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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?

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2 Answers 2

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The angular momentum of the rod is 0 at the beginning because it is not rotating.

I would proceed like that:

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

  2. with that given, calculate the final angular momentum of the rod

  3. 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".

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As we are in beta, I would like to know if you guys think it is a good way to answer homework question (hope I did not make any mistake in the answer itself). –  Cedric H. Nov 7 '10 at 21:41
Wait so, I already have the answers for 1 and 2 (right?) So what am I missing? –  maq Nov 7 '10 at 21:44
The answer to point 1 is the answer of (3). Then how can you calculate the angular momentum, with the rotational speed given ? –  Cedric H. Nov 7 '10 at 21:56
So does that mean that the answer I put in number 4 is incorrect? I multiplied the final angular velocity by the total moment of inertia of the system, which is .3023 and that multiplied by 26.03 is just 7.87 –  maq Nov 7 '10 at 22:01
The answer to question 4 should be "automatic": by conservation of angular momentum this has to be the same as answer 1. To answer "my question 2" you just multiply the rotational speed by the momentum of inertia of the rod (not the total one). –  Cedric H. Nov 7 '10 at 22:03

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.

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Well, the question is about AVERAGE torque, not implying that it needs to be constant. –  Thomas Themel Nov 29 '10 at 13:54

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