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Homework Help: Rolling motion- experimental data differs from expectation

  1. May 19, 2013 #1
    1. The problem statement, all variables and given/known data

    An experiment was done to test the validity of the equation a = 2/3 g sin∅ for a rolling cylinder. A hockey puck was rolled down a wooden ramp at 5 different inclination angles, and the time it took to roll down the length of the board was recorded. ∅ was found using trigonometry, acceleration via L= 1/2 at^2, with L the length of the board. The five points were plotted and linear best fit line was drawn.

    2. Relevant equations

    Expected result:

    a = 2/3 g sin∅

    Experimental results:
    a = 0.92 gsin ∅ -0.05

    3. The attempt at a solution

    I can't quite figure out why such a large difference arose. The value for slope should be closer to 0.67, but it's actually much larger. If anything, I would have thought it would be smaller due to friction acting on the puck, but the puck actually accelerates faster than it should. Granted, the measured times were very small, the largest less than 1.5 seconds, so I suppose it could be that all of the times were a bit short but this still seems like a rather large discrepancy. I've already done the experiment twice and I keep getting similar results.
  2. jcsd
  3. May 19, 2013 #2
    If the puck accelerates faster than expected, are you sure the moment of inertia isn't less than expected? Is the puck a uniform cylinder?
  4. May 19, 2013 #3
    I believe so, what else could it be? That is the closest shape I can think of, unless the non-uniformity of it drastically changes the moment of inertia. In any case, the experiment specifically asks for a comparison between the equation for the rolling motion of a solid cylinder so I believe it's meant to be done assuming that's what it is? This question is driving me bonkers.
  5. May 19, 2013 #4
    I've never seen a hocky puck up close but I would bet it is not a true uniform cylinder, there is no other explanation other than measurement error. Something like ice sliding down a flat surface would have little moment of inertia and should accelerate much faster.
  6. May 19, 2013 #5
    Please share both sets of data with us. The time vs the angle will be adequate. Thanks.
  7. May 20, 2013 #6


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    Gold Member

    How are you measuring the time from start to finish?
    If you are using a stopwatch, perhaps your start of the stopwatch is always after the puck has started rolling and you click stop( in anticipation ) before the puck has actually reached the end.
  8. May 20, 2013 #7
    That's it, thanks! I re-did it, beginning the stopwatch just as the puck began motion and it worked. Since the times were so small, the measurement error was screwing it up quite a bit. Thanks for all the help!
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