# Gravity question

## Main Question or Discussion Point

does gravity only have a certain amount of force it can have to pull matter towards it?

for example lets say there is a planet with nothing on it but natural features (like mars) and you put one person on it... the gravity of mars is now using its force on that person to keep them planted on the ground. is the fact that the gravity is controlling that person, now weaken the gravity as a whole? I know one person would not have any noticible difference, but that is just an example.. lets say jam packed on the entire surface, you have people. is the force now less because it is being used?

sorry if its confusing.. it makes more sense when i think about it than type it

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chroot
Staff Emeritus
Gold Member
No. There is not some "gravity storage" inside Mars, of which Mars uses a little to hold each person. All that matters in gravity are these things:

1) The mass of the first body. Say, Mars. Call it M.
2) The mass of the second body. Say, Arnold Schwarzenegger. Call it m.
3) The distance between them. Say, the radius of Mars. Call it r.
4) The gravitational constant, which is a feature of the entire universe (think of it as simply a conversion factor that relates our human units.) Call it G.

The formula for force (F) is:

F = (G M m) / r^2

As you can see, as long as you don't change the mass or size of Mars, it can hold as many people on it as you can pack in.

- Warren

F = (G M m) / r^2
Yes, this is the traditional understanding of gravity. But do not forget that all objects have the same acceleration in free fall. The reason is that gravity is from its origin not a force but an acceleration.

We know very precisely that the speed of light is reduced in a gravitational field. As a consequence every fast moving object (like a photon) is subject to refraction. It will be bended towards the source of gravitation (e.g. the Mars).

Also the particles which oscillate inside of an elementary particle are subject to refraction. This causes the gravitational acceleration.

chroot
Staff Emeritus
Gold Member
Originally posted by Albrecht
Yes, this is the traditional understanding of gravity. But do not forget that all objects have the same acceleration in free fall. The reason is that gravity is from its origin not a force but an acceleration.
Wrong. Gravitation is from its origin a result of geometry. This has nothing to do with the question posed, which is entirely explainable within the bounds of Newtonian gravity.
We know very precisely that the speed of light is reduced in a gravitational field.
Wrong. This does not happen at all. All observers, everywhere, even those deep inside gravitational wells, will measure c the same.
As a consequence every fast moving object (like a photon) is subject to refraction. It will be bended towards the source of gravitation (e.g. the Mars).
Wrong. Light is subject to gravitation because gravitation is the result of the curvature of spacetime. Light follows the shortest paths in curved spacetime, and thus bends.
Also the particles which oscillate inside of an elementary particle are subject to refraction. This causes the gravitational acceleration.
I have no idea what this means. Elementary particles are not composite -- by definition, and we have no currently acceptable theory of gravitation at the scale of elementary particles.

- Warren

chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
This is a good question. I think the answer is yes. Just like
protons in the nucleus only has enough force to support a
certain number of negative charges, when the force of gravity
is being used to move some mass, there becomes less in the system
to be used- basic conservation of energy.
Why post what you THINK? How useful are your THOUGHTS in the General Physics forum? If you want to debate your THOUGHTS, go to a theory development forum.

The fact is there is no conservation of force, only of energy. What kleinma suggested is absolutely, profoundly wrong.

- Warren

Eyesee
Originally posted by kleinma
does gravity only have a certain amount of force it can have to pull matter towards it?

for example lets say there is a planet with nothing on it but natural features (like mars) and you put one person on it... the gravity of mars is now using its force on that person to keep them planted on the ground. is the fact that the gravity is controlling that person, now weaken the gravity as a whole? I know one person would not have any noticible difference, but that is just an example.. lets say jam packed on the entire surface, you have people. is the force now less because it is being used?

sorry if its confusing.. it makes more sense when i think about it than type it
Yes, if energy is actually conserved. It's like the situation
with the protons in the nucleus- it can only support so many
electrons (since energy is conserved).

chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
Yes, if energy is actually conserved. It's like the situation
with the protons in the nucleus- it can only support so many
electrons (since energy is conserved).
This is also incorrect. All atoms can be ionized, both positively and negatively. Stop posting nonsensical crap.

- Warren

Eyesee

Originally posted by chroot
Why post what you THINK? How useful are your THOUGHTS in the General Physics forum? If you want to debate your THOUGHTS, go to a theory development forum.

The fact is there is no conservation of force, only of energy. What kleinma suggested is absolutely, profoundly wrong.

- Warren
Force has to be conserved if it depends on the particles
emanating the force. Imagine the situation in the atom-
if the force of attraction of protons arent' conserved,
you can pack an infinite amount of electrons in an atom
using just one proton.

chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
Force has to be conserved if it depends on the particles
emanating the force. Imagine the situation in the atom-
if the force of attraction of protons arent' conserved,
you can pack an infinite amount of electrons in an atom
using just one proton.
Newsflash: YOU CAN.

- Warren

Eyesee

Originally posted by chroot
This is also incorrect. All atoms can be ionized, both positively and negatively. Stop posting nonsensical crap.

- Warren
Yes, but atoms can also be neutral, why? Because the positive
force of the protons are all used up. You have to ionize
(get rid of an electron) the atom for there to be more
force available for use.

chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
Yes, but atoms can also be neutral, why? Because the positive
force of the protons are all used up. You have to ionize
(get rid of an electron) the atom for there to be more
force available for use.
Saying it's true again and again doesn't make it true.

- Warren

Eyesee

Originally posted by chroot
Newsflash: YOU CAN.

- Warren
No you can't, a hydrogen atom can only support 2 electrons
before it becomes unstable.

chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
No you can't, a hydrogen atom can only support 2 electrons
before it becomes unstable.
Wrong.

- Warren

Eyesee

Originally posted by chroot
Saying it's true again and again doesn't make it true.

- Warren
My saying it true doesn't make it true but the fact
that a positive charge can only support two negative
charges (hydrogen atom) and not 3 or more, makes it
true. If the force wasn't conserved, a proton should be
able to support more than 2 electrons.

chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
My saying it true doesn't make it true but the fact
that a positive charge can only support two negative
charges (hydrogen atom) and not 3 or more, makes it
true. If the force wasn't conserved, a proton should be
able to support more than 2 electrons.
Why do you think this is true? Who told you this? What reference do you have?

- Warren

Eyesee

Originally posted by chroot
Why do you think this is true? Who told you this? What reference do you have?

- Warren
My reference is logical interpretation of the physical
experiments. Do you deny that a proton can only support
two electrons, as in the situation with the hydrogen
atom? If so, how can you say that force is not conserved?
Where is the positive force outside of a hydrogen
atom? Or for that matter, where is the negative force? They
are all being used to stabilize the atom. Only after
you ionize the hydrogen atom (get rid of an electron) can you have
more force to attract an (and only enough for one) electron.

Last edited by a moderator:
chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
My reference is logical interpretation of the physical
experiments. Do you deny that a proton can only support
two electrons, as in the situation with the hydrogen
atom? If so, how can you say that force is not conserved?
Where is the positive force outside of a hydrogen
atom?
Yes, I vehemently deny that. Show me an experiment that corroborates your position, or shut up.

- Warren

chroot
Staff Emeritus
Gold Member
You provided links to online periodic tables. I see nothing that corroborates your assertion that H-- can't exist.

- Warren

Janus
Staff Emeritus
Gold Member
Originally posted by kleinma
does gravity only have a certain amount of force it can have to pull matter towards it?

for example lets say there is a planet with nothing on it but natural features (like mars) and you put one person on it... the gravity of mars is now using its force on that person to keep them planted on the ground. is the fact that the gravity is controlling that person, now weaken the gravity as a whole? I know one person would not have any noticible difference, but that is just an example.. lets say jam packed on the entire surface, you have people. is the force now less because it is being used?

Actually, the force acting on each person on the jam-packed world wouldbe greater than that working on the one standing on the planet alone. Because now not only does each person have Mars pulling on them, put they also have the mass of each of the other people pulling on them.

Think about it, if the force of gravity weakened by how much it had to hold, then larger planets would have weaker gravity because more and more of the force of gravity it has would be used up just to hold the planet itself together. But this is not the case, as we see that larger planets have stronger gravity.

Originally posted by chroot
No. There is not some "gravity storage" inside Mars, of which Mars uses a little to hold each person. All that matters in gravity are these things:

1) The mass of the first body. Say, Mars. Call it M.
2) The mass of the second body. Say, Arnold Schwarzenegger. Call it m.
3) The distance between them. Say, the radius of Mars. Call it r.
4) The gravitational constant, which is a feature of the entire universe (think of it as simply a conversion factor that relates our human units.) Call it G.

The formula for force (F) is:

F = (G M m) / r^2

As you can see, as long as you don't change the mass or size of Mars, it can hold as many people on it as you can pack in.

- Warren
so if you use the radius as the distance, that would mean that gravity originates from the very center of the planet correct? has to do with the spherical shape planets have and all that right??

chroot
Staff Emeritus
Gold Member
Hi kleinma,

that would mean that gravity originates from the very center of the planet correct?
That's a very astute observation -- I wasn't sure if my saying it would confuse you earlier, so I left it out. I'm happy that you arrived at that conclusion on your own, because it is very remarkable and very correct.

In general, any body can be viewed as a point mass when you're far enough away from it. What I mean is that, when you're far away from a body, it's force on you is exactly as if all its mass is concentrated at one point. This point is called, creatively, the center of mass.

For a uniform density sphere, the center of mass is right at the center of the sphere. So, when you're outside the sphere, you will be attracted to it just as you would be to a point mass at the same distance.

As an example, let's figure out the acceleration due to gravity on Mars. You know the acceleration due to gravity on Earth is 9.8 m/s^2, and you know that Mars is quite a bit smaller and less massive than the Earth, so we should expect a smaller acceleration.

Let's imagine a test mass, say, 1 kg, at a distance r from the center of Mars. If it's sitting on the surface of Mars, it is 3,397 km from its center of mass. Mars is known to have a mass of 6.4219 x 10^23 kg.

So that's all we need to calculate the force: the masses of the two bodies and their separation. Let's call the test mass's mass m, Mar's mass M, and the distance r, as usual. The force experienced by both Mars and the test mass is:

F = (G M m) / r^2 = approximately 3.71 newtons of force.

That's right -- the test mass is pulled toward the center of Mars, and Mars is pulled toward the center of the test mass with the same force! However, such a small force can scarcely accelerate Mars due its enormous mass, so we generally don't worry about its movement. In contrast, our little test mass moves quite a bit and can hurt your toes and such.

The test mass experiences an acceleration of

a = F / m

Plugging in F as found above leads us to:

a = (G M m) / (m r^2)
= (G M) / (r^2)

That's right -- the test mass's mass cancels. Just as Galileo found, it doesn't make any difference what the test mass is -- all bodies fall the same speed in the absence of air resistance.

So what is the resulting acceleration? Plugging in the numbers yields, of course, 3.71 meters per second squared, which agrees quite nicely with the numbers in the books.

Does all this make sense? Feel free to ask questions if you have them.

- Warren

Eyesee

Originally posted by Janus
Actually, the force acting on each person on the jam-packed world wouldbe greater than that working on the one standing on the planet alone. Because now not only does each person have Mars pulling on them, put they also have the mass of each of the other people pulling on them.

Think about it, if the force of gravity weakened by how much it had to hold, then larger planets would have weaker gravity because more and more of the force of gravity it has would be used up just to hold the planet itself together. But this is not the case, as we see that larger planets have stronger gravity.

No, in both cases the larger planets would have stronger gravity
since the larger planets have larger surface areas. It's just
that in one scenario, the gravitational constant would be less. The conceptual difference between the two views is that in one, gravitational force penetrates matter, while my view is that it doesn't. How can gravitational force react with matter if
it goes through it? If you drilled a hole through a ball so that
you can push a thread through it, how can you pull the ball assuming

chroot
Staff Emeritus
Gold Member

Originally posted by Eyesee
...while my view...
You view is not supported by any theory or evidence, and does not belong in this forum.

- Warren

Eyesee
Or think of it a different way, as fishing lines. If you only
have 5 fishing poles, how many fish can you catch? If after hooking
5 fish, suddenly 5 more fishing poles appear right before your
eyes so that you can catch 5 more fish, isn't that magic? It certainly doesn't conserve the number of fishing poles that's for sure.