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Science Forum

  1. Standard member sonhouse
    Fast and Curious
    16 Jan '09 08:24
    This constant comes up in so many gravity field and lens formula's, just wondered if anyone knew how it was derived and what is it anyway?
    I know it's 6.67 E-11 and all that but what does that mean?
  2. 18 Jan '09 20:11
    It's just a proportionality constant that arises because of the use of SI units. You can transform to a different set of units and get no G.
  3. Standard member sonhouse
    Fast and Curious
    18 Jan '09 21:22
    Originally posted by KazetNagorra
    It's just a proportionality constant that arises because of the use of SI units. You can transform to a different set of units and get no G.
    What do you mean by 'no G'? You mean G would be 1 or something in some other units system? What units would that be? I guess it would have to be 6.7 E+11 times bigger, eh to be equal to 1 if that's what you mean.
  4. 18 Jan '09 23:26
    Originally posted by sonhouse
    What do you mean by 'no G'? You mean G would be 1 or something in some other units system? What units would that be? I guess it would have to be 6.7 E+11 times bigger, eh to be equal to 1 if that's what you mean.
    Yes, it could be 1 in some other unit system. It doesn't really matter how you would define those units as long as you understand it's an artifact of SI units.
  5. Standard member Ramned
    The Rams
    19 Jan '09 19:47
    Originally posted by sonhouse
    This constant comes up in so many gravity field and lens formula's, just wondered if anyone knew how it was derived and what is it anyway?
    I know it's 6.67 E-11 and all that but what does that mean?
    The constant is derived experimentally, first done by I believe Cavendish. It was a torsion pendulum set-up, you can probably find it on the internet.
    6.67e-11 indicates that it is very weak.
  6. Standard member PBE6
    Bananarama
    19 Jan '09 20:02
    Originally posted by Ramned
    The constant is derived experimentally, first done by I believe Cavendish. It was a torsion pendulum set-up, you can probably find it on the internet.
    6.67e-11 indicates that it is very weak.
    Hey Ramned, whatever happened to your physics question thread? It was fun.
  7. Standard member Ramned
    The Rams
    19 Jan '09 21:22 / 1 edit
    Originally posted by PBE6
    Hey Ramned, whatever happened to your physics question thread? It was fun.
    It's hidden in the Posers and Puzzles somewhere, I left the site for awhile. Though the other day when I was tutoring I ran into a good one - in fact, it relates to gravitation.

    "Show how, guided by Kepler's law of periods, Newton could deduce that the force holding the moon in its orbit (assumed circular) depends on the inverse square of the moon's distance from the center of the Earth."
  8. Standard member sonhouse
    Fast and Curious
    20 Jan '09 10:32
    Originally posted by Ramned
    It's hidden in the Posers and Puzzles somewhere, I left the site for awhile. Though the other day when I was tutoring I ran into a good one - in fact, it relates to gravitation.

    "Show how, guided by Kepler's law of periods, Newton could deduce that the force holding the moon in its orbit (assumed circular) depends on the inverse square of the moon's distance from the center of the Earth."
    Would that be in terms of earth radii? Calling the earth's radii about 4000 miles and the moon 240,000 miles out, 60 radii? 1/60^2 or about 1/3600 of surface pull, so the acceleration of gravity from earth on the moon would be about 9.8/3600 M/S^2 or 2.7E-3 M/S^2 at the moon. I don't have time to go further, am at work, midnight shift, am tending a lot of testing equipment, we produce laser modulators for fiber optic networks, 40 Ghz and up (and down So a one Kg weight on earth would be 0.36 grams at the distance of the moon if there were an elevator that high