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Homework Help: Can anyone explain why this happens? mechanics

  1. Dec 12, 2007 #1
    Ok this is an experimental result that has stumped me (I posted a while ago on the same topic but received no replies so this is a more specific question.)

    This picture shows what happens:


    The experiment involves dropping a mass (through a tube, visible in the top right of each shot) onto a pencil that is balanced on the edge of the table. The picture is video print-outs.. I realise the picture quality isn't great but basically, in this case the pencil jumped up backwards instead of the usual forwards (clockwise from this direction) rotation and movement.

    So yeah, if anyone can explain WHY or give me some clues on how to start that would be great! Thanks.
  2. jcsd
  3. Dec 12, 2007 #2
    hmm if the pencil was sharpened, it's possible that the mass hit it on the cone part, making the mass "slide off" before it delivered all of its force. That would explain why the pencil did not complete a revolution. It's also possible that the eraser (i assume that's what it is on the left side of the pencil) was a large enough source of clockwise torque to prevent the pencil from flipping forward.
  4. Dec 12, 2007 #3


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    I would think shape of objects is not important, force, angle, and motion can be like actions and reactions on a pool table.
  5. Dec 12, 2007 #4


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    Weight of the pencil act at the center of the pencil. When the mass hits the pencil, the reaction at the edge of the table produces an anticlockwise moment which is greater than the clockwise moment due to weight of the pencil.It is the cause of the backward jump of the pencil. If you perform the same expefriment with a heavy lead rod of the same shape, you may no get the same result.
  6. Dec 13, 2007 #5

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    Hi Zoe-b,

    In your earlier post you had said that this backward flipping happens when only a small fraction of the whole length is sticking out. Is that always true? Also, in that case, at which point on the pencil is the ball dropping on?
  7. Dec 13, 2007 #6
    Hey, thanks for the replies!
    No, sorry I didn't explain very well, but the pencil was the other way round, with the sharpened bit on the table. It doesn't have an eraser, the red-ish thing you can see is coloured paper wrapped round with sellotape; I used different colours in the centre and on each end in order to make my videos clearer- I got the same effect before adding the paper.

    Not quite.. it's probably not obvious from the pictures but the direction of rotation is actually always clockwise. In the above film, the pencil spins forwards but moves backwards.

    Yeah, as far as I can tell it is. As the fraction sticking out decreases, the pencil is more likely to go upwards, and eventually, backwards. The current theory I have (gained from talking to my teacher) is that the weight hits the very end of the pencil and therefore tilts when hitting the pencil. It moves forwards, and by newtons 2nd (? hehe) law the pencil therefore moves backwards. Consider a metre ruler with a few centimetres hanging off the edge of the table... there's no way it'll spin forwards, and if you hit it right on the end it jumps backwards. This effect is similar but magnified because the pencil is lighter...

    I think!
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