# Oscillations - A puzzling demonstration

## Main Question or Discussion Point

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? Please explain.

Thank you.
Elixer

Last edited by a moderator:

Related Other Physics Topics News on Phys.org
alxm
Cute.

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?

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.

Integral
Staff Emeritus
Gold Member
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:
sophiecentaur
Gold Member
Good lecture - great little demo.
I wonder how many of the students came up with the answer "in the shower".

Aha! I feel foolish now . 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.

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?

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.