Why Is Gravitational Force Always Attractive ?

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#1 petrushkagoogol

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Posted 25 November 2017 - 06:10 AM

Why is gravitational force always attractive, as opposed to electrostatic forces, which is are both attractive and repulsive ?

#2 exchemist

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Posted 25 November 2017 - 11:52 AM

Why is gravitational force always attractive, as opposed to electrostatic forces, which is are both attractive and repulsive ?

It just is, according to observation. And of course, in general relativity (which is currently our best model) it is not really a force at all, but a manifestation of the curvature of spacetime by mass.

#3 Dubbelosix

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Posted 29 December 2017 - 12:37 PM

It just is, according to observation. And of course, in general relativity (which is currently our best model) it is not really a force at all, but a manifestation of the curvature of spacetime by mass.

This is actually a good reply and does explain why things are always attracted to each other.

Gravity is a pseudo force and can be understood in the following (neat) and (concise and short) way:

$\frac{d^2x^{\mu}}{d\tau^2} + \Gamma^{\mu}_{\nu \lambda} \frac{dx^{\nu}}{d \tau} \frac{dx^{\lambda}}{d\tau} = 0$

where

$\Gamma^{\mu}_{\nu \lambda} = \frac{\partial x^{\mu}}{\partial \eta^{a}}\frac{\partial^2 \eta^a}{\partial x^{\nu}\partial x^{\lambda}}$

or more compactly

$\Gamma^{\mu}_{\nu \lambda} = J^{\mu}_{a} \partial_{\nu} J^{a}_{\lambda} = J^{\mu}_{a} \partial_{\lambda} J^{a}_{\nu} \equiv J^{\nu}_{a} J^{a}_{\nu \lambda}$

which represents a pseudo force for gravity which makes it in the same league as the Coriolis and the Centrifugal forces.

Notice though, there are other reasons why gravity is always attractive. We are yet to find a negative mass in nature which repels mass. You can understand then, that Coulombs law and the Newton attractive force equation are not true analogues of each other because charges can repel or attract while mass strictly (so far) is attractive by definition.

Edited by Dubbelosix, 29 December 2017 - 12:56 PM.

#4 julianasanabri

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Posted 07 January 2018 - 10:59 AM

According to the Law of Gravitation, every mass in this world is attracted by the other mass. The mass of earth is very greater than the other things. That's why the force of Earth is always attractive. With the help of this concept, we will also be able to get an idea that what is the difference between mass and weight

#5 studentgary

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Posted 07 January 2018 - 07:18 PM

The real "problem" about understanding gravitation is the same as when Newton was asked, "What is it?"

All we really have are descriptions and suppositions about how it behaves.

#6 exchemist

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Posted 08 January 2018 - 03:29 AM

The real "problem" about understanding gravitation is the same as when Newton was asked, "What is it?"

All we really have are descriptions and suppositions about how it behaves.

I suppose so, but in the end that is all we have about any of the fundamental features of physics.

Ultimately science produces models to describe how nature behaves, but does not answer the deepest questions about the nature of reality.

#7 Dubbelosix

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Posted 20 January 2018 - 08:58 AM

The real "problem" about understanding gravitation is the same as when Newton was asked, "What is it?"

Except, Newton actually had no clue and attributed it to mystical forces. Einstein on the other hand found it was related to the geometry of spacetime caused by massive objects. So it's not really a case of ''what is it?'' Maybe the question should be ''what else could it be?''

Relativity is well proven by now as well.