Why Is Gravity Always Attractive?

[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.