# What gives gravity its power to accelerate objects?

What is the physical property that gravity has that enables it to make an object go faster or, if it is resting on the ground, have weight?

That means it can do work: it gives objects kinetic energy. Where does that ability come from and where does the energy (the gravitational potential energy, I suppose it is called) come from and how is it transformed into the kinetic energy or the weight of the object?

• Delta2

russ_watters
Mentor
What is the physical property that gravity has that enables it to make an object go faster or, if it is resting on the ground, have weight?

That means it can do work: it gives objects kinetic energy. Where does that ability come from and where does the energy (the gravitational potential energy, I suppose it is called) come from and how is it transformed into the kinetic energy or the weight of the object?
Potential energy of any kind comes from the location of objects in a force field. So it originates with the placement of the objects - for GPE, the formation of the universe is the ultimate source. Converting it to another form just requires motion through the field, acted on by the force.

• ohwilleke
Delta2
Homework Helper
Gold Member
Classically the gravitational force field is created in an "automatic" kind of way in the space that surrounds every mass. There is no deeper mechanism according to classical physics(though general relativity and quantum gravity suggest a deeper mechanism of the gravitational force field creation). Once we accept that the gravitational force field around some mass M (this mass M is the source of the field) exists, its basic property is that it exerts a force to any other mass m that is located within the field at some point of space (that's why it is called a force field). This force does work when there is motion of the mass m through the field and it is this work of this gravitational force that converts the gravitational potential energy of the mass m to kinetic energy of the mass m.

• ohwilleke
CWatters
Homework Helper
Gold Member
Not perfect but perhaps see..

Gravity is a force. Forces accelerate objects. As the final step of the syllogism, gravity can accelerate objects. Hence the answer to you first question may be as simple as the property that gravity has to accelerate objects and give them energy is the "property" that gravity is a force. The electric force can also accelerate objects, and the magnetic force, etc.

As to the general question of why forces accelerate objects, Newton said bodies move at a constant speed in a straight line unless acted upon by a force.
This is of little help as natural scientists at that time also recognized the definition of force is that which will deviate a body from moving in a straight line at constant speed. Hence, you can see the definition of force and Newton's first law is somewhat circular.

Newton proposed something like "Hypothesis non fingo" meaning he did not know the answers underlying these deep questions. He simply applied his laws of motion and universal gravitation and optics to compute orbits, discover light is composed of a mixture of frequencies, as a by-product, invent the calculus, and contribute to, the calculus of variations with the brachistochrone etc.
Not a bad achievement considering he avoided considering the "fundamental" questions, like why is gravity a force? or why do forces accelerate objects? why is gravity actually a field? If gravity is a field, why do fields accelerate objects. If particles are actually following a minimum interval path is a curved space-time continuum (a la Einstein), why do they do that. In short, it is always possible to eventually get to an unanswerable question.

One possible answer to your original question may be that our universe just behaves this way where gravity, higher electromagnetism, etc are forces, and there are or maybe other universes out there or long ago that have different rules. Perhaps, life itself or humans or physicists are excluded from evolving in those other universes that follow different rules.

• Delta2
To get closer to my puzzlement - in General Relativity (as I understand it) objects in free fall move in geodesics. Thus an orbiting satellite moves in the curve around the planet that is the geodesic for it given its velocity, etc. If it is in a circular orbit it does not gain or lose kinetic energy (ignoring orbital decay). If it is in any other orbit it gains and loses velocity and its kinetic and gravitional potential energy are constantly rebalanced. It is this latter process whose origin puzzles me. If it helps to get to an answer that satisfies me, there is the fact that it moves at constact velocity - c - in spacetime, and its local time constantly changes speed (or changes some quantity).

russ_watters
Mentor
To get closer to my puzzlement - in General Relativity (as I understand it) objects in free fall move in geodesics. Thus an orbiting satellite moves in the curve around the planet that is the geodesic for it given its velocity, etc. If it is in a circular orbit it does not gain or lose kinetic energy (ignoring orbital decay). If it is in any other orbit it gains and loses velocity and its kinetic and gravitional potential energy are constantly rebalanced. It is this latter process whose origin puzzles me. If it helps to get to an answer that satisfies me, there is the fact that it moves at constact velocity - c - in spacetime, and its local time constantly changes speed (or changes some quantity).
You didn't ask any follow-up questions.... Could you explain what this "puzzlement" is?

Also, please note this is the Classical Physics forum. You don't need Relativity for this.

Also, please note this is the Classical Physics forum

My mistake. I didn't notice that until I read your post. I will re-post it in the Relativity forum if I am allowed to.

russ_watters
Mentor
My mistake. I didn't notice that until I read your post. I will re-post it in the Relativity forum if I am allowed to.
Sure, but I'm not sure there is any added value to that.