# Newton's gravitational constant, what is it?

sonhouse
Science 16 Jan '09 08:24
1. 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. 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. 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. 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. Ramned
The Rams
19 Jan '09 21:221 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. 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 downðŸ™‚ So a one Kg weight on earth would be 0.36 grams at the distance of the moon if there were an elevator that highðŸ™‚