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Oscillations - A puzzling demonstration

  1. May 18, 2010 #1
    I watched MIT OCW PHYSICS 8.01 lectures, and in lecture no.13, i saw a puzzling demonstration. Link :http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/detail/embed13.htm" [Broken]

    I don't understand, what can be the reason for the oscillations of ball on the track with the smaller radius to have a larger time period than the oscillations of the red object on the larger track? :confused:Please explain.

    Thank you.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 18, 2010 #2


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    Well, to repeat the question that was asked, but in a more leading way:
    How is a ball on a curved track not like a pendulum?
  4. May 18, 2010 #3
    I think the answer lies somewhere in the fact that the metal ball rolling on a track is actually in contact with the track, but I'd have to think it through some more. This is a very cool demonstration though.
  5. May 18, 2010 #4


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    What are the balls doing on the track that is different from a pendulum?

    Edit: I have now actually watched the demo, I'll rephrase my question.

    What is the ball doing on the track that the air car is not?
    Last edited: May 18, 2010
  6. May 18, 2010 #5


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    Good lecture - great little demo.
    I wonder how many of the students came up with the answer "in the shower".
  7. May 18, 2010 #6
    Aha! I feel foolish now :frown:. The answer is because the ball is rolling and has an additional angular momentum, while the air car has zero angular momentum. I should have caught that right away, but i was sort of right in the sense that the ball rolls because it's in contact with the track.
  8. May 18, 2010 #7
    So the ball has an angular momentum as well.
    But , consider the following

    The ball on the circular track has an angular velocity
    w = squareroot(g/R)[where R = radius of circular path ,g = acceleration due to earth's gravity]
    So, as the radius of the path the ball travels decreases, w should increase hence time period should decrease, why did T increase?
  9. May 18, 2010 #8
    T increased because the ball has an additional angular momentum, and thus less linear momentum than if the ball had slid down the track without rolling.
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