Energy for fundamental forces

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  • #1
adjacent
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Gravity pulls things down.Things then gain kinetic energy.When you lift something it has gravitational potential energy.I know that potential energy is transformed into kinetic but according to the law of conservation of energy, potential energy must also be transformed from some other type of energy.As energy cannot be created or destroyed,from where is this potential energy coming from?This is also same for magnetic forces.
 

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  • #2
Bandersnatch
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The law of conservation of energy states that in a closed system energy is conserved.

When you lift something against gravity, you're doing work, which is adding energy from outside the system.
 
  • #3
adjacent
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You did not explain anything about where is the potential energy coming from
 
  • #4
Bandersnatch
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You did not explain anything about where is the potential energy coming from
Was your question not: where does the change in potential energy come from when lifting a stationary object?
As I said, you are supplying it from outside the system by doing work on it.

If you want to know where does the energy for doing that work come from, then you can trace it through the chemical energy stored in your muscles, through your food, to the plants, to the Sun, and ultimately to the gravitational potential energy of the cloud of gas that collapsed to form the solar system.

If this doesn't answer your question, then you need to restate it in more precise manner.
 
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  • #5
adjacent
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my question was how does gravity pulls things down,does it need energy to do that work?if so how?I know that we need energy to lift,how do gravity get energy to pull?
 
  • #6
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my question was how does gravity pulls things down,does it need energy to do that work?if so how?I know that we need energy to lift,how do gravity get energy to pull?
If you have a perfectly efficient pulley and you lift an object up with this pulley, you need energy to do this. Ask yourself this: Due to conservation of energy, where is this energy going?

You're putting energy into the system. The moment you release the pulley, an energy equivalent to what you put in the system makes the object fall back to the ground.
 
  • #7
adjacent
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For example,a stationary body is in the space.Then a star comes near it.It falls to the star even when it was not bothered.I did not hold it.Then how did it gain kinetic energy?
 
  • #8
HallsofIvy
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It loses potential energy, of course. Why do you have a problem with this?
 
  • #9
russ_watters
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Perhaps you are looking for the Big Bang as your answer...?
 
  • #10
CWatters
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I have a feeling "adjacent" is indirectly asking how gravity works. The answer is we don't know. One theory is that particles called gravitons cause objects to be attracted to each other. Gravitons haven't actually been observed.

I suppose one unsatisfactory answer would be to say that the big bang and the expansion of the universe seperated all matter and gave everything PE with respect to everything else. After that it's all "down hill".

Edit: Looks like Russ beat me to it.
 
  • #11
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In your example of a star coming towards a stationary body and falling, the energy comes from the gravitational field.

Gravitational potential energy is a function of space, its like a number each position has. So, before the star was there the gravitational potential energy had a certain value, I suppose it would be zero since there is no gravitational field strength (of course fields extend infinitely but it would at least be super tiny). I suppose its important to mention that nearby, say in a 1m radius, the potential is also zero. Agreed?

Gravitational force is the (negative of the) derivative of gravitational potential energy with respect to space. So since all around the stationary object there is no change in the potential, its derivative and therefore its force is also zero.

What would happen if a star approached? At some point the star would be close enough to a location near the object that that location would have a non-negligible value for gravitational potential energy. This would happen because the gravitational field strength would become large enough; in general it would get more non-zero the closer the star came.

So at the object you might have a potential of zero, but at the nearby location you have a potential of say -0.0005. Now all of a sudden you have a non-zero derivative, you have a force. The force is proportional to this difference; again no difference means the derivative is zero.

Just g by itself is linearly proportional to the source of the field's mass. So lets pretend these values were for a sun-like star, and the object was earth-like. Now we also pretend all of a sudden we wipe out the existence of the star with unnatural, god-like powers. What happens to the gravitational potential energy at our location? Nothing instantaneously, since no information can travel faster than light the changed situation (namely that there is no longer a gravitational field strength, no more star's mass) can't be known to us yet. This change has to propagate through the field, and does so even when the source is annihilated, and so the gravitational field has its own existence independent of the source.

Similar things happen with other forces. Really, energy as a concept is just a simplified way of dealing with forces and fields. Energy is a scalar, fields/forces are vectors. Scalars are typically easier to work with than vectors (compare arithmetic to vector calculus). Physically speaking, there is no physical mechanism in energy that "causes forces to arise" (at least that I know of). But for descriptive purposes it works out that the negative derivative of potential is equal to force.

You might check out this simulator. It is for charge and electric field, but can give you a familiarity with fields:

http://phet.colorado.edu/en/simulation/radiating-charge

But I should mention, gravitational waves have never been detected.
 
  • #12
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For example,a stationary body is in the space.Then a star comes near it.It falls to the star even when it was not bothered.I did not hold it.Then how did it gain kinetic energy?
Is the kinetic energy not acquired by freefalling inside the gravity field?

As you stated, an object has potential for acquiring kinetic energy, but that energy is a latency and will not be realized until the object acquires speed and kinetic energy along with it.

Potential is a "latent excellence" which may become reality. But until activated by a specific event, potential remains dormant.
 
  • #13
Drakkith
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my question was how does gravity pulls things down,does it need energy to do that work?if so how?I know that we need energy to lift,how do gravity get energy to pull?
Forces do NOT work because of energy. Energy is the result of forces acting on objects. The fundamental forces of nature do not expend energy to attract or repel or anything like that.

Consider a book on a table. It is being pulled down by a force, gravity. Since it isn't moving, no energy is being transferred and no work is being performed. Yet, that does not mean that gravity isn't pulling the book down, it most certainly is! And then you have the fact that the table and book exert forces on each other that keep them seperate, with no work is being performed there either.

The difference between gravity and our muscles is that our muscles are inefficient machines. You put some quantity of energy in, you get less energy out as work, with the remainder being turned into heat.
 
  • #14
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Gravity pulls things down.Things then gain kinetic energy.When you lift something it has gravitational potential energy.
gravity pulls things 'up' as well. When either happens, when two bodies move closer together, potential energy decreases...equal in some conserved [like frictionless] situations to the change in kinetic energy.


I know that potential energy is transformed into kinetic but according to the law of conservation of energy, potential energy must also be transformed from some other type of energy. As energy cannot be created or destroyed,from where is this potential energy coming from?This is also same for magnetic forces.
As already noted, all energy originated in the big bang...kinetic, nuclear, electromagnetic, and potential....AT one time it is believed all forms were 'unified' in a single high energy, uniform but very unstable vacuum energy. See spontaneous symmetry breaking [from the big bang] to get some ideas....but ultimately, nobody REALLY knows where energy originates....
it IS everywhere around us in the form of 'vacuum energy' which does have some quantum explanations....and a form of that could even be the source of the big bang....but even if that is someday verified, you can still ask "where does THAT come from'.....
 

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