Angular Momentum and Average Torque 
Refer to number 6. This is the one I'm stuck on. So angular momentum is conserved right, so initial angular momentum is equal to final angular momentum. Initial is 7.87 so final must be 7.87, right? And so average torque is just change in angular momentum / change in time, so 0/7=0. What am I doing wrong?
 A: The angular momentum of the rod is 0 at the beginning because it is not rotating.
I would proceed like that:


*

*by conservation of angular momentum, calculate the final rotational speed of the rod

*with that given, calculate the final angular momentum of the rod

*You have that the torque gives the variation (with time) of the angular momentum. So if the torque is constant you just have "torque = angular momentum / $\Delta t$".
I can be more specific if you want. Tell me where you find a problem.
Edit: Apparently the steps are done in the previous questions, so this should just be a "put everything together question".
A: I take it that the idea for part 6 is that a non-rotating rectangular bar is dropped onto the rotating disk, but initially does not notate with it (why not?).  Then (not by friction or eany other simple coupling I can imagine where the torque on the bar would typically be proportional to the angular velocity difference, but by some other unstated mechanism,) the rod is angularly accelerated with constant torque (weird no?), somehow coupling to and using the rotational energy of the disk, and that frictional energy losses can nevertheless be neglected.    No wonder our student was confused.   This is a rather unphysical or unusual situation, no?   What might be a mechanism to do this?   Does anyone else find this difficult to visualize as an actual situation?   To me, at a minimum, little more clarity and detail in the initial description of the problem might have been helpful for our student's initial visualization.
