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Estimating mass of earth

  1. Jul 5, 2006 #1
    i know nothing about astronomy, so please excuse my silly question here.

    how did people originally estimate the mass of earth?

    i sense our current data of earth mass (10^24 kg? or so) has been there for quite sometime. how did they do it?
     
  2. jcsd
  3. Jul 5, 2006 #2

    tony873004

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    I forget the name of the experiment, but ~100 years ago, a scientist suspended a rod from a thread. He knew the resistance the thread would have in keeping the rod from spinning. Then he put a large object of known mass near the rod and watched how much the rod moved. Kinda like how much the Earth's magnetic field makes a compass needle turn. Except this "compass needle" responded to gravity.

    Measuring how far it moved and knowing how hard the thread would resist this motion, he calculated G, the gravitational constant. With that, he used the formula g=GM/r^2, re-written as M=gr^2/G. g is known. It's 9.81 m/s/s. r is known. It's 6378000m. Now that he knew G, M is computable.

    ** edit **
    Cavendish experiment. Just Google for "weighing the Earth" or "Cavendish experiment" and you'll get a whole collection of web sites describing the experiment in detail. I pretty much got it right. But it was ~200 years ago.
     
    Last edited: Jul 5, 2006
  4. Jul 6, 2006 #3
    thanks.

    i feel it is very strange that gravitational force can be observed in that way. if it were just an experiment proposal, i would definitely say it wouldn't work.
     
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