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Walter Lewin's best performance was the pendulum demonstration, and I copy the transcript now:

Would the period come out to be the same or not?

[students respond]

Some of you think it's the same.

Have you thought about it, that I'm a little bit taller than this object and that therefore maybe effectively the length of the string has become a little less if I sit up like this? And if the length of the string is a little less, the period would be a little shorter.

Why would the length decrease? Shouldn't it increase, as the string is stretchable?

If this concerns centre of mass, a diagram would be much appreciated!

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up vote 2 down vote accepted

The length of the string should be measured from the pivot to the centre of mass. Prof. Levin sitting on top of the bob raises the centre of mass of the new bob + Prof. Levin system as compared to the bob's centre of mass. That is why the length of the string is effectively shorter.

crude diagram

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ok, and a simple pendulum is supposed to have point sized bob, right? How can this be a simple pendulum then? – Saurabh Raje Mar 12 '13 at 3:03
Because the point sized bob is just the center of mass. Simple pendulum just means yu only consider the center of mass – Martin Beckett Mar 12 '13 at 4:58

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