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Equal Forces - Bullet and train (opposite directions)

  1. Aug 30, 2007 #1
    Hi ive been having an argument with a few friends about a theory of mine (based on what i can remember from my physics lessons and common sense (i think lol).

    My theory is this.

    There is a train travelling at 100mph in one direction

    There is a man with a gun standing at the back of the train pointing the gun exactly horizontal in the opposite direction to the trains direction of travel (ie pointing out the back of the train). the gun that he has fires bullets out at exactly 100mph

    The man manages to fire his gun at the exact point at which the train passes through a station and passes a by-stander standing on the platform.

    My question is - what does the by-stander see happen to the bullet? (ignoring wind resistance, and any spinning of the bullet caused)

    My answer is, the by-stander would just see the bullet fall to the floor.

    Can anyone confirm that this is true?


  2. jcsd
  3. Aug 30, 2007 #2


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    You are correct.
  4. Aug 30, 2007 #3
    :-) any idea what i can say to my 'doubters' that will proove im right and that they cant argue with?

  5. Aug 30, 2007 #4


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    Do you know anything about frames of reference ? How about adding vectors ?
  6. Aug 30, 2007 #5


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    Tell them the burden of proof is on them. Tell them they must demonstrate why the bullet would do anything other than drop to the ground.

    I suggest, BTW, that, rather than a gun, you switch the scenario to a baseball pitcher, who can throw at 100mph. They will likely have a much easier time grasping the results intuitively. In fact, it may well lead them to perform a scaled down experiment. You'll be a lot closer to winning them over.
  7. Aug 30, 2007 #6


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    There's was a previous thread about this, and there was a link to a video that was 90% introduction and 10% actual example of the experiment, but basically a ball was shot off the back of an open pickup truck and the ball basically just drops down.

    If someone here can remember the thread and/or the link to that video.
  8. Sep 28, 2007 #7
    There is some thing I don't understand here. If I were to throw a ball to someone at 20m/hr inside a train moving at 20m/hr (the direction of throw is opposite to the direction of the train) you mean to say the ball would not move?

    From experience when inside an airplane if I were to toss something to a passanger at the back, it difnetly does reach them!!
  9. Sep 28, 2007 #8

    Doc Al

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    It will not move relative to the ground, so a bystander on the ground will just see the ball fall down. Of course it moves relative to the train!

    Motion is always relative to some frame.
  10. Sep 28, 2007 #9
    But if the person I throw the ball to were to catch the ball what would the by stander see? Would he see the ball fall or the person catch the ball?
  11. Sep 28, 2007 #10

    Doc Al

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    He'd see both! From the bystander's viewpoint, one person (the thrower) moved away from the ball while another person (the catcher) moved towards the ball. From his viewpoint, the ball itself didn't move (except to fall down a bit).
  12. Sep 28, 2007 #11
    There could only be one outcome and that is the other person catches the ball, but to the stationary observer it is the catcher who (along with the train) moves towards the ball.
  13. Sep 28, 2007 #12
    So if I'm directly behind the train on the ground, and someone fires a gun from the back of the traveling train at the same speed, I won't get hit by the bullet...

  14. Sep 28, 2007 #13
    Yes. Thats the general idea.

    However, the scenario that you just mentioned implies the bullet is moving just as fast as the train (specify these things or ur gonna get shot) :biggrin:.
  15. Sep 29, 2007 #14
    I get the picture.

    However I have another doubt. Will the energy required to throw the ball inside the train at 20m/hr (since it is moving along with the train) be the same as the energy required if the train were stationary?
  16. Sep 29, 2007 #15


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    Yes, and consider the fact that if this were at the equator, then the earth is rotating at 1040mph, and that the earth is orbiting the sun at 65000+mph, and the sun is orbiting in the galaxy at 486000 mph.
  17. Aug 18, 2009 #16
    Found a link to the aforementioned video.

    http://www.spikedhumor.com/articles/109706/Physics-Experiment-With-Baseball.html" [Broken]
    Last edited by a moderator: May 4, 2017
  18. Aug 18, 2009 #17


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    Oh, very cool! All in Japanese, but still very clear indeed.

    I especially like the building drama and excitement as the little truck speeds along the race track at the apparently enormously exciting speed of 100 km/hr. The thrill and danger is emphasized with the the driver wearing a whopping big crash helmet...

    ... but with his mate sitting bare-headed on the tray outside. :surprised
    Last edited by a moderator: May 4, 2017
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