Chronology Current Month Current Thread Current Date [Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

# Re: [Phys-l] How did Newton estimate the Gravitational constant?

On Oct 29, 2010, at 2:56 PM, Herbert Schulz wrote:

On Oct 28, 2010, at 9:59 PM, Jack Uretsky wrote:

So Newton had a value for M_{E}G. He could estimate M_{E} from knowing g
at the earth's surface.
Regards,
Jack

Howdy,

I guess I don't understand this. How do you get M_{E} from M_{E}G/r_{E}^2=g without already knowing the value of G? Maybe I just missed something.

My impression is that knowing the value of M_{E}G by knowing r_{E} and g at the surface of the Earth is all you need to estimate the period of the moon around the earth and that pretty much agrees with measurement.

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)

Howdy,

Maybe I should have been more explicit and stated that you need M_{E}G and the estimated radius of the Moon's orbit around the Earth to find the expected period of the Moon around the earth:

T = 2\pi r\sqrt{r/M_{e}G}

where r is the radius of the Moon's orbit. This assumes a circular orbit but still gives a reasonable value for the period. Note that you don't need the mass of the Moon which drops out of the equation.

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)