Pendulum experiment contradicts mechanical energy conservation

[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.

 

[pervect]

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.

 

[futb0l]

the experiment was done during the end of the video ...
anyways - thanks for the hint - i will think about it.

 

[Tide]

That's way too long to watch - just tell us about the experiments.

 

[futb0l]

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! "

 

[futb0l]

That's way too long to watch - just tell us about the experiments.

It's on the last 5 minutes of the video.

 

[Tide]

Yes - but I'm working with low bandwidth and it will take ages to get there.

 

[futb0l]

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.

 

[futb0l]

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...

 

[Tide]

From what you described, pervect's explanation sounds right on!

 

[Tide]

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 $\frac{2}{5}Mr^2$ which would increase the period by a factor of about $\sqrt{1+\frac{2}{5}}$.

 

[futb0l]

ok thanks.