Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    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

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    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

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    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

    Tide

    User Avatar
    Science Advisor
    Homework Helper

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

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?