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Annoying Physics Question, Plz Help

  1. Jun 7, 2006 #1
    Annoying Physics Question, Plz Help!!!

    Hey guys,

    I am having trouble understanding why when sitting in a car, and a tennis ball is thrown up that you do not go in front of it and why the tennis ball returns back perfectly to your hand without you having to adjust to the fact that you have moved forward and not the ball.

    Now i understand that general relativity states that the laws of motion are neglible and that the laws of physics are the same regardless, but i do not understand why are.

    You are moving forward yet the ball is going up, the ball should land behind you!


    This question is bugging me BIG TIME!

    ANY HELP would be appreciated.

  2. jcsd
  3. Jun 7, 2006 #2
    You don't need relativity to understand that. The main point, I think, is your misconception that the ball is just going up. From your point of reference, yes the ball is just going up, but then you are not moving either - you are always stationary from your reference frame. From an outsider's point of view, the ball is not just moving up; it always has a horizontal velocity component same as the train's (and yours) even before it is thrown.

    Hope that helps.

    Wai Wong
  4. Jun 7, 2006 #3
    The vertical and horizontal components of moving objects are independent of each other. This is why when someone jumps on a moving skateboard they will land in the same spot on the skateboard. In this case, the ball (and you) are moving at the same horizontal velocity as the car. When you throw the tennis ball up while travelling in the moving car, both you and the ball have the same inital velocity (as well as horizontal velocity), only the ball has that added vertical component.

    Similarly, firing a rifle bullet in a horizontal direction at some height h will strike the ground at the same time as dropping it from that height h. Or on a smaller scale, throwing a ball horizontally and dropping it from that same distance will mean that they both strike the ground at the same time.

    In terms of equations of parabolic motion, Vx = Voxt (the x direction has no horizontal direction), and Vy = Voyt - gt (the y direction does have acceleration, in this case gravity). Of course the other kinematic equations for 2D motion can be found by a quick google search.
    Last edited: Jun 7, 2006
  5. Jun 7, 2006 #4


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    Staff: Mentor

    You are not just throwing the ball up, you are throwing the ball up and forward. (with respect to the ground)
  6. Jun 7, 2006 #5
    THANKS for help guys.


  7. Jun 7, 2006 #6


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    Thanks!!! Although I had nothing to do with it. I just wanted to add one perspective. If you were on a flatbed as opposed to being in a boxcar, wind resistance would definitely make the ball land behind you. For any real-world scenario, you have to factor in everything.
  8. Jun 14, 2006 #7


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    In an accelerating car, the ball would not return to your hand, you would move forward in relation to the ball. In a car moving at a constant speed: the car, the ball, and your hand are all moving at the same speed...
  9. Nov 14, 2010 #8
    Re: Annoying Physics Question, Plz Help!!!

    from 2006 no less...Motia

    Similarly, firing a rifle bullet in a horizontal direction at some height h will strike the ground at the same time as dropping it from that height h.

    Now here is a myth that has been around for a while - and I have no idea why. The only reason that I can think of is that too many people have just taken it for granted and not really thought about it or taken a pencil to paper and see the fallacy in the idea. There are no tricks or sneaky little 'other things' - just plain down to earth physics. "...down to earth", should be enough of a hint.
  10. Nov 14, 2010 #9

    Vanadium 50

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    Re: Annoying Physics Question, Plz Help!!!

    A good reason to close it.
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