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Rotating Disk and two students?

  1. Apr 14, 2015 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    Read below

    3. The attempt at a solution

    A) D. As the mass of the students gets further away from the axis of rotation, the rate slows down (remember the skater with arms out).
    B) T. No explanation currently...
    C) ????????
    D) T. The students exert a force on the plate (and the plate on the students) when walking out. Thus, that force over the distance walked causes word to be down. Consequently the force of the plate on the students is what slows the rotation rate.
    E) S. Angular momentum is conserved.

    Have no idea about C I tried both T or F.

  2. jcsd
  3. Apr 14, 2015 #2
    What is your explanation for (B). I mean you must have at least thought of something while saying its true which however it is not. Consider this. Take your disc as one system and the two students as one system. If there is no net torque on the disc, why is it slowing down? Moreover try drawing Free Body Diagrams of each element of your disc+students system; you will know what I am talking about.
    Moreover, your explanation for (A) is unsatisfactory. You must know why the rate of rotation decreases. Its not like you know similar situation where it decreases, so it must decrease in this case as well. You must be 100 % sure and must be able to explain why while you give your response.
    For (C), moment of inertia about the axis perpendicular to the plane is given by $$ J = ∫r^2 dm $$ I think that will give you an idea.
    (D) As a matter if fact, when you will analyse the free body diagrams, you will notice that there is no net force exerted by the two students system on the disc. However, you do know that even torque might do work. So figure this out yourself.
    (E) Yes that is correct.

    And I will advise you to make free body diagrams while solving problems. It is an important tool. Further if you say something, you must be able to prove it. If you cannot answer why, don't use it.
  4. Apr 14, 2015 #3


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    To be fair, MaximumPhysics didn't say there was. The statement
    can be read as referring to each student individually.
  5. Apr 15, 2015 #4
    Ya each "student" will exert force on the disc. However this force is not the reason for slowing down of the disc. Net force applied by the "students" is zero but not the net torque. So there is no point talking about this force. And the question cleary asks about students doing work not about each student. Please pardon me but i hate manipulation of words in physics
  6. Apr 15, 2015 #5

    Most of my mistakes in physics are due to manipulation of words.
    Some time I feel that I have to memorize the question and the answer instead of understanding it.

    English is not my first language, so those kind of questions always get me in trouble.

    Do you know of any way where I can strength my ability in those type of questions where they play with words?
  7. Apr 15, 2015 #6
    Yes there is one way. Take every word of a question by its face value or literal meaning. What I mean is, students means students. If they wanted to ask about individual student they must have mentioned each student or individual student. See if you take every word of the question by what it says and then there is some hidden meaning or manipulation, that is not your fault. You need not worry about it.
  8. Apr 15, 2015 #7
    Thanks and I believe I am still stuck, but I believe DTFTS is correct I can't see any other answer.

    Could you tell me what I have incorrect? last 2? first 2?
  9. Apr 15, 2015 #8


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    Yes it is. It doesn't matter whether you consider the effect of each force separately and add them, or combine them into a torque. Either way slows the disc just the same. We could have had one student moving out first, then the other, to get the same result.
    You are making a linguistic distinction where none exists. Both readings are valid. Fortunately, the work done cannot depend on whether you analyse separately and add or treat them as a unit. The only place in the questions where it matters is B, and there it makes clear it is concerned with the net of the forces.
    No. As Vatsal asked, please give your reasoning for your answer on B.
  10. Apr 15, 2015 #9
    Yes I agree that it will be the same for both the cases. However there need to be linguistic distinction if we wish to report our results. Isn't it? All I opposed what Maximum Physics gave the reason of his answer and also the fact he has no where mentioned about the torque on the disc.
  11. Apr 15, 2015 #10
    Dude! If you trying to match your answer with some answer script, I must tell you that those are not always correct. Form your own answers and whatever you write, you must know the proper reason behind it. If you can prove what you are saying, you are correct.
  12. Apr 15, 2015 #11


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    There is a problem with the proposed answer to D, but it's that MaximumPhysics has not shown that each student exerts a force in (or at least, partly in) the direction of movement.
  13. Apr 15, 2015 #12
    Obviously there is a problem with his answer to D) That is why I am asking him to re analyse his solution.
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