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  1. #1
    Bar0n janvdl's Avatar
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    Physics Help

    This problem makes use of Newtons Universal Gravitation Formula (F = (G.m1.m2)/(r^2))

    In an experiment, using Cavendish's Apparatus(whatever this is!!!) to measure G, the mass of each small sphere is 1,0 kg and the mass of each bigger sphere is 5,0 kg.

    The distance between the midpoints of the smaller spheres and the midpoints of the bigger spheres is 80 millimetres. (From the edges of the horizontal beam)

    A force of 10^-7 Newton is noted.

    Calculate G, with the mass of the beam excluded.


    I have no idea what this Cavendish Apparatus is, so i cant imagine what this thing looks like or how it works.
    Last edited by janvdl; May 5th 2007 at 06:23 AM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by janvdl View Post
    This problem makes use of Newtons Universal Gravitation Formula (F = (G.m1.m2)/(r^2))

    In an experiment, using Cavendish's Apparatus(whatever this is!!!) to measure G, the mass of each small sphere is 1,0 kg and the mass of each bigger sphere is 5,0 kg.

    The distance between the midpoints of the smaller spheres and the midpoints of the bigger spheres is 80 millimetres. (From the edges of the horizontal beam)

    A force of 10^-7 Newton is noted.

    Calculate G, with the mass of the beam excluded.


    I have no idea what this Cavendish Apparatus is, so i cant imagine what this thing looks like or how it works.
    See here for a description of the apparatus and experiment.

    RonL
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  3. #3
    Bar0n janvdl's Avatar
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    Thank you very much Captain Black.
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  4. #4
    Bar0n janvdl's Avatar
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    Help Again Please!

    Ok, so:

    F = G(1 x 5)/(0,08m)^2
    10^-7 = G(5)/0,0064

    G = (10^-7 x 0,0064) / 5

    But now my answer is = 1,28 x 10^-10

    And the answer should be: 6,4 x 10^-11

    If only the number under the line was 1000 instead of 5 it would have worked. Where did i go wrong??
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  5. #5
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by janvdl View Post
    Ok, so:

    F = G(1 x 5)/(0,08m)^2
    10^-7 = G(5)/0,0064

    G = (10^-7 x 0,0064) / 5

    But now my answer is = 1,28 x 10^-10

    And the answer should be: 6,4 x 10^-11

    If only the number under the line was 1000 instead of 5 it would have worked. Where did i go wrong??
    There are TWO sets of spheres in the Cavendish experiment, so
    F = 2*G*m1*m2/r^2

    So divide your answer by 2.

    Also, when you put in 1 x 10^{-7} into your calculator you put in "10EE-7" didn't you? That's equivalent to 10 x 10^{-7} which is one power of 10 too large. Thus your exponent should be reduced from -10 to -11. This will generate your correct answer.

    -Dan
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  6. #6
    Bar0n janvdl's Avatar
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    Topsquark you are a genius! Thanks a lot!

    I had a choice last night of whether to struggle with physics or do maths... You can guess what i chose
    Last edited by janvdl; May 6th 2007 at 03:41 AM.
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