# Gravity as a force

Why is gravity always attractive in nature ? mfb
Mentor
Physics cannot answer "why" questions on a fundamental level. If you ask "why" long enough, you'll always get to "because we observe that the universe is like that".

As simplified description, gravity acts on the energy of objects - most objects have their mass as largest contribution to their total energy. Energy is always positive (this is an observation - but a universe with negative energies would look completely different), so gravity always attracts.

• NowsTheTime and petrushkagoogol
For the same reason time always moves from past to future.

• Fervent Freyja
Andrew Mason
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Physics cannot answer "why" questions on a fundamental level. If you ask "why" long enough, you'll always get to "because we observe that the universe is like that".

As simplified description, gravity acts on the energy of objects - most objects have their mass as largest contribution to their total energy. Energy is always positive (this is an observation - but a universe with negative energies would look completely different), so gravity always attracts.
I agree that, at least for now, gravity appears to be a fundamental phenomenon. I think I know what you are saying about energy being positive (as in ##E = \sqrt{(pc)^2 + (mc^2)^2}##) but of course energy itself is often expressed as a negative quantity (e.g. binding energy, potential energy).

For the same reason time always moves from past to future.
I think we know why time always moves from the past to the future. I don't think we yet fully understand why (or perhaps even if) mass always attracts other mass by gravity.

AM

• petrushkagoogol
Maybey because their is no other attraction force in our galaxy and not only because the Gravity pull of one is more powerfull than the other.

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Khashishi
Because masses are always positive.

Andrew Mason
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Because masses are always positive.
So are positive charges. But they don't attract each other.

AM

• davenn
phinds
Gold Member
Maybey because their is no other attraction force in our galaxy and not only because the Gravity pull of one is more powerfull than the other.
This makes no sense. Can you clarify what you are saying? Do you not believe that positive and negative charges attract each other?

I agree that, at least for now, gravity appears to be a fundamental phenomenon. I think I know what you are saying about energy being positive (as in ##E = \sqrt{(pc)^2 + (mc^2)^2}##) but of course energy itself is often expressed as a negative quantity (e.g. binding energy, potential energy).

I think we know why time always moves from the past to the future. I don't think we yet fully understand why (or perhaps even if) mass always attracts other mass by gravity.

AM
expressing potential energy as a negative quantity is a convention....not a requirement

Andrew Mason
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expressing potential energy as a negative quantity is a convention....not a requirement
Yes. But unless you want to treat gravitational potential energy as decreasing with increasing separation distance, gravitational potential energy has to be expressed as a negative. So it is not an arbitrary convention.

AM

jbriggs444
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Yes. But unless you want to treat gravitational potential energy as decreasing with increasing separation distance, gravitational potential energy has to be expressed as a negative. So it is not an arbitrary convention.
The point being made is that only differences in gravitational potential energy are physically significant. The numeric value is irrelevant. One is free to choose a baseline potential energy of zero at the surface of the gravitating object. Then gravitational potential energy is positive everywhere [above the surface] and is still increasing with increasing separation.

Andrew Mason
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The point being made is that only differences in gravitational potential energy are physically significant. The numeric value is irrelevant. One is free to choose a baseline potential energy of zero at the surface of the gravitating object. Then gravitational potential energy is positive everywhere [above the surface] and is still increasing with increasing separation.
Then it is negative below the surface. The point is that energy is often considered negative for real physical reasons - hence the need to at least qualify the statement "a universe with negative energies would look completely different".

AM

mfb
Mentor
Gravitational potential energy can be expressed as negative value, but it does not have to be - just add mc2 and everything is fine. This choice also gives the total energy added to the system by adding the object, which is always positive.

Free fall and inclined plane experiments since Galileo show the uniform acceleration of falling bodies regardless of their compositions, shapes, sizes, and distances. The important fact that has been overlooked is, gravitational acceleration is independent of composition, shape, size, and distance. However, the paradox is, attracting acceleration is dependent upon composition, shape, size, and distance. Therefore, isn't gravity not attraction force, or it does not pull?

mfb
Mentor
The important fact that has been overlooked is, gravitational acceleration is independent of composition, shape, size, and distance.
It does depend on distance.
However, the paradox is, attracting acceleration is dependent upon composition, shape, size, and distance.
There is no paradox. It would be odd if the force on an object would not depend on the total mass and the mass distribution of the other object.
Therefore, isn't gravity not attraction force, or it does not pull?
What?
Gravity is clearly an attractive force.

It does depend on distance.There is no paradox. It would be odd if the force on an object would not depend on the total mass and the mass distribution of the other object.What?
Gravity is clearly an attractive force.

Doesn't the hammer and feather drop performed by David Scott in Apollo 15 show the independent of mass and gravitational acceleration?

mfb
Mentor
(a) the experiments showed nothing new, countless other experiments have shown the same before (we have vacuum chambers on Earth... but other experiments are much more sensitive). The Apollo spacecraft could not have reached the Moon otherwise, for example.
(b) the experiment gave an example of the general principle that the gravitational acceleration of an object is independent of its own mass. The acceleration still depends on the mass of the Moon and the distance of the objects to it, but those two things were the same for both falling objects.

PeroK
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Gold Member
2020 Award
Doesn't the hammer and feather drop performed by David Scott in Apollo 15 show the independent of mass and gravitational acceleration?

If he had dropped another moon what would have happened?

Andrew Mason
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Just to follow up on the subtle point made by PeroK that Galileo's principle that gravitational acceleration of mass m toward a body M is independent of m is really an approximation. It applies only where M>>>>m (such that the acceleration of M toward the centre of mass of M and m is negligible). Newton's law of universal gravitation is the correct law. Perhaps Mr. Huang could clarify what concerns he has about Newton's formulation.

AM

mfb
Mentor
The acceleration of the individual objects does not depend on their mass - this is exact in Newtonian gravity. The acceleration relative to the ground can depend on mass, if we don't neglect the acceleration of this ground.

Andrew Mason
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The acceleration of the individual objects does not depend on their mass - this is exact in Newtonian gravity. The acceleration relative to the ground can depend on mass, if we don't neglect the acceleration of this ground.
The force on body A does not depend on its own mass for a given separation from body B. So acceleration relative to the centre of mass of that two body system (A and B) only depends on that separation. But that force/acceleration during a "fall" depends on the rate of change of separation, ie. the magnitude of the acceleration of A relative to the centre of mass plus the |acceleration| of B relative to that point. The latter does depend on the mass of body A. So, in that respect, acceleration of a body is not necessarily independent of its mass.

AM

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mfb
Mentor
The force on body A does not depend on its own mass for a given separation from body B.
The force does depend on the mass.

At any given point in time, if the other constraints are the same (in particular, distance to the source mass), acceleration is the same. If you compare two setups where you replace one object by a heavier object, the other constraints won't stay the same, but that is a different statement. An object also has a different trajectory if it is so large that it is standing on the ground.

Andrew Mason
Homework Helper
The force does depend on the mass.
Of course. I should have said "force per unit mass".

At any given point in time, if the other constraints are the same (in particular, distance to the source mass), acceleration is the same. If you compare two setups where you replace one object by a heavier object, the other constraints won't stay the same, but that is a different statement. An object also has a different trajectory if it is so large that it is standing on the ground.
I was just following up on Perok's question "If he had dropped another moon what would have happened?". And the answer is that 1. the time of fall would be shorter and 2. the average acceleration over that time would be greater.

AM

(a) the experiments showed nothing new, countless other experiments have shown the same before (we have vacuum chambers on Earth... but other experiments are much more sensitive). The Apollo spacecraft could not have reached the Moon otherwise, for example.
(b) the experiment gave an example of the general principle that the gravitational acceleration of an object is independent of its own mass. The acceleration still depends on the mass of the Moon and the distance of the objects to it, but those two things were the same for both falling objects.
First of all, I appreciate and enjoy this discussion very much. Thank you!

(a) Indeed, the experiments showed nothing new as many historical experiments. However, I believe the universe does not lie to us, but interpretations could vary from time to time and person to person.

(b) You said the gravitational acceleration of an object is independent of its own mass, doesn't it mean the gravitational acceleration of the feather, the hammer,... and the Moon are all independent of their own masses? Since total mass of (Moon + hammer) is larger than the total mass of (Moon + feather), wouldn't the attraction force between the Moon and the hammer be greater? Why the acceleration of the hammer is uniform with the feather? I believe the acceleration is dependent on the strength of the gravity field, not necessary the mass. For example, a larger battery does not necessary produce stronger electromagnetic force. The composition (or structure) of the battery could be a significant factor, isn't it?

(c) Logically, Moon would accelerate toward the hammer and the feather by mutual attracting force even it is undetectable, isn't it? I believe the easiest attraction force to experiment is magnetic attraction. For example, a free-moving large magnetic ball will also accelerate toward a free-moving small magnetic ball, only not as much, isn't it? If the small one is anchored, the free-moving larger one would have to accelerate the total distance, isn't it?

(d) By the way, there are many magnetic slim experiments in Youtube. I believe they show the slow motion of attraction force at work, which is dependent on mass, composition, shape (surface), and distance. One observation I would like to mention is, the frontal part of the slim tears away faster. What if David Scott set the feather on the top end, further from the ground, of the hammer, would the hammer accelerate away from the feather?

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If he had dropped another moon what would have happened?
Do you mean performing the same experiment on different moon (a), or dropping a moon along with the hammer and the feather on our Moon (b)?

(a) I believe the difference is the atmosphere and the strength of it's gravity field of other moon. If there was no atmosphere, the hammer and the feather would have to fall in uniform acceleration. However, the rate of acceleration could be different if the strength of it's gravity field is different from our Moon. If there was atmosphere on other moon, isn't Earth better place?

(b) Does it make any difference if he had dropped the Lunar Lander, instead of the hammer, along with the feather on our Moon? If it was the size of another moon, I believe it would be a Moon-quake, and/or cracking of both moons. Maybe Armstrong can do it, just kidding:). My point is, do the sizes of objects matter?