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Moment of inertia: Answer without calculations

  1. Jul 3, 2011 #1

    Femme_physics

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    To this shape

    http://img10.imageshack.us/img10/2301/nocalculations.jpg [Broken]

    Uploaded with ImageShack.us

    I'm asked to calculate the estimate of the static moment of inertia of the planar shape towards the x axis, that goes through its center of gravity. And then I'm told that I'm asked for an answer without calculations!

    That's a bit taking aback doesn't it? How can I solve it without any calculations?
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jul 3, 2011 #2

    rock.freak667

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    I am not too sure if you can estimate it without doing any sort of calculation.
     
  4. Jul 3, 2011 #3

    I like Serena

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    I think the keyword here is "estimate", meaning to do it with a minimum of calculations.

    What you need to know is that moment of inertia is a summation of small areas times their distance squared.

    For a rough estimate you can take the total area times the distance of the centroid.

    So you would need to make an estimate of the total area.
    Make an estimate of the distance of the centroid.
    And calculate the area times distance squared.
     
  5. Jul 5, 2011 #4

    Femme_physics

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    But I'm told to "find the answer without calculations" -- literally. That contradicts you telling me to calculate!
     
  6. Jul 5, 2011 #5

    I like Serena

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    The problem asks for a number. Afaik you can only get a number with calculations from other numbers.

    So if you're not allowed to calculate at all, then I've got no clue either. :smile:


    Btw, I just reread the problem, and it says: "the static moment of inertia of the planar shape towards the x axis, that goes through its center of gravity."

    I do not understand which axis is meant here.
     
  7. Jul 5, 2011 #6

    Femme_physics

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    Let's agree it's a stupid, idiotic, pointless question then whose author needs to be slapped in the tush for it?
     
  8. Jul 5, 2011 #7

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    Hmm, there's is usually a point to questions, even though they might seem pointless at first.

    Perhaps if you work it out with regular calculations, while keeping an eye out for a shortcut, you'll find something.
    I really have no clue at this stage, but that's what I would do.
    Even if there's nothing there, it's good practice. :smile:
     
  9. Jul 5, 2011 #8

    Femme_physics

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    I got some more important practices to do before the test, but thanks for your PoV!
     
  10. Jul 5, 2011 #9

    I like Serena

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    As you wish. I'll see if I can make some sense of it this evening. ;)
     
  11. Jul 5, 2011 #10

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    I've thought about it some more and decided it is an idiotic problem.

    I was thinking that perhaps you'd have to rearrange the shape around the axis in such a way that it would become a shape that you have listed.

    But I don't see what shape that would be, and even calculating the centroid is a complex calculation with no simple answer.

    I give up! :grumpy:
     
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