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Brain,
The French revolution hadn't happened. This gave us the metric
system, plus Newton's calculus, Fluxion Theory, was based more on geometry
then algebra we use today. In geometry the usual approach to calculations
was to do it by ratio and proportion. The Gravitational constant doesn't
appear until Boys measurements in the late 19th century. Cavendish
experiment determined the density of the earth and not big G.
Hope this helps. Our twenty twenty hindsight often distorts the way we
perceive history.
Gary
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:
phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of brian whatcott
Sent: Thursday, October 28, 2010 7:00 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] How did Newton estimate the Gravitational constant?
On 10/28/2010 5:00 AM, Brian Blais wrote:
Hello,(haven't looked at the Principia in a long time, but always found the
A student asked me this question in class yesterday, and I wasn't sure
arguments a bit hard to follow). I imagine he could do it from a rough
estimate of the mass of the Earth, mass of the Moon, and distance to the
Moon. With the Moon's period you could get a value for G. Is this how he
did it? I know that the direct measurement wasn't done until later, by
Cavendish.
I quickly reviewed Book 1 Section 12 and saw propositions given in the
Further, did he have any way of estimating the distance to the Sun? I
couldn't think of one that was available at his time, but he was more
clever than I. :)
thanks,
bb
usual geometric way of that time, that an inverse square law for force
operates between bodies in elliptical orbit depending on their joint masses
and inversely as the distance squared. I did not see an estimate
for the scaling constant, but I had no time to spare. Is it
established that Newton gave this estimate?
Brian W
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Forum for Physics Educators
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