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Question about objects and motion

  1. Feb 20, 2009 #1

    cvw

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    Hi guys,

    Please could you help with with a puzzling question for a non physics person. I am sure it has a simple answer.


    Today at the park, my kids and I discovered a strange physical phenomenon while playing with a tennis ball and a large stainless steel tube (the kind you crawl through).

    Whizzing the ball around the inside of the tube at 90 degrees to the tube long axis, the ball goes round and round the inside circumference until it runs out of steam. This continues at 90 degrees, staying on the orginal plane. Good.

    Firing the ball into the tube, while giving it some rotation too I expected it to spiral all the way to the other end. Along plus Round equals Corkscrew?

    No.

    If you fire the ball in at an angle, not only does it travel around the circumference but it tries to return to the thrower at a similar angle. This means that after a few rotations, the ball ends up travelling around the inside of the tube at 90 degrees to the long axis again.

    It seems impossible to get the ball to corkscrew to the other end AND get a number of rotations around the tube - the ball always returns to the 90 angle of travel.

    This behaviour must have a name. What can I look for to find out more?

    Cheers


    Rob

    and two small children
     
    Last edited: Feb 20, 2009
  2. jcsd
  3. Feb 20, 2009 #2
    That was quite difficult to understand, but I think I got there in the end. Basically, whenever you try to get the ball to corkscrew through the tube to come out the other end, it ends up just spiralling inside the tube but moving nowhere?

    Edit: If that is the case, my initial guess would be that it's to do with the spin of the ball. It's really hard to explain, but if you think about how the ball is rolling as it starts its ascent up the side of the tube, its spin will be such that it acts to stop the ball from moving along the tube. Try to follow the direction of the spin as it moves through the tube and you might catch my drift. Basically, by the time the ball is at the top of the tube it is spinning in the wrong direction to the direction of travel acting to brake the balls movement. I've never considered this before. It's really quite interesting...
     
    Last edited: Feb 20, 2009
  4. Feb 20, 2009 #3

    A.T.

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    Gold Member

    It is a consequence of http://en.wikipedia.org/wiki/Angular_momentum" [Broken] conservation. If the ball rolls within a plane, it's spin axis is constant. But rolling in a spiral would require the spin axis to change, and the ball resist this change. Just like a spinning top refuses to fall over. If the ball was sliding, not rolling, it would spiral trough.
     
    Last edited by a moderator: May 4, 2017
  5. Feb 21, 2009 #4

    cvw

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    Thanks for the replies. Your understanding of the setup was correct and the ball definitely rolls around, rather than skidding.

    Although the ball didn't do what I expected, it make some kind of sense. The ball sent in at an angle, the round tube still caused the "round and round" motion but as an elongated elipse rather than a circle. As the elipse becomes less elongated each turn, the rotation axis swings back toward the 90 degree mark.

    When we are next at the play park, we will try the same thing with a sliding object like a coin. This surely should corkscrew.

    Thanks very much once again.

    Rob and the kids
     
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