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*From*: Jack Uretsky <jlu@hep.anl.gov>*Date*: Thu, 28 Oct 2010 22:15:03 -0500 (CDT)

The Mechanical Universe text and videos describe the Cavemdish experiment,

moer picturesquely, as "weighing the earth."

Regards,

Jack

"Trust me. I have a lot of experience at this."

General Custer's unremembered message to his men,

just before leading them into the Little Big Horn Valley

On Thu, 28 Oct 2010, Daryl Taylor wrote:

Ahh, I stand corrected... Sorta... Cavendish's GOAL was to determine the

density of the earth in 1798, but he was the first to experimentally

determine the actual force between two masses in a laboratory and thereby

actually calculated 'G' implicitly without stating it as a number. (Even

though my AP textbook says he did... sigh...) He used ratios of two forces

which allowed for calculations without 'G'. His experiment did indeed lead

others to accurately calculate 'G'. According to the wonderful world of

Wiki, the first mention of 'G' as an accepted numerical value was in 1873 by

Baille & Cornu, roughly 75 yrs after Cavendish did his work.

See? I lied to my own students... Now, will I be big enough to correct this

next class? Of course...

On Thu, Oct 28, 2010 at 8:21 AM, Karshner, Gary <gkarshner@stmarytx.edu>wrote:

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

Phys-l@carnot.physics.buffalo.edu

https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

--

Daryl L Taylor

(Sent from my Droid using 100% certified recycled electrons...)

_______________________________________________

Forum for Physics Educators

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**References**:**[Phys-l] How did Newton estimate the Gravitational constant?***From:*Brian Blais <bblais@bryant.edu>

**Re: [Phys-l] How did Newton estimate the Gravitational constant?***From:*brian whatcott <betwys1@sbcglobal.net>

**Re: [Phys-l] How did Newton estimate the Gravitational constant?***From:*"Karshner, Gary" <gkarshner@stmarytx.edu>

**Re: [Phys-l] How did Newton estimate the Gravitational constant?***From:*Daryl Taylor <daryl261@gmail.com>

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