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Conservation of Mechanical Energy

  1. Sep 5, 2004 #1
    I watched [Lecture 13] of the Newtonian Physics class in here . On the last part of the lecture, they did 2 experiments on pendulums (conservation of mechanical energy, etc.) and the first experiment followed the prediction however, the 2nd experiment did not follow the prediction.

    I have thought about it and are unable to come up with an answer, I hope you guys can give me a hint.
  2. jcsd
  3. Sep 5, 2004 #2


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    I looked in the index rather than sat through the video - was this problem related to the difference between the equations of motion of a sliding puck and a rolling ball?

    If so, you might want to think about the moment of inertia of the ball.
  4. Sep 5, 2004 #3
    the experiment was done during the end of the video ...
    anyways - thanks for the hint - i will think about it.
  5. Sep 5, 2004 #4


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    That's way too long to watch - just tell us about the experiments.
  6. Sep 5, 2004 #5
    oh - oops - actually it wasnt on the puck and the rolling ball
    it was the last thing on http://ocw.mit.edu/OcwWeb/Physics/8...tures/detail/Video-Segment-Index-for-L-13.htm

    "The known radius of a circular air track is used to predict the period of oscillation of a sliding object (small angles!), and a measurement is made to confirm this. The process is repeated for a ball bearing rolling in another circular track. The period of oscillation can now not be predicted in a similar way as was possible in the case of the air track. Why? ==> No, it has nothing to do with friction! "
  7. Sep 5, 2004 #6
    It's on the last 5 minutes of the video.
  8. Sep 5, 2004 #7


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    Yes - but I'm working with low bandwidth and it will take ages to get there.
  9. Sep 5, 2004 #8
    it's pretty difficult for me to explain but here it is:

    experiment #1:

    an object is put on the air track with a radius of ~115m and was released at the starting point and the professor predicted the period using the principles of mechanical energy and simple harmonic oscillation.

    experiment #2:

    an ball is put on a curvature much smaller than the air track radius ~85cm and it was tested using the same principle of experiment #1 but the result did not agree to the prediction.
  10. Sep 5, 2004 #9
    btw - it's better to watch the experiment yourself since i might miss some detail ...
    and pervect said that it might be through the rotation of the ball - but i am not quite sure...
  11. Sep 5, 2004 #10


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    From what you described, pervect's explanation sounds right on!
  12. Sep 5, 2004 #11


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    I let the video run in the background while I was doing other stuff and managed to see the experiments. Yes, absolutely, the rotation of the second object is what causes the "error!"

    The moment of inertia for a sphere (solid & uniform) is [itex]\frac{2}{5}Mr^2[/itex] which would increase the period by a factor of about [itex]\sqrt{1+\frac{2}{5}}[/itex].
  13. Sep 6, 2004 #12
    ok thanks.
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