# Conservation of Mechanical Energy

1. Sep 5, 2004

### futb0l

I watched [Lecture 13] of the Newtonian Physics class in http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/index.htm [Broken] . 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.

Last edited by a moderator: May 1, 2017
2. Sep 5, 2004

### pervect

Staff Emeritus
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.

3. Sep 5, 2004

### futb0l

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

4. Sep 5, 2004

### Tide

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

5. Sep 5, 2004

### 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 [Broken]

"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! "

Last edited by a moderator: May 1, 2017
6. Sep 5, 2004

### futb0l

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

7. Sep 5, 2004

### Tide

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

8. Sep 5, 2004

### futb0l

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.

9. Sep 5, 2004

### 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...

10. Sep 5, 2004

### Tide

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

11. Sep 5, 2004

### Tide

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 $\sqrt{1+\frac{2}{5}}$.

12. Sep 6, 2004

ok thanks.