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Disk rotation probelm

  1. Aug 14, 2014 #1
    Ok here is the question that is being asked. Note that I know how to solve it one way, but when I go about trying to solve this problem another way that I think should give the the same result, I don't end up with the same result.

    A disk of mass M and radius R rotates at angular velocity ω0. Another disk of mass M and radius r is dropped on top of the rotating disk, in its center, causing both disks to spin at a new angular velocity ω. Assuming a negligible loss of energy to friction what is ω?

    Ok so this problem is pretty easy just set initial angular momentum to final angular momentum

    I(large disk)(ω0)=I(large disk)(ω)+I(small disk)(ω) and just solve for ω.

    which gives me an answer of ω=((R**2)ω0)/(R**2+r**2)

    So I know that is the correct answer. But then I also thought that I should be able to solve this using conservation of energy. I set this up like so

    (1/2)I(large disk)(ω0**2)=(1/2)I(large disk)(ω**2)+(1/2)I(small disk)(w**2)

    however when I solve I get


    I checked my work multiple times and cannot find an error. I just don't know where my logic is going wrong. I mean if there is conservation of angular momentum doesn't that imply conservation of energy? Or since you are adding mass to the system can I not set up my energy conservation equation that way because I am not accounting for the rest energy of the second disk? I guess it is an inelastic collision when you drop the second disk on the top of the first so maybe that is why as well. But they say assuming a negligible loss to friction which throws me off. I mean isn't there significant loss of friction in inelastic collisions or do they just mean a small loss compared to the total rotational energy.

    Thanks for reading
    Last edited: Aug 14, 2014
  2. jcsd
  3. Aug 14, 2014 #2

    Doc Al

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    Staff: Mentor

    That last sentence is problematic. The only way for both disks to end up rotating together is for friction to act, and that means kinetic energy is not conserved. It's an inelastic collision.

    Was that sentence really part of the problem?

    No, not at all!
  4. Aug 14, 2014 #3
    Yeah that's exactly what I was just thinking. Yeah it is a problem in a conquering the physics GRE book by Yoni Kahn and Adam Anderson. Well I guess not word for word. The last sentence is actually. "Assuming negligible a negligible loss of energy to friction, what is w?" I figured the first negligible was a typo.
  5. Aug 14, 2014 #4

    Doc Al

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    Staff: Mentor

    Yikes. They messed up!
  6. Aug 14, 2014 #5

    Doc Al

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    Staff: Mentor

  7. Aug 14, 2014 #6
    Wait sorry I read wrong. It says "Assuming negligible a negligible loss of energy to friction, what is w?" I figured the first negligible was a typo but maybe my english reading is wrong and that changes the sentence.
  8. Aug 14, 2014 #7
    AHH THANK you sooo much. You just solved another problem I had with the book as well that I thought was weird!!!! Just solved two problems that I was stuck on. Saved me from posting for the other. Thanks so much!
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