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where does the energy for gravity come from?

shortylickens

No Lifer
Jul 15, 2003
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I got to thinking about terminal velocity when watching a movie with dudes falling to their deaths. then I thought about gravity. then I realized I have no idea where the energy for gravity originates.
 

sandorski

No Lifer
Oct 10, 1999
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I think that is one of the things about Gravity that is not well understood.
 

PowerEngineer

Diamond Member
Oct 22, 2001
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Well, I cannot pretend to answer this question as a technical expert... However, if we can stay with Newtonian physics then gravity is a force. Energy comes into play because it takes energy to move an object a distance against a force.

In your example, the "dudes" presumably took some action to reach the height from which they subsequently fell. Maybe they climbed a mountain, or maybe they took an elevator to the top floor. However they did this, there was energy being used to move them "up" against the downward force of gravity. That energy is effectively stored up in the new position they have arrived at that is further away from the center of gravity. This stored or (more properly called) potential energy is equal to the mass of the object moved times the distance moved times a gravitational constant (for earth it is about 9.81 meter/sec2). When the "dude" trips and falls (so that the force of the floor no longer counteracts the force of gravity) then he will start accelerating at the rate of the gravitational constant. The kinetic energy he gains (0.5 times his mass times his velocity squared) on the way to his death is essentially a release (or conversion) of the potential energy. You can think of it as being similar to storing energy by winding up a clock spring and then letting that potential energy be later converted into kinetic energy in the motion of the clock mechanism.
 

Paperdoc

Golden Member
Aug 17, 2006
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PowerEngineer above is right. More thoughts. Gravity just is. It is a property of all matter, resulting in a force of attraction between objects. It is not a form of energy.

The energy associated with gravity is in the fact that the two (or more) objects are separated over a distance, and there is an attractive force between them. That is called Potential Energy because it may be "used up" at some point. For example, if the objects move closer to each other, it uses some potential energy to cause the motion, and decreases the amount of potential energy stored in that system of separated bodies. But energy can be neither created nor destroyed, so the reduction in potential energy is transformed into an increase in Kinetic Energy - that is, one or more of the bodies is accelerated to a new higher speed, and it will keep on moving at the new speed (and direction) until some other force acts on it again. The Potential Energy is not "used up", it is converted to another form.

So, where did that original energy come from? We now use the whole model of the "Big Bang" which almost instantaneously created ALL matter AND did that in a huge explosion that gave all of those objects kinetic energy to move away from each other. It also gave the components of those objects many forms of potential energy at the atomic and sub-atomic level. Throughout time in our universe, many of those potential energy forms have been converted into other forms: unstable isotopes decaying to new isotopes or atoms and releasing radiation of several forms, hot items releasing infrared radiation to be absorbed by other objects, rocks falling down mountains. Meanwhile all the matter dispersed in that process keeps on spreading out because it has its original kinetic energy and keeps going. The intriguing part, though, appears to be that calculations indicate that the attractive forces of gravity between objects in our universe ought to slow those things down, and eventually cause them to reverse towards each other, and that is NOT happening. In fact, it appears the reverse is happening - the speed of expansion of the universe appears to be increasing. That can happen only because there are other forces that repel objects more than gravity causes them to be attracted. That's just one a many puzzles we do not understand. And in fact, nobody has any understanding of the Big Bang process, nor of where the energy for THAT came from. It just was, and still is.
 
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YuliApp

Member
Dec 27, 2017
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ELI5: it is just like a string between every objects, bigger the object, stronger the string. Bigger the distance, more energy is stored. If you exceed the break up energy, string breaks and you fall out to the space :)
 

deustroop

Golden Member
Dec 12, 2010
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Most of these replies and analysis are based on Classical physics /Newtonian concepts ( "attraction" ) and two hundred years too late to address the question.Gravity is not a concept in E= MC 2 and therefore does not require energy As far as I can make out Einsteins Theory of General Relativity analyses gravity as a geometric property of space and time or four-dimensional spacetime. Briefly put, matter shapes spacetime ( space is curved) and gravity is the effect on their motion as bodies approach each other..When I release an apple it collides with the earth because the shape of space here creates the direction for smaller falling bodies that close to the earth.
 
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