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Thread: Big G

  1. #1 Big G 
    Forum Sophomore biohazard87's Avatar
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    Hey does anyone know anything about big G. I was just put in a physics based research class and I need to know about this constant.


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  3. #2  
    Forum Radioactive Isotope mitchellmckain's Avatar
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    The universal gravitation constant is used in Newton's law of gravity
    F = G m1 m2 /r^2
    where F is the force of gravity between two masses m1 and m2 and r is the distance between the centers of the two masses.

    G = 6.67259 x 10^-11 m^3 / (kg s^2)

    Apparently in some cosmological theories this value is not constant but is slowly decreasing with time.


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  4. #3  
    Forum Sophomore biohazard87's Avatar
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    How do I measure it without having crazy established equipment like lazers?
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  5. #4  
    Forum Radioactive Isotope mitchellmckain's Avatar
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    I don't know what lasers would have to do with it.

    How you would measure it depends on what information you allow yourself to start with. For example do you accept previously measured values of the mass and radius of the earth? If you do you could use the value for g=9.8m/s^2 (or measure it yourself with a pendulum), and use it in the above equation to get G.

    g = 9.8m/s^2 = G Me / re^2

    Using the mass of the earth Me = 5.9676 x 10^24 kg and the radius of the earth re = 6378000 m.

    9.8 m/s^2 = G x 1.467 x 10^11 kg/m^2

    Now all you have to do is solve for G.

    This is probably not an independent confirmation of the value of G since the mass of the earth is probably computed using G. The only other thing I can think of doing without this is a best fit statistical analysis on the moons of different planets, because their orbital periods depend on their distances from the planet, the mass of the planet and G.
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  6. #5  
    Forum Sophomore biohazard87's Avatar
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    What about the "Cavindish Experiment"? I don't really understand how it works but I think it would probably be the easiest way to measure big G.
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  7. #6  
    Forum Radioactive Isotope mitchellmckain's Avatar
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    With a torsion balance? Looks pretty difficult. Does this satisfy your condition for no special equipment? It looks as if you would be lucky to get accuracy to within 10%.
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