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## Homework Statement

A thin, uniform, rigid disk of mass M, radius R is welded to a light, elastic shaft of radius r, length L, shear modulus G. Phi is the torsional oscillation.

http://img38.imageshack.us/img38/9811/unledlf.png [Broken]

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## Homework Equations

Phi=TL/GI

I=pi/2*r^4(polar area moment of inertia)

## The Attempt at a Solution

Since the shaft is "light", I assumed it to be massless and considered it a spring instead. I'm not sure if this is the right train of thought. I'm trying the newtonian approach of equating torque:

I(total)*phi"=torque of shaft+torque of disk

somehow though, I couldn't figure out the torque of the disk. If the shaft is a spring, it's torque would be k*phi.

Am I even on the right track here? Should I do conservation of energy instead?

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