Spinning and heating a metal rod

[psyhprog]

Homework Statement

There's a metallic rod of length l1 which is spinning around a vertical ax which passes through its center. The ends of the rod are spinning with ω1 angular speed.

Homework Equations

Determine the temperature at which to heat the metal rod so that the angular speed of the end ω2 doubles.
Also, how to generally approach problems involving torque or angular momentum? (How to figure out when to use them, and how to use the formulas/equations in order to solve the problem.)

The Attempt at a Solution

The only idea I have is the following: ω1 and ω2 are constant, so the net force is 0, and so the angular momentum r⃗ ×mv⃗ is conserved. How should I approach this? And in general, for problems relating to torque or angular momentum, how should I approach those?

 

[CWatters]

Sounds like a spinning ice skater.

http://www.bsharp.org/physics/spins