the speed of light, is it possible, from the physics perspective, to hitch a ride?

    The facets to that are that the object either has to have no mass, which would get totally screwed up by having a hitchhiker for baggage, or the object already has infinite mass ( I don't really know what the implications are for being in the same universe as an object with infinite mass, so let me know also if that's as suicidal as the stepping on a train scenario) in which case jumping on won't be affecting the mass at all.
  2. jcsd
  3. Oct 30, 2009 #2
    Hi there,

    I don't know if I understand your first question very well, but to answer your underlying question, with the train idea.

    When you step on the speeding train, two things happen. Firstly, your speed will suddenly increase. But the train will slow down a tiny bit.

    Extrapolating to hitching a ride on a object at the speed of light. To understand this, only from a classical point of view, you have to look at the two body system.

  4. Oct 30, 2009 #3
    On your first Q, it depends on what you are measuring, 'you' might suddenly be at the speed of the train, but your centre of gravity wouldn't due to deformation. In a perfect world with no deformation, similar to Archimedes balls (I think that is it's name, that little clicky pendulum desk toy thing) then it would be just as the other poster said with some speed gain and drop from both you and the train, depending on relative masses, but I see no reason why there would be any delay with the speed changes.
  5. Oct 30, 2009 #4


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    To change from being stationary to moving at the train's speed, you will need an impulse. The impulse (Force times Time) is actually equal the change in your momentum. There will always be a finite time (as your body is squashed against the front of the train) - the shorter the time, the greater the force. That's why a seat belt makes stopping much more comfortable than hitting the car windscreen; the time for your change of speed is extended and so the force on you is reduced. Let's hope someone strapped a pile of mattresses to the front of your train!
    btw, Archimedes certainly had balls but wasn't it Newton who had the cradle?
  6. Oct 31, 2009 #5


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    Too right.
  7. Oct 31, 2009 #6
    What you say reminded me of the old cartoon. One calls phone in the phone box falling from the cliff top. Just when the phone box reached the ground, he finished phone and came out of the door. The phone box crashed and he is safe. In your case I think a passenger is smashed by a train wall.
  8. Oct 31, 2009 #7
    Oops! :-)
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