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Does the ball rotate about its axis?

  1. Jul 13, 2010 #1
    Does the ball rotate about its axis?!?!?!

    Hi Newbie here,

    I have a disagreement with an associate.

    I say that a ball on a string moving in a horizontal circular path rotates about an axis perpendicular to the said circle.

    He insists that it doesn't as it would wind up the string if it did that.

    Which of us is right, and can anyone explain why?

    Many thanks.
     
  2. jcsd
  3. Jul 13, 2010 #2

    jtbell

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    Re: Does the ball rotate about its axis?!?!?!

    Consider a similar example: the moon orbiting around the earth. It always keeps the same face turned towards the earth, the same as a ball going around your head on a string.

    At any point on the moon's surface except at the poles, the sun rises and sets once every (approximately) earth month, because the moon is rotating around its axis.
     
  4. Jul 13, 2010 #3
    Re: Does the ball rotate about its axis?!?!?!

    My associate and I agree with the moon rotating about its axis as it orbits the earth, but I cannot get him to accept that the ball on a string rotates about its axis as it moves around the circle.

    Can you provide an explanation that even a complete fool could understand, as I have tried all sorts of ways of explaining it to him without success.
     
  5. Jul 13, 2010 #4

    Doc Al

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    Re: Does the ball rotate about its axis?!?!?!

    Draw yourself a diagram of the ball + string at various points along its path as viewed from above. Label various points on the ball's surface for reference (for example, label four arrows N, S, E, and W). The side of the ball where the string attaches always points toward the center. As the ball completes half its revolution about the center, what happens to the orientation of the ball? Do the arrows on the ball still point in their original directions?
     
  6. Jul 13, 2010 #5
    Re: Does the ball rotate about its axis?!?!?!

    Hi Doc,

    I tried that one already, and it doesn't convince him.

    I am at a complete loss as to how to open his eyes to the truth of the situation.
     
  7. Jul 14, 2010 #6
    Re: Does the ball rotate about its axis?!?!?!

    A version of jtbell's example should work. Tie a string to a peg and the other end to a ball. paint the half of the ball that always faces the peg white and the other side black. Rotate the ball around so the white side always faces the peg. Have him stand at a distance and watch the side of the ball facing him change back and forth between white and black.

    Many people will not make admissions like this once their ego has been challenged.
     
  8. Jul 14, 2010 #7
    Re: Does the ball rotate about its axis?!?!?!

    I tried this too, I am at my wit's end trying to get this person to understand.
     
  9. Jul 14, 2010 #8
    Re: Does the ball rotate about its axis?!?!?!

    Sounds to me like his ego has been challenged past his capacity for admission.
     
  10. Jul 14, 2010 #9

    Doc Al

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    Re: Does the ball rotate about its axis?!?!?!

    Have your associate draw his own version of the diagram. If the ball does not rotate, have him explain how the string doesn't wrap around the ball. Put the burden on him.

    (Don't discount the possibility that he realizes his error and is too embarrassed to back down now, as my_wan has pointed out.)
     
  11. Jul 14, 2010 #10
    Re: Does the ball rotate about its axis?!?!?!

    He's just yankin' yer chain.
     
  12. Jul 14, 2010 #11

    Doc Al

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    Re: Does the ball rotate about its axis?!?!?!

    That's what I think. :wink:
     
  13. Jul 14, 2010 #12
    Re: Does the ball rotate about its axis?!?!?!

    I have tried that too, it seems he is unwilling to even try.

    So I guess you and my_wan could be right and he is just too stubborn to admit he is wrong.
     
  14. Jul 14, 2010 #13
    Re: Does the ball rotate about its axis?!?!?!

    But its not only me he is annoying, but loads of others who have tried to get him to see it the right way.
     
  15. Jul 14, 2010 #14

    Doc Al

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    Re: Does the ball rotate about its axis?!?!?!

    Stop trying. Get him to convince you by drawing a diagram as I suggest in post #9. The burden is on him.

    Did you draw your own diagram? If so, then it's his turn to show why yours is wrong.
     
  16. Jul 14, 2010 #15
    Re: Does the ball rotate about its axis?!?!?!

    Burden-of-proof arguments are often used by people in a weak or losing position to try to place the obligation on the other party and then refuse to accept the 'proof'.

    Simply point out that by democratic consensus, he has lost the argument and refuse to discuss it further.

    In fact, come to think of it, why don't we all do that?
     
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