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Moment of Inertia project

  1. Sep 5, 2004 #1
    Hi, I am doing a physical project and i encountered a vital problem. I need to calculate the moment of inertia of hollow paper block(beam of a trebuchet(type of a catapult)). I was looking on the internet and there is no explanation of this particular case.
    So, would anyone know how to calculate the moment of inertia for hollow paper object? (block, pyramid with no base and cut top with a hole in it for example)
    Is there any formula for it?
    Thanks for potential responses
    Last edited: Sep 5, 2004
  2. jcsd
  3. Sep 5, 2004 #2


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    Perhaps you could describe the shape a little more accurately or provide a drawing? For example, it's not clear from your description whether the shape has any type of symmetry which would be quite helpful in calculating the moment(s) of inertia.
  4. Sep 5, 2004 #3
    Well first of all you need to define yourself some refrence point, as the moment of inertia is always relative to some point. Then ( I think), you can sum the moments of inertia of each face of the block relative to that point. So if its a cube, it requires finding the moment of inertia about each face individually, and then summing them up. I think this would work becuase the moment of inertia is just the sum of the mass times the radius squared, and you can do this for each face and sum the faces. This should yeild the same result. But have someone else verify what im saying before you bother to do any calculation, I could be wrong.
  5. Sep 5, 2004 #4
    Well, it is a long hollow paper arm(four sides), the two opposite are always identical trapezoids(symmetrical ones)
    So, I can take any point? Well, I'll choose the fulcrum(it's a lever actually) as the point, that should be ok.
  6. Sep 5, 2004 #5
    Just make sure to take it with respect to a point that is usefull! It does you no good to take it at a point that will not be where its rotating! You picked a very good point, as that would be were the motion would be about.
  7. Sep 5, 2004 #6
    Ok, thanks to you both!
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