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Moment of Inertia, Please, Please Help Today!

  1. Nov 16, 2006 #1
    Hello,

    I have some questions:

    1. Do you think the calculated moment of inertia is larger or smaller than the true moment of inertia? Explain.

    for static moment of inertia I have:.0202kgm^2
    for dynamic " ":.0195 kgm^2

    In this case, it's smaller but I don't know why.

    2. Is the percent difference in accord with your answer to #2? Give plausible explanations/causes to account for the difference. for the percent difference I got 3.6% which is small so it does agree with #2. I am not sure why there is a difference though.

    3. The wheel could replace the pulley on newton's law's 2nd law. Try to rethink an answer to the question: "If the pully has a fairly large mass, how will the result of this experiment be affected? " :confused:
     
  2. jcsd
  3. Nov 16, 2006 #2

    Doc Al

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    You might want to state the full problem exactly as given so we can understand your questions.
     
  4. Nov 16, 2006 #3
    I'm sorry, those are the questions my professor gave us.:frown:

    the question:
    2. Is the percent difference in accord with your answer to #2? Give plausible explanations/causes to account for the difference. for the percent difference I got 3.6% which is small so it does agree with #2. I am not sure why there is a difference though.

    It's talking about question #1 on this board, not #2 (was question two on the paper).
     
  5. Nov 16, 2006 #4

    Doc Al

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    Moment of inertia of what?

    What do you mean by "static" moment of inertia?

    You need to describe the problem you are trying to solve.
     
  6. Nov 17, 2006 #5
    calculated moment of inertia in general.

    You never hear of static or dynamic moment of inertia?
     
  7. Nov 17, 2006 #6

    OlderDan

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    These concepts only make sense for an object that deforms when it rotates, like a speed governor. Since you seem to be unwilling to respond to Doc Al's request that you tell us what the problem is talking about, there is no way we can help you.
     
  8. Nov 17, 2006 #7

    Doc Al

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    What about it? Is calculated moment of inertia larger or smaller than the true moment of inertia? That would depend on how you're calculating it, wouldn't it?

    Not in the context of simple rigid body problems. Since a pulley is mentioned, I assume this is some kind of experiment with masses and pulleys. Why don't you describe the experiment?

    Edit: OlderDan brought up deformable bodies; in such a case, static and dynamic moments of inertia will indeed be different. Are you talking about deformable bodies?
     
    Last edited: Nov 17, 2006
  9. Nov 17, 2006 #8
    experiment: In this lab you will determine the moment of inertia of a solid disk in two different ways and compare the results. It is somewhat like Newton's Second Law experiment where you determined the total mass of a system, statically by "weighing" it and then dynamically by measuring accelerations vs. force and taking the inverse of the slope of a line. There are differences, in that you will measure a single linear acceleration and the torque due to the tension in the tape will have to be considered. In addition, you will be required to "figure out" the expression to use in the determination of the "dynamic" moment of inertia. Refer to the sample problem done in class and there is another in the text, both of which contains the physics you need to know and all of the required equations. You just need to put the equations together in a different way suitable to calculate the moment of inertia using the dynamical results.

    okay, my actual (which is the dynamic) is larger than the calculated moment of inertia (static). For actual, I got 0.0195, and the theoretical I got 0.0202. I was wondering the calculated moment of inertia is smaller than the actual. Does it have to do anything with the center mass of the wheel?

    Also, when I did the percent difference (actual-theo/actual)X100 I got 3.6%. Does this account for the the calculated moment of inertia being small?

    formula for static is I=1/2MR^(2)
    formula for dynamic: I=R^(2)m(g-at)/at

    It is confusing, I think this experiment was talking about deformed bodies, yes.
     
    Last edited: Nov 17, 2006
  10. Nov 17, 2006 #9

    Doc Al

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    So far, I see no reason to think that your experiment involved deformable bodies. What was the wheel made out of? How fast did it spin?

    It looks like you are hanging a mass m from the wheel and letting it fall.

    In this context, you are using "static" to mean: Calculated based on some model of the disk and measurements of R and M. How well did you measure those quantities? Was the disk perfectly uniform? (Unlikely, as it must have some axle to rotate.)

    And you are using "dynamic" to mean: Calculated based on measurements of kinematic quantities like at (the acceleration of the falling mass), as well as m and R. Again, how carefully did you measure those quantities? What did you actually measure? Time to fall a given distance?

    In both cases, the rotational inertia should be the same. Any differences are most likely due to errors in your model or measurements.
     
    Last edited: Nov 17, 2006
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