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Yes, it is my homework - Moment of Inertia of a composite sphere...

Discussion in 'science and environment' started by stuff_it, May 29, 2012.

  1. stuff_it

    stuff_it Nerd burglar

    I can't seem to get anything like what they are asking for in part i. I realise tht I may be some sort of muppet, but no one else who has had a look at it for me can get that either so I'm throwing it out to the SF for some help with methodology.

    Taking a simple model of the Earth as a spherical iron core of radius 3490 km with an average density of 13 000 kg m, surrounded by rocky material with an average density of 4000 kg m
    out to a radius of 6400 km, answer the following:
    1. Show that the moment of inertia of the Earth is about 8 × 10^37 kg m2.
    2. Calculate the rotational kinetic energy of the Earth.
    3. Calculate the kinetic energy of the Earth in its orbit round the Sun, given that the orbit is approximately circular, with a radius of 1.5 × 10^11 m.
  2. Greebo

    Greebo Czekałam na Ciebie

    To start with answering any of those 3 questions, you need to work out how the mass (how much in kg) the iron core is (which you'll need to know to, and its volume.
    Then you need to work out the total volume of the entire sphere Earth, subtract the volume of the iron core, and then work out the mass of the outer layer.

    Sorry, after that, what I can remember about physics fades out. But I do remember that you need to know mass to work out inertia and kinetic energy.
  3. xes

    xes F.O.A.D

  4. stuff_it

    stuff_it Nerd burglar

    Yes I know the mass and the volume, that's not the problem. It's combining the MOIs of the two masses to get the 8 × 10^37 kg m2.

    They must be on the same course as me. :hmm:
  5. xes

    xes F.O.A.D

    Quite possibly, does the answer make any sense to you? Looks like hyroglyphs to me :D
  6. stuff_it

    stuff_it Nerd burglar

    I will have a good look over it later, I'm just putting the rest of it in electronic first. I'm at my mum's and the cleaner is here so a bit hard to concentrate... I thought I would utilise the time to rinse out her broadband downloading mixes instead. :D
  7. Greebo

    Greebo Czekałam na Ciebie

    The answer's useless if you don't understand why you needed to use those particular equations to work it out.
  8. 2hats

    2hats these hands were made for making

    And not having got the answers for K.E. on a plate (s)he rephrased it and asked again. Top marks for effort. They'll go far... in maybe politics or law, but not physics (at that rate).

    PS Get smart arse marks by computing the orbital moment of inertia as well since they didn't specify rotational only in the original first part of the question (if it's as exactly is as written here).
  9. stuff_it

    stuff_it Nerd burglar

    TBF it's only part i. that I'm struggling with.
  10. Greebo

    Greebo Czekałam na Ciebie

    Your recent coursework should have included something about calculating momentum (moment of inertia). The end result has been provided, AFAIK what you're being asked to do is to show how you'd go about working it out.
  11. stuff_it

    stuff_it Nerd burglar

    Yes, I know what it's asking. :rolleyes:
  12. Greebo

    Greebo Czekałam na Ciebie

    Sorry.
  13. stuff_it

    stuff_it Nerd burglar

    Success! :cool:
    Greebo likes this.

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