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Experiment doubt on physics

  1. Dec 8, 2008 #1
    Hey i have a doubt . i have not tried this experiment but i wanted to know what would happen. If we take a tube bending at the end and joining to a 5 km long pipe which is frictionless (assume) and there is no air to offer air resistance . So when i start rolling the ball will it stop or not . You might tell no but please take into consideration about the ball hitting the surface and producing a sound and hence losing energy.Even if the fiction is not hitting the object , the object should also be hitting the surface ( Every Force has equal an opposite reaction) . Please tell with reasons
  2. jcsd
  3. Dec 8, 2008 #2
    Assuming that you take a ball and throw it down the 5km long pipe (what is the bent tube at the start for?) which is frictionless, than the ball will keep rolling with no resistance. The 'ball hitting the surface and producing sound' is part of what we call friction (unless the ball is bouncing in the tube, but I assume it is just rolling).

    The ball is touching the surface and produces a force on it (it's weight). The surface pushes back with a equally strong but opposite force (which is called the normal force). But if the pipe is completely straight (no bumps etc) and horizontal, then this normal force only acts vertically and thus cannot make the ball move faster or slower horizontally.
  4. Dec 8, 2008 #3
    It can't roll if it is frictionless :)
  5. Dec 9, 2008 #4
    Yep that's something I didn't even think about lol... Oh well, just replace 'roll' with 'slide' in my post then :p
  6. Dec 9, 2008 #5
    Anyway my question is incomplete ! the bending tube is attached to a long pipe which is 5 km and i hope the rest is correct

    How is this possible ? Why wont it roll ?

  7. Dec 9, 2008 #6
    How is that posibble can u explain a liitle more about it ?
  8. Dec 9, 2008 #7
    For a ball to roll it needs friction on the surface that it's rolling on.

    Think of it like this: Imagine you slide a ball across some surface. Because the surface causes a little friction on the part of the ball touching the ground, that part is slowed down. However, the rest of the ball 'topples over' the part on the ground: the ball starts to roll.

    Now, when that surface is frictionless (think of an extremely slippery ice for example), there is nothing to slow the part of the ball touching the ground down, and the ball does not roll. It only slides over the surface.
  9. Dec 10, 2008 #8
    Thanks a lot !
  10. Dec 10, 2008 #9


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    If the ball was already rolling before it entered the frictionless tube (perhaps because of the initial gradient), it would continue to roll, since no force would be acting on it to stop it rolling.
  11. Dec 10, 2008 #10
    True. But I took the original post as a 'reference' as to how the ball would be made to move: by a bent tube. I can remember from another thread that even a ball falling down a frictionless slope would not roll and hence the ball will not roll for the entire length of the tube (assuming ofcourse that the person who dropped the ball in didn't give it a slight rotation to start it rolling...)
  12. Dec 10, 2008 #11
    It sounds like a ball in a frictionless tube would basically act like a ball floating in space. Even if the bend in the beginning got it to spin, the spin would have nothing to do with the frictionless part of the tube.

    The idea of "frictionlessness" sounds like it may not be physically possible unless many restrictions are placed on the ball.
  13. Dec 10, 2008 #12
    No, a frictionless environment is impossible to realize. Even in deep space there still is some resistance, because there is also no such thing as a perfect vacuum.
  14. Dec 15, 2008 #13
    In deep space? Does that mean that you can crash into vacuum fluctuations until you stop? That would seem to imply a cosmic reference point of motion, which I thought wasn't possible.

    Or is it more like a galactic "atmosphere", where a high pressure zone would mean one hydrogen atom per cubic inch?
  15. Dec 15, 2008 #14
    Even if there is only one particle you bump into, that is already some resistance.
    I don't know how many particles there are in deep space, but I'm sure there's more than zero...
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