Conservation of Mechanical Energy

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Discussion Overview

The discussion revolves around the conservation of mechanical energy as demonstrated in two experiments involving pendulums on an air track. Participants explore the discrepancies between predicted and observed outcomes in these experiments, focusing on the implications of different physical properties such as moment of inertia.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes two experiments from a lecture: one involving an object on an air track and another with a ball on a smaller curvature, noting that the second experiment did not align with predictions based on mechanical energy principles.
  • Another participant suggests considering the moment of inertia of the ball as a potential factor affecting the results.
  • A later contribution mentions that the rotation of the ball may be responsible for the discrepancy, referencing the moment of inertia for a solid uniform sphere.
  • Some participants express difficulty in accessing the video due to bandwidth issues, leading to requests for more detailed descriptions of the experiments.
  • There is an acknowledgment that watching the experiments may provide better insights into the details that could affect the outcomes.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the reasons for the discrepancies observed in the experiments. While some suggest the moment of inertia and rotation of the ball as factors, others remain uncertain and seek further clarification.

Contextual Notes

Participants express limitations in understanding due to potential missing details from the video and the complexity of the experiments. There is also a noted dependence on the definitions and principles of mechanical energy and oscillation.

futb0l
I watched [Lecture 13] of the Newtonian Physics class in http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/index.htm . 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.
 
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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.
 
the experiment was done during the end of the video ...
anyways - thanks for the hint - i will think about it.
 
That's way too long to watch - just tell us about the experiments.
 
oh - oops - actually it wasnt on the puck and the rolling ball
it was the last thing on http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/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! "
 
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Tide said:
That's way too long to watch - just tell us about the experiments.

It's on the last 5 minutes of the video.
 
Yes - but I'm working with low bandwidth and it will take ages to get there.
 
Tide said:
Yes - but I'm working with low bandwidth and it will take ages to get there.

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.
 
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...
 
  • #10
From what you described, pervect's explanation sounds right on!
 
  • #11
futb0l said:
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...

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].
 
  • #12
ok thanks.
 

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