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Torque and moment of inertia

  1. Jan 7, 2007 #1
    The moment of inertia of Earth was recently measured to be 0.331MR^2. what does this tell you ablut the distribution of mass inside the earth?
    that was the problem
    my attempt/answer was that it tells us that it is widely distributed because Earth is so large and round. (honestly i dont really know, i am kinda confussed on this problem).

    2. A different problem: two forces equal in m agnitude but opposite in direction act at the same point on an boject. is it possible for there to be a net torque on the object? explain.

    my attempt/answer was: yes, it is possible for there to be a net torque equal in magnitude but opposite in direction acting at the same point on an object. this is because if there is two opposite forces with the same magnitude they will cancel each other out.

    those are two different problem with two different answers from me. if you think i am right or wrong or have a hint or answer for either or both please let me know that would be very helpful. i want to understand my physics better so i am asking for help in order to do so. thank you for taking your time to read this.

    - physics student:confused:
  2. jcsd
  3. Jan 7, 2007 #2


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    Can you think of any objects with a moment of inertia similar to this?
    You may wish to reconsider this answer. What is the definition of a torque (i.e. the formula for calculating it)?
  4. Jan 7, 2007 #3


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    For the first question compare the Earth's moment of inertia with those you might expect it to be like (i.e. spheres solid and hollow).

    For question two i'm not sure you've got the correct jist of the problem. Do you know the equation for torque?
  5. Jan 7, 2007 #4


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    Your argument says that if there are two forces with opposite magnitude acting at the same point on an object, then they wil cancel out. That is, the net force on the object at that point is zero. If the net force at a point is zero, what can you say about the torque at that point?

    (I think your question should state that the two forces are the only forces acting at that point, and that there are no other forces acting at any other point on the object).

    edit: woah.. didn't realise two people got in before me, sorry!!
    Last edited: Jan 7, 2007
  6. Jan 7, 2007 #5
    Now you are right in comparing earth with a SOLID spere. that is 2/5=.4 instead of .331. So earth has less rotational inertia then a homgeneous spere of equal mass. So you should understand that moment of inertia is sort of a measure of how hard it is to spin something; with that in mind, what sort of mass distribution would lower moment of inertia (make it easier to spin).
  7. Jan 7, 2007 #6
    just a little obvious not on your second question, if you push equally hard on both sides of some spot on a door, is it gonna move? (yes that was redundant)
  8. Jan 7, 2007 #7
    Thank you! Thank you to everyone that responded to these two problems. All of your suggestions and imput were very helpful. when i answered these questions the first time i was greatly confussed and just had to guess but with each one of you giving me a little hint or something to think about it made me be able to have a more educated answer to the problem. i just wanted to thank you for this. With your help it made me understand the problems better. Thank you soooooooooo much. i appreciate it greatly for you to take your time to read my problems and look at them and respond to help me. THANK YOU
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