# Understanding gravity: mass and gravity relation

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• Alexa1618
In summary, the force that two objects of different mass exert on each other is equal, as explained by the formula F=GMm/r^2. This is true on both a small and large scale, with the Earth exerting a stronger gravitational force on objects on its surface due to its larger mass. The concept of equal and opposite forces is easier to observe when the masses are less dissimilar, such as the Earth and the moon. General relativity explains gravity as the curvature of spacetime, providing a stronger explanation for why this equal force exists between objects.
Alexa1618
<< Mentor Note -- thread moved from the Introductions forum to the technical forums >>

Hello there,
This is my first post, I find myself often wondering about the universe and how it works, and there are a couple of things I just can't wrap my head around.

According to wikipedia the force that 2 objects of different mass exerce on each other is equal? How can that be? Is it only speaking on the surface of the Earth because the Earth has a way stronger gravitational force on both the objects? But on a bigger scale I would think that for example the Earth would exerce a much weaker force on the sun than the sun on the earth?

I'm just an amateur no physics background not even science but I am absolutely fascinated by the subject.

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Neeraj Chandran
Welcome to PF.

We do not ask or answer technical questions under "New Member Introductions"

General physics is the place for questions about gravity.

Alexa1618 said:
Is it only speaking on the surface of the Earth because the Earth has a way stronger gravitational force on both the objects?
The force the Earth exerts on a nearby object (like a dropped ball) is exactly equal to the force the object exerts on the earth; the ball is pulling the Earth towards it with the same force that the Earth is pulling the ball towards it. However, the Earth is way more massive than the ball, so the ball accelerates rapidly towards the Earth (we call this "falling") while the motion of the Earth under the influence of this force is completely undetectable.

The equal-and-opposite effect is easier to see when the two masses are less dissimilar; for example, it is easy to observe that the gravitational force that the Earth exerts on the moon is equal and opposite to the force that moon exerts on the earth.

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Ibix
Alexa1618 said:
According to wikipedia the force that 2 objects of different mass exerce on each other is equal?
True.
Alexa1618 said:
How can that be?
In terms of Newtonian gravity, there isn't really an answer. It just is that way. You can make up plausibility arguments suggesting it's so, but they're post hoc justifications.

For example, it's 93 million miles to the Sun. Think about the force between two atoms 93 million miles apart. The situation is symmetric so the forces each one exerts on the other must be the same. Now stick another atom to one of the two. Each one exerts the same force on the single atom, so the force on the single atom is doubled. But each one feels the same force from the single atom, so the total force on the pair of atoms is doubled. Keep adding atoms and the argument repeats, until you have a Sun in one place and a planet or a person at the other - both exerting the same force on each other.

Of course, my mass is a lot less than that of the Sun, so the acceleration I get from the Sun is enough to curve my path into an orbit, whereas the same force applied to the Sun won't even make it wobble.

General relativity doesn't model gravity as a force, but rather as spacetime curvature. That ends up in a rather stronger explanation for why any theory that treats gravity as a force must be the same between any pair of bodies, but the explanation is a bit more involved.

Alexa1618 said:
<< Mentor Note -- thread moved from the Introductions forum to the technical forums >>

Hello there,
This is my first post, I find myself often wondering about the universe and how it works, and there are a couple of things I just can't wrap my head around.

According to wikipedia the force that 2 objects of different mass exerce on each other is equal? How can that be? Is it only speaking on the surface of the Earth because the Earth has a way stronger gravitational force on both the objects? But on a bigger scale I would think that for example the Earth would exerce a much weaker force on the sun than the sun on the earth?

I'm just an amateur no physics background not even science but I am absolutely fascinated by the subject.
Yes you are right... The two forces are same and is of the magnitude F=GMm/r^2 . Actually both the object gets dragged to the point of their centre of mass. As given in one example a ball excerts the same force as that of the Earth and both of them gets closer to its Centre of mass under their mutual force of attraction. The ball comes faster under the force because of less mass and Earth is too slow due to its high mass even if the same force is acted (in this case centre of mass lies very near to the centre of Earth due to Earth's high mass.Therefore the movement is sooo small for the earth. If Force is constant then accileration is inversely proportional to mass. Here also it obeys)
I think you may have understood it. Let me give you another impact of this. It is that Jupiter doesn't orbit sun. Actually both sun and Jupiter and orbiting a common point called barycentre. Because Jupiter has a comperable mass as that of the sun even if it is too small. So the Centre of mass lies outside the sun and it explains the above phenomenon.

Neeraj Chandran said:
Actually both sun and Jupiter and orbiting a common point called barycentre.

well, all planets and the sun do this, as do all the planets and their moons

davenn said:
well, all planets and the sun do this, as do all the planets and their moons
Yes it is correct...i just said an example to explain the asked question as in the solar system, sun and Jupiter have its barycentre ouside both the bodies and are vice versa for others.

## 1. What is gravity?

Gravity is a fundamental force of nature that causes objects with mass to attract each other. It is responsible for keeping planets in orbit around the sun and objects on Earth from floating off into space.

## 2. How does gravity work?

Gravity works by creating a force between two objects with mass. The strength of this force is determined by the mass of the objects and the distance between them. The larger the mass and the closer the distance, the stronger the force of gravity.

## 3. What is the relation between mass and gravity?

The relation between mass and gravity is that the greater the mass of an object, the greater its gravitational pull. This means that objects with larger mass will have a stronger force of gravity and will attract other objects with greater force.

## 4. How does mass affect gravity?

Mass affects gravity by determining the strength of the gravitational force between two objects. The more massive an object is, the stronger its gravitational pull will be. This is why larger objects, such as planets, have a stronger gravitational pull than smaller objects, such as rocks.

## 5. Can gravity be explained by Einstein's theory of relativity?

Yes, Einstein's theory of relativity provides a more accurate and comprehensive explanation of gravity compared to Newton's theory. It explains gravity as the curvature of space-time caused by the presence of mass, rather than a force between two objects. This theory has been proven through various experiments and observations.

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