Talk:Gravitational constant

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In imperial units

What is the gravitational constant (6.67408 * 10^-11) converted to imperial units (in ft and lb instead of m and kg)? 212.186.0.174 (talk) 12:58, 19 May 2019 (UTC)[reply]

Alright, I figured out that the imperial gravitational constant is 1.0691 * 10^-9 ft^3 lb^-1 s^-2. Maybe one should add it to the page. 212.186.0.174 (talk) 16:43, 1 June 2019 (UTC)[reply]

No one with a sane mind uses imperial units for scientific calculations. --mfb (talk) 23:13, 1 June 2019 (UTC)[reply]

And I'm obviously so insane that I have to ask you "why not?" 212.186.0.174 (talk) 04:51, 2 June 2019 (UTC)[reply]

Science is done basically exclusively in metric because it is much easier to convert units there and because 95% of the world population uses metric everywhere. --mfb (talk) 05:06, 2 June 2019 (UTC)[reply]

With pounds there are two competing definitions. One is that it is a unit of mass and one is that a unit of force, and which is more common has shifted over the years (IMHO). You'll get a different value for

G

in imperial units depending upon which definition you are using. —

Q

uantling (talk | contribs) 13:57, 28 March 2023 (UTC)[reply]

With force pounds,

G = 3.4397 \times 10 -8 ft 4 lbf -1 s -4

. —

Q

uantling (talk | contribs) 16:30, 29 March 2023 (UTC)[reply]

Sign of the force

In a comment, @TowardsTheLight: writes "the text defines F as the attractive force. No negative sign is needed for this case where the force is defined to be attractive". I agree that no negative sign is needed when the force is defined to be attractive. However, my experience (and maybe yours too?) is that Gmm/r^2 gravitational force is almost always written with a negative sign. I don't think that we are doing the reader any favors by going counter to this convention. I would rather go with convention by giving the minus sign in the equation, and adjust the text so that it works with the equation. Might something along those lines work? — $Q$ uantling (talk | contribs) 21:46, 27 March 2023 (UTC)[reply]

As far as I recall, most books don't write the minus sign. See, for instance MacDougal. And Feynman, Ohanian and Hewitt don't do it either in their basic books. - DVdm (talk) 22:12, 27 March 2023 (UTC)[reply]

Ohanian & Ruffini Gravitation and Spacetime has the minus sign. Halliday and Resnick does not. —

Q

uantling (talk | contribs) 22:25, 27 March 2023 (UTC)[reply]

I have Ohanian's Physics, vol 1, Second Edition here before me, chapter 9, section 9.1, eq (1), page 212. No minus sign. Just the (standard) non-vectorial form. - DVdm (talk) 22:35, 27 March 2023 (UTC)[reply]

Textbooks are what matter when it comes to Wikipedia, but I can't help venting anyway. Gravitational potential energy is negative,

-{\frac {GMm}{r}}

; we can at least agree on that, right? And force is the negative gradient of the potential. And

-{\frac {d}{dr}}\left(-{\frac {GMm}{r}}\right)=-{\frac {GMm}{r^{2}}}

, with the minus sign. Sure, calling the force as positive but in the opposite direction works, but it will confuse the student who is learning the canonical ways of dealing with potential energy, conserved forces, Hamiltonians, etc. —

Q

uantling (talk | contribs) 01:42, 28 March 2023 (UTC)[reply]

My view is that defining the force so that it is positive in the direction of r increasing is more mathematically logical. The equation then requires a minus sign. It is, however, a more difficult concept for readers who are fairly new to the concepts of the article. I can see that there is an argument in favour of defining the force as attractive, so as to avoid having a minus sign in the equation, given that the article is about the gravitational constant rather than the Newtonian law of gravitation.

However, I'd like to say that the article as a whole is quite messy. TowardsTheLight (talk) 12:49, 28 March 2023 (UTC)[reply]

Apologies for reverting the text, but the edit changed the meaning in a way that was incorrect. The text had been written in a way that meant that the force F was positive, so the formula does not have a minus sign. TowardsTheLight (talk) 12:41, 28 March 2023 (UTC)[reply]

What if we insert "magnitude" into the text? Because the formula doesn't give the force vector, that might be appropriate even without the debate about the sign. Using "magnitude" justifies the use of the positive sign to people like me who would otherwise be supporting the negative sign.

According to Newton's law of universal gravitation, the magnitude of the attractive force ( $F$ ) between two point-like bodies is directly proportional to the product of their masses, $m 1$ and $m 2$ , and inversely proportional to the square of the distance, $r$ , between their centers of mass:

Thoughts? —

Q

uantling (talk | contribs) 13:05, 28 March 2023 (UTC)[reply]

I would be happy to have either a reference to the magnitude of the force, or the current wording. TowardsTheLight (talk) 15:35, 28 March 2023 (UTC)[reply]

Perhaps "strength" would be more accessible than "magnitude"; when talking about force, they mean pretty much the same thing.

According to Newton's law of universal gravitation, the strength of the attractive force ( $F$ ) between two point-like bodies is directly proportional to the product of their masses, $m 1$ and $m 2$ , and inversely proportional to the square of the distance, $r$ , between their centers of mass:

Is that better? @TowardsTheLight: are you using "reference" in a strict sense, meaning that you'd want the modification to be within <ref>...</ref> markup, or would the change I am proposing meet with sufficient approval? —

Q

uantling (talk | contribs) 16:18, 29 March 2023 (UTC)[reply]

That would be fine. Personally, I would prefer magnitude to strength, because magnitude is something that novices would understand as a word, even if they do not understand its mathematical meaning, while people with a proper understanding of the subject would understand magnitude in its full meaning. But either strength or magnitude would be fine. And I meant "reference" in a general sense, not as a source of information. TowardsTheLight (talk) 19:24, 29 March 2023 (UTC)[reply]

I edited the article and went with "magnitude". If you revert, please say why here. Thank you —

Q

uantling (talk | contribs) 19:17, 30 March 2023 (UTC)[reply]

That's absolutely fine. The statement is correct in all all respects. TowardsTheLight (talk) 19:39, 1 April 2023 (UTC)[reply]

"Magnitude" is fine. Wcmead3 (talk) Wcmead3 (talk) 04:12, 25 April 2023 (UTC)[reply]

Time variation

The "controversial" suggestion about a 5.9 day variation in G is not worth including. There is better evidence against it than for it. Plus, it is theoretically unsupported.

I suggest removal.

The astronomical evidence against time variation of G is worth mentioning. Wcmead3 (talk) Wcmead3 (talk) 04:09, 25 April 2023 (UTC)[reply]

Discrepancy in G measurements

This is an intriguing puzzle that has persisted for several years. If there is significant research reported that helps resolve the matter, it should be described and referenced in this article. Wcmead3 (talk) Wcmead3 (talk) 04:15, 25 April 2023 (UTC)[reply]