ok... We were given the following question... (I apologize for it's length... please bear with me)(adsbygoogle = window.adsbygoogle || []).push({});

A thin long cylindrical rod (mass M radius R and length l) is initially rotating about its long axis with angular momentum L. There are no external forces or torques however interestingly Ke is usually not conserved.

a) Express the initial Ke of the system in terms of M R l and L

b) Describe how the system can lower its energy while maintaining constant angular momentum.

c) Calculate the ratio of the final to initial energy

d) How thin does a rod of radius R have to be at least so that one can consider it as "thin and long"

Ok. First of all... I have no idea what the formula is for a thin long rod about it's long axis. (The only formulas I can find are for through the cm and perpendicular to the long axis) so I assumed I = 1/2 MR^2 (inertia for a solid cylinder). If that's right, I get Ke = (L^2)/MR^2

And that's fine. Except that I don't have an l term. Which means that when I go to do part d... I can't find the ratio of l to R necessary.

Also I have no idea what the "final" energy would be because I have no idea how MUCH Ke is lost based on the question...

Any help you can give would be greatly appreciated!

Thanks!

Nicola

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# Homework Help: Rotational Motion. Apparently, I have no idea what I'm doing.

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