Gravity just a theory?

1. Aug 1, 2010

sketchy21

HI all, I have been in a debate with a friend over this for some time and I braught up the fact gravity is just a theory and a theory created by humans and humans are 'unstable' variables with many errors. Thus the question was spawned:
Is the value of gravity always constant and she said yes (as most would) but how can gravity (or anything for that matter) remain a CONSTANT value when there are so many variables that are not constant in this universe that would impact on the value, for example there are 7 different (atleast 7) ways inwhich the earth is spinning (centripital force) and the distance from the centre of the earth is not the same all over the earth? so my question to you all is,
what do you think, do you believe that the value of G is ALWAYS 9.81 m/s squared or do you agree with me that gravity can change and most likely does change depending on many things and we are talking about the value of gravity here. thank you
Sketch

2. Aug 1, 2010

xxChrisxx

Well this is a tough one, as you are right, but for the wrong reasons.

Gravitational constant in Newtonian description of gravity IS constant. The 'G':
F = GMm/R^2
The only variables that affect the 'pull' felt and the two masses and the distance of the objects.

But like with all mathematical models it doesn't describe the phenomenon fully. When you get to very large or very fast scales you have to use General relativity.

However, the value for gravity ie 9.81m/s^2 is by no means constant. 9.81 is just a convenient number that people use for gravity, like we use 293 K @ 1atm for standard temperaute and pressure. In reality the value of gravity changes even on the earth.

You also have to be very careful, as the spin of the earth doesn't affect the acutal pull of gravity, only its net effect. So for example if you were pulling in 1 direction with 100N and I was pulling in the other with 50N, measurements would show 50N net force. This doesn't mean you are pulling less.

http://en.wikipedia.org/wiki/Gravity_of_Earth

3. Aug 1, 2010

sfsy1

The gravitational constant, G, as its name implies, is always constant. However in reality, there are many other factors that influences the net pull on an object, such as the non-uniform density of the Earth, imperfect spherical shape of the Earth and even the gravitational pull from other objects such as the moon.

4. Aug 1, 2010

D H

Staff Emeritus
First off, you are confusing big G, the universal gravitational constant, and little g, the gravitational acceleration toward the Earth.

Strictly speaking, both are constant (as far as we can tell). Little g is constant because it is a defined value: g=9.80665 m/s2 by definition.

Big G, the universal gravitational constant is about 6.6743x10-11 m3kg-1s-2. Whether G truly is a constant of nature, is like any scientific theory, only tentatively correct. It is a physical constant in both Newtonian mechanics and in general relativity, and that it is a constant has been tested and retested and re-retested at many different scales.

So, back to little g. This has multiple meanings, one of which is the rather artificially defined value of 9.80665 m/s2 (exactly). Another meaning is the gravitational acceleration toward the Earth, and this is not constant -- even on the surface of the Earth. The artificially defined value is often denoted as g0 to distinguish it from this varying value g. This g varies with latitude, altitude, the local density of rock, etc. g is about 9.789 m/s2 at the equator at sea level while it is about 9.832 m/s2 at the poles. In Mexico City g is about 9.779 m/s2. In general, g decreases with distance from sea level. For an astronaut aboard the space station, g is about 0.9 g0. The value drops off the further one gets from the Earth.

5. Aug 1, 2010

sketchy21

THANK YOU ALL SOO MUCH!!! I knew I was right (I'm the worst at explaining but in your explanations is my argument) thank you! I knew it, my argument was that G is not constant it's just what people use but in reality it is not and would you all agree that NOTHING in the known universe is at all constant?
I'm not talking about mass being constant if you discount air resistance or something like that because in reality... we simply cant "say" something is discounted and it makes it so which is what most people commonly think and so they disclose variables at will and that is why some of my teachers hated me because they couldn't accept my views that we shouldn't just disclose "our" chosen variables but in all situations there a so many variables and thus things CAN be constant.... IF WE SAY THEY ARE but not in reality.
Thank you all

6. Aug 1, 2010

D H

Staff Emeritus
Whoa! As noted by everyone above, G is a constant. Using the correct terminology is very important. You are reading things into our posts that were not there.

7. Aug 1, 2010

sketchy21

ok sorry, I wasn't, I'm just bad a explaining. I was opening another queston, I'm stating that nothing is constant which by the G being constant statement I believe you consider me wrong?

8. Aug 1, 2010

sketchy21

also you said something about g being about 0.9 in space, wouldn't this too be ever changing (if by the slightest) depending on where you are in space or distance from the sun?

9. Aug 1, 2010

D H

Staff Emeritus
Yep. I am disagreeing with you. As far as we can tell, G is constant. G is the universal gravitational constant (google that phrase) -- and "constant" here means that it does not vary over time, over distance, or from object to object.

You appear to still be confusing little g and big G. These are very different things.

Last edited: Aug 1, 2010
10. Aug 1, 2010

sketchy21

how can anything be constant (non changing and non affect and always the same)?

11. Aug 1, 2010

Staff: Mentor

Why should anything not be constant?

Note also that you are misunderstanding what "just a theory" means as well as misunderstanding that gravity is both a theory and a factual phenomena.

12. Aug 1, 2010

Dr Lots-o'watts

If physics, the people who are right are those who can show things experimentally and come out with useful equations.

Many people have kept showing over many years that G being constant is part of a very useful theory.

Unless (or until, if you prefer) there is any valid reason to complicate things, you don't.

13. Aug 1, 2010

I think as D H pointed out you may be a little confused over the terminology being used. As you travel further away from the Earth, the Earth's gravity will "pull" you less. Gravity itself will not change.

There are some things that don't change no matter how we observe them. Many times, we can even see the effects of these phenomenon when we observe unrelated things.

The "How is this possible?" part of the question is philosophical in nature. You might not find an answer that satisfies you.

Last edited by a moderator: May 4, 2017
14. Aug 2, 2010

ACPower

How can nothing be constant? If everything always changes, then change itself is a constant, which is, presumably, impossible. So at least one thing should be constant.

Sophistry aside, I believe there are some cosmological models that have posited a value of G that has changed over time...

15. Aug 2, 2010

mikelepore

I've never heard of such a thing. Do you have a link to some literature online? The value of g depends on what planet or moon you are on. Even if you put "planet earth" into the definition, it depends on your altitude, and even the density of the ground beneath you. Even if you put "the average on the surface of the earth" into the definition, then the actual number is an empirical measurement.

16. Aug 2, 2010

xxChrisxx

Little 'g' it's sometimes written g0 is a standard value which is defined as 9.80665m/s^2. It's not a constant in the same way big G is, it's a defined value. This is by FAR the most common usage when you see g, so much so that noone ever bothers putting g0 any more, or defining that it's on earth.

It's standardised for precisely the reasons you stated above, so you don't have to bother figuring out altitude or where you are.

EDIT: Also read DH's entire post, this is all VERY clearly explained in the exact same post.

Last edited: Aug 2, 2010
17. Aug 2, 2010

Staff: Mentor

Ehh, not really, Dave. A law is a a mathematical relation in a model and there is no implied level of 'correctness' to a law. It can be severely limited/badly flawed. Scientifically, it is inferior to a theory.

"Gravity" is an observed phenomena for which there exists:
-Newton's law of gravitation. This is a mathematical relation that Newton found which is limited/flawed. It is superceded by:
-Einstein's theory of gravitation. This has a mathematical model which is much more accurate than Newton's, contains an idea about where gravity comes from, and makes predictions beyond what is already known.

One needs to be specific about which 'gravity' they are talking about - the law, the theory, or the phenomena. But since the law is largely obsolete, it isn't saying much about our level of understanding of gravity to refer to it.

18. Aug 2, 2010

DaveC426913

Why, whatever are you talking about Russ?

19. Aug 2, 2010

mikelepore

If you "define" a value for such a measurement, then it's very precise, but it refers only to itself and and not to the natural object. I could say that the height of Mt. Everest is exactly h0=10.0 km by definition, and then I could point out everyone's right to define something called h0=10.0 km, but that doesn't reflect the mountain itself.

20. Aug 2, 2010

D H

Staff Emeritus
From http://www.bipm.org/en/CGPM/db/3/2/,
1. The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram;
2. The word "weight" denotes a quantity of the same nature as a "force": the weight of a body is the product of its mass and the acceleration due to gravity; in particular, the standard weight of a body is the product of its mass and the standard acceleration due to gravity;
3. The value adopted in the International Service of Weights and Measures for the standard acceleration due to gravity is 980.665 cm/s2, value already stated in the laws of some countries.
Aside: Note that (2) conforms with the meaning of "weight" as defined in some (but not all) physics texts and some (but not all) countries. In the US, for example, "weight" is legally a synonym for mass, not force.

21. Aug 3, 2010

xxChrisxx

But Everest isn't 10km exactly, and there is only one Everest it doesn't make sense to define it as anything else. I understand what you are saying (it was just an odd axample to use).

It's like having a hundered eggs of similar but marginally different sizes, you could them define a standard 'egg' for use in all further calcualtions. This is what happens in cook books. The standard value of 'large egg' doesn't define the egg itsself but makes conveying recipies more convenient.

This was DH's point, the big G really IS a constant. Little 'g' in everyday useage is a defined value as we usually only talk about g on earth so is effectively constant for practical use, but he then explains that real gravity values alter depending on a huge list of variables.

22. Aug 3, 2010

D H

Staff Emeritus
Of course. I specifically did not say that acceleration due to gravity everywhere on the Earth is 9.80665 m/s2. I very explicitly gave some examples to the contrary (the equator, the poles, and Mexico City).

Nonetheless, a very specific defined value comes in very handy, and is absolutely essential for some systems of measure. Newton's second law in SI units is F=ma. Newton's second law is not F=ma in general. Newton's 2nd says force is proportional to the product of mass and acceleration, or F=kma. The constant of proportionality is 1 kgf/(9.80665 kg·m/s2) if you express force in kilogram-force and mass in kilograms. A similar nastiness occurs in the English system. The pound force is defined as the force needed to accelerate a mass of one pound (avoirdupois) by 9.80665 m/s2, exactly. And yes, the official US definition of the pound force does mix units.

23. Aug 4, 2010