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Jumping inside a moving train

  1. Nov 11, 2006 #1
    Why when you jump inside a moving train you land exactly in the same place, instead of landing closer to the tail of the train?

    I know it a stupid question but Im not able to find what physical law applies in this case.

    Has it something to do with momentum cancellation? (I remember the question about firing a bullet backwards at 1km/sec on a train going forward at the same speed)

    Links are welcome, thank you.
     
  2. jcsd
  3. Nov 11, 2006 #2

    Doc Al

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    How about Newton's first law?
     
  4. Nov 11, 2006 #3
    you move with the train, then you jump, your body still moves with the train regardless of the disconnection between you and the train, and you will land on the exact same point on the train since you and the train did the same distance while you've been disconnected...

    all that is in the reference to the ground the train is moving on.

    usually, i would explain it otherwise, but those new to this concept always refer motion to the ground...
     
  5. Nov 11, 2006 #4
    How come when the train stops suddenly you go flying forward?
     
  6. Nov 11, 2006 #5

    Doc Al

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    Again: Newton's 1st law. (The train stops, but you keep moving.)
     
  7. Nov 11, 2006 #6
    But if the train is accelerating, when I am in the air after jumping, will I land in the same place?
     
  8. Nov 11, 2006 #7

    Hootenanny

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    What do you think? :wink:
     
  9. Nov 11, 2006 #8
    I think I will land somewhere backwards, there's no force acting on me so the train during that time is traveling faster than me. But I could be wrong, that is why I asked. :wink:
     
  10. Nov 11, 2006 #9

    Hootenanny

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    Indeed you are correct. If the train is accelerating, the frictional force between your shoes are the floor is providing the force required to accelerate you at the same rate as the train. As soon as you leave the floor this force no longer exists, therefore the train is accelerating and you are not (from the reference frame of the tracks of course). If we assume that the train is accelerating uniformly at a rate of a m/s2; we can say that if you are in the air for t seconds, that you will land s meters from your initial position (on the train) such that;

    [tex]s =-\frac{1}{2}at^2[/tex]
     
  11. Nov 11, 2006 #10
    I am trying to prove that a bug can come into your window while you are driving
    at 60 mph and hit you. Is there a physical law to explain this as a
    fact?
     
  12. Nov 11, 2006 #11

    DaveC426913

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    Pretty straightforward.
    The bug had a velocity relative to the ground of between 0 and 5mph.
    You had a velocity of 60 mph.

    mmmPOW!


    Seriously, the folly here is to use the jumping passenger on a train as an analogy. They're not the same.

    That the bug was stationary wrt ground, not wrt your car. As it enters your car window, it does not instantly accelerate to the speed of the car. Thus, when it hits you in the face, it is with a relative v of ~60mph.
     
  13. Nov 11, 2006 #12
    Thank you, and I did understand that they were two different scenarios...just thought that the subject was similar enough to find an answer among the participants. More specifically though, would there be an angle of entrance imperative as the difference in speed was so drastically different? Would the draft created by the vehicle come into play? Playing devil's advocate, would it not more likely pass by the vehicle rather than enter it?
     
  14. Nov 12, 2006 #13
    i would imagine it would be hard for the bug to enter the car since the air inside would have a higher velocity wrt the air outside and effectively be like a wall, similar to if u use a leaf blower to blow a tennis ball in the air, it stays relitively in the same place and doesnt fall out of the airflow to the side. however wrt the angle of enterance i would say it almost certainly sould have to be normal to the car since, 60mph~27.78m/s and therefore if the window is 45cm long it would have ~ 0.016s to fly in the window assuming the velocity of the bug is << that of the car and therefore if it flew alongside te car it wouldnt increase the tme by much
     
  15. Nov 12, 2006 #14

    DaveC426913

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    So, an open window at 60mph DOESN"T cause a roaring wind inside the car? Window up, window down makes NEGLIGIBLE difference to the air flow inside the car?


    Where would you say all that air is coming from? If one of those bits of air happened to have a bug in it, what would you feel then?
     
    Last edited: Nov 12, 2006
  16. Nov 16, 2006 #15
    If you was on top of the train with very little wind would the same laws apply as being inside the train?
     
  17. Nov 17, 2006 #16
    Hi,

    you didn't mention which way the train does move. As for my experience you will not land on the same place when the train moves, but closer to the tail as the train moves forward. o:)

    Steve
     
  18. Nov 17, 2006 #17

    Danger

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    That 'experience', however, would involve air resistance if the train was moving fast enough for relative motion to be noticeable (unless there was a tail-wind that matched the train's speed).
     
  19. Nov 17, 2006 #18

    Hootenanny

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    And if you were inside the carriage it would depend (as Doc Al said) on if the train were accelerating, or travelling up an incline.
     
  20. Nov 18, 2006 #19
    Hello!

    I'm sorry Gentlemen, I find your attitude a bit nit-picking. As I went to bed yesterday
    I kept thinking about the thread and as it was put to the forum by the thread starter.

    Sure one has to scrutinize the according situations carefully. But after all, if that train
    consists from flat panel wagons, you could jump on easily, then you would be landing
    on a position more towards the back of the train depending on the trains speed while
    moving forward. In my mind this was without no doubt. Let us assume all the other
    conditions are approaching the same each time.

    It is by the way a very complex situation and therefore hard to measure at all. But if
    the wagon would pass a specific (same ) point where a signal would be set off
    making you jump, the landing position will be further to the trains tail the faster the
    train moves. :smile:

    Steve
     
    Last edited: Nov 18, 2006
  21. Nov 18, 2006 #20

    Danger

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    Hello, Steve;
    Welcome to PF. With no offense intended, your post makes absolutely no sense. Could you possibly express it in other terms that we might understand?
     
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