I Why Is Gravity Always Attractive?

[petrushkagoogol]

Why is gravity always attractive in nature ?

 

[mfb]

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.

 

[David Lewis]

For the same reason time always moves from past to future.

 

[Andrew Mason]

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

 

[TahirGorgen]

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.

 

[Khashishi]

Because masses are always positive.

 

[Andrew Mason]

Because masses are always positive.

So are positive charges. But they don't attract each other.

AM

 

[phinds]

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?

 

[lychette]

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]

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]

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]

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]

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.

 

[Cres Huang]

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]

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.

 

[Cres Huang]

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]

(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]

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]

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]

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]

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 https://www.physicsforums.com/posts/5586519/editf a body is not necessarily independent of its mass.

AM

 

[mfb]

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]

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

 

[Cres Huang]

(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?

 

[Cres Huang]

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?

 

[PeroK]

(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?

It's perhaps not a very important point, but if you are talking about relatively small objects then they take the same time to fall. But, if both objects are large then they both fall towards each other significantly and will take less time to collide, given the same starting distance, than the case of a small object falling towards one of them.

 

[mfb]

(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?

Yes. F=MmG/r², and if you look at the acceleration of one of the objects (e.g. the one with mass m), then a=F/m=MG/r² which does not depend on m any more.

Since total mass of (Moon + hammer) is larger than the total mass of (Moon + feather)

The sum of masses does not matter. Only the product appears in formulas.

wouldn't the attraction force between the Moon and the hammer be greater?

The force is larger, but acceleration is force divided by mass, so the mass cancels in the calculation.

I believe the acceleration is dependent on the strength of the gravity field, not necessary the mass.

The strength of the gravitational field is given by the mass of the moon and the distance to it.
The mass is exactly the relevant part of the "composition".

(c) Logically, Moon would accelerate toward the hammer and the feather by mutual attracting force even it is undetectable, isn't it?

Sure.

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?

Depends on the relative orientation of the magnets. Magnets are not a good model for this reason.

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?

Theoretically yes, but the difference is completely negligible. At that level of precision the attraction between hammer and feather could be relevant as well.

My point is, do the sizes of objects matter?

No.

 

[Cres Huang]

magnetic slim

Correction, magnetic slime.

the experiment gave an example of the general principle that the gravitational acceleration of an object is independent of its own mass.

David Scott's hammer feather drop, along with all other falling body experiments, shows:

The gravitational acceleration of an object is independent of it's shape, size, and surface; in addition to it's own mass, isn't it?

 

[Andrew Mason]

Correction, magnetic slime,

David Scott's hammer feather drop, along with all other falling body experiments, shows:

The gravitational acceleration of an object is independent of it's shape, size, and surface; in addition to it's own mass, isn't it?

Gravitational acceleration is independent of its mass. It is not necssarily independent of its mass distribution, or size. For example, a 100 T mass in the form of a thin wire stretched from a satellite to the Earth surface will experience different gravitational accelerations than a 100T mass located at the midpoint of the stretched wire.

AM

 

[Cres Huang]

Gravitational acceleration is independent of its mass. It is not necssarily independent of its mass distribution, or size.

Can you explain:
- How abut the surface and shape?
- Didn't David Scott's hammer fell in sync with feather, the mass, size, shape, and surface are clearly very different?
- The different size, and weight, of the balls fall in uniform acceleration in all experiments I have learned, when the friction of the air is neglected?
- Galileo's inclined plane experiments show the uniform acceleration of different sizes of spheres, by times-squared law regardless of the sizes? I believe he had done very intensive studies on this, if not the most. Great man like him would not give up finding the truth in my view.
- I believe there are countless experiments out there. However, I have been searching for years. Would you kindly direct few falling body experiments that show otherwise? Video clips would be perfected, but anything will do. Thank you. Help, anyone?

My point is, do the sizes of objects matter?

No.

a 100 T mass in the form of a thin wire stretched from a satellite to the Earth surface will experience different gravitational accelerations than a 100T mass located at the midpoint of the stretched wire.

What kind of difference, which mass would have higher gravitational acceleration, one at midpoint?
I wouldn't think it is feasible to perform such an experiment considering these reasonings:
- Mose satellites travel at much higher speed then surface of the Earth. Satellite tied to a stretched wire anchored on the ground would be a big issue, even geosynchronous satellite. I don't expect our technology will meet the challenge any time soon, do you?
- Mass at the altitude of satellite is about weightless. It would not fall unless it is pushed down and break the surface of the atmosphere. Isn't the interference is introduced by altering the initial free-fall?
- It would introduce the surface interactions of the object and the atmosphere upon reentry. Could it bounce off?
- Object at midpoint would begin at lower centrifugal force.
- Object at the satellite has next to zero air resistance to begin with. It's starting and en-route environment is different from the one at midpoint.
- Gravity anomaly, air, atmosphere friction, altitude, and weather over large distance would compound the issues. For example, temperature, humanity, and wind speed would be constant. Wire would sway, stretch, expand, and bend even it could hold. The set-up and environment of the experiment can not be controlled.
- Loss of mass could void the experiment. Wouldn't the one from satellite burn out beforehand?
- Isn't this similar experiment to Galileo's inclined plane, but high-tech overkill?
- Do you think the experiments already done, even Galileo and David Scott, cannot provide clear and sufficient clues?

 

[Andrew Mason]

Can you explain:
- How abut the surface and shape?
- Didn't David Scott's hammer fell in sync with feather, the mass, size, shape, and surface are clearly very different?
- The different size, and weight, of the balls fall in uniform acceleration in all experiments I have learned, when the friction of the air is neglected?
- Galileo's inclined plane experiments show the uniform acceleration of different sizes of spheres, by times-squared law regardless of the sizes? I believe he had done very intensive studies on this, if not the most. Great man like him would not give up finding the truth in my view.
- I believe there are countless experiments out there. However, I have been searching for years. Would you kindly direct few falling body experiments that show otherwise? Video clips would be perfected, but anything will do. Thank you. Help, anyone?

The Galileo experiment, and astronaut Scott's demonstration of it, was not accurate enough to show variations in gravity due to mass distribution. A 15 kg dumbell and a .15 kg baseball held above the moon surface will accelerate toward the moon surface at the same rate unless you can measure the difference to about 1 part in 10^25. But if you could measure the acceleration to that level of accuracy, you would find that the dumbell's acceleration will vary slightly depending on its orientation, whereas the baseball's acceleration would not.

AM

 

[mfb]

- Galileo's inclined plane experiments show the uniform acceleration of different sizes of spheres, by times-squared law regardless of the sizes? I believe he had done very intensive studies on this, if not the most. Great man like him would not give up finding the truth in my view.

An inclined plane is not a free fall. On an inclined plane, you have to consider the moment of inertia of the rotating objects.

 

[Cres Huang]

The Galileo experiment, and astronaut Scott's demonstration of it, was not accurate enough to show variations in gravity due to mass distribution. A 15 kg dumbell and a .15 kg baseball held above the moon surface will accelerate toward the moon surface at the same rate unless you can measure the difference to about 1 part in 10^25. But if you could measure the acceleration to that level of accuracy, you would find that the dumbell's acceleration will vary slightly depending on its orientation, whereas the baseball's acceleration would not.AM

I believe you understand the reason why David Scott dropped hammer and feather, not 15 kg dumbell and .15 kg baseball.

1 part in 10^25 is a ridiculous claim. Variance that small you can not say it is the result of gravity, magnetic field, temperature, humility, spiral momentum of the Earth/Moon/Solar System, trillions and trillions of passing solar particles, electrons, neutrinos, and all possible variations of the environment as well as the testing objects, devices, including your breath, heartbeat, body temperature, shaking hands, and so forth... Even you can prove, say 1 part in 10^10, you have to prove the testing environment, devices, and procedures are in perfect constant. Scientifically, you did not disprove Galileo, Scott, and all other experiments. The same principle applies to all experiments short of significance.

• How can you disprove gravitational acceleration is independent of the surfaces of the hammer and feather?
• How can you disprove gravitational acceleration is independent of the shapes of the hammer and feather?
• How can you disprove gravitational acceleration is independent of the sizes of the hammer and feather?
• Action at a distance of gravity remains debatable.

Other than that, attracting acceleration also has to be proven independent of it's mass, otherwise, gravitational acceleration is not equal to attracting acceleration.

An inclined plane is not a free fall. On an inclined plane, you have to consider the moment of inertia of the rotating objects.

Yes, you have to consider the moment of inertia of the rotating objects falling in spherical (3D directions) in mid space as well. There is no such thing as perfect sphere. However, it is a slow motion of gravity acceleration. I think it is brilliant to obtain the mathematic equation of gravitational acceleration. It is the same principle of high-speed photography in science.

 

[sophiecentaur]

If he had dropped another moon what would have happened?

And if he had dropped the feather from the mid (neutral) point and if the moons had identical masses. The net gravitational force on the feather would be zero.The two moons would move towards each other, squashing the feather between them as they met. In the frame of one moon, the feather would have fallen half the distance that the other moon would have 'fallen' in the same time.
But the law that's in operation in the standard hammer and feather situation implies a uniform gravitational field (massive / flat planet); a much simpler case than for two moons.

 

[Andrew Mason]

Scientifically, you did not disprove Galileo, Scott, and all other experiments.

It is not a matter of disproving Galileo's "equal times of fall" experiment. It is a matter of correctly stating its level of accuracy. If Newton law of universal gravitation is correct: ie. $F = GMm/r^2$ is correct, then it is just a matter of applying that law.

AM

 

[PeterDonis]

For the same reason time always moves from past to future.

This can't be right, because gravity is still attractive if you reverse the direction of time. There are physical processes we know of that violate time reversal invariance, but gravity is not one of them.

 

[PeterDonis]

The Galileo experiment, and astronaut Scott's demonstration of it, was not accurate enough to show variations in gravity due to mass distribution.

I assume you are talking about tidal gravity here? The general law that acceleration of a test object is independent of its mass (and mass distribution) assumes that tidal gravity is negligible. If tidal gravity is not negligible then you are right that this general law is no longer exactly correct.

 

[PeterDonis]

If he had dropped another moon what would have happened?

This isn't really a comparable situation, because there is no way to drop another moon towards the moon while ignoring the effects of tidal gravity, whereas, as I noted in another post just now, the general law under discussion is only correct if tidal gravity is negligible. Also, another moon would not qualify as a "test object", and the general law under discussion only applies to test objects. Test objects, by definition, have negligible gravitational effects themselves, and that is obviously not true for another moon.

 

[Andrew Mason]

I assume you are talking about tidal gravity here? The general law that acceleration of a test object is independent of its mass (and mass distribution) assumes that tidal gravity is negligible. If tidal gravity is not negligible then you are right that this general law is no longer exactly correct.

Yes. Tidal forces. I assume that is what you mean by tidal gravity. Tidal forces, due to differences in the gravitational forces/unit mass acting on different parts of a body due to differences in position in the gravitational field of the other gravitating body, have the tendency to pull a body apart, which is the opposite of how one thinks of gravity.

AM

 

[PeterDonis]

Tidal forces. I assume that is what you mean by tidal gravity.

Yes.

Tidal forces, due to differences in the gravitational forces/unit mass acting on different parts of a body due to differences in position in the gravitational field of the other gravitating body, have the tendency to pull a body apart

Not always. Radial tidal forces in the field of a spherical mass do this, but tangential ones don't; they have a tendency to push a body together. But both of these are different from the effect of the Newtonian "force" of gravity, which has no effect at all on the internal structure of a body, neither pulling apart nor pushing together.