View Full Version : Gravitational Potential (Separation of Masses)

Canis Lupus

2017-Jan-04, 10:50 PM

From another thread, but a divergence from the OP.

It is possible that the quantum foam is linked to dark energy, but nobody really knows.

Also, please note the one and only source of negative energy in our universe is gravitational potential(separation of masses).

Can you or anyone else explain this principle in more detail?

grapes

2017-Jan-05, 01:41 PM

It's all relative.

Gravitational potential energy in an isolated system (one main body) is defined to be 0 at infinity. As a particle approaches the body, its kinetic energy increases until its velocity becomes the escape velocity at the surface of the body. But that means its gravitational potential energy is always decreasing, and it is always negative. I'm not sure that that's what ShinAce was talking about.

https://en.m.wikipedia.org/wiki/Potential_energy

ShinAce

2017-Jan-05, 06:06 PM

Can you or anyone else explain this principle in more detail?

Someone like grapes probably could, but not me.... :)

As an example, take two magnets. Put the south poles facing each other and press the magnets together. When you let go, they will fly apart. Where did the energy to make them fly apart come from???

It came from you pushing the magnets together. The concepts of work and energy are inseparable. The work you do with your fingers on the magnets is stored in the magnetic field. That energy can then make the magnets fly apart.

Gravity is the same concept. The only difference is that instead of trying to make two south poles fly apart, it wants to pull everything in. Like grapes said, this reversing of the direction of the force in terms of the work done is what gives us a minus sign for gravitational potential. Again, like he said, the closest you'll get to a positive number is when the masses are infinitely far apart, and then the potential is just zero(not positive). [Important edit: the potential energy being 0 when the masses are infinitely far apart is not an arbitrary choice. Since the force at that point is zero for any mass, then the potential at that point must be a constant. This is because the potential is obtained by integration of the force. Since it must be the same number for any mass infinitely far apart, zero is the only choice.]

Imagine that we put a minus sign on every dollar bill. The money in your wallet is a negative, and the balance of your loans are positive. What would this change? Nothing. When you apply a negative bill to a positive loan, the balance does down. As it should, because you made a payment.

The total energy of the universe is zero. The universe is flat. These are the same thing. That was the whole point of saying gravitational potential is negative. Since we know the net sum is zero to begin with, then now it really makes no difference if you call a $20 bill a +$20 bill or a -$20 bill. Once you count them all, you'll get zero.

Canis Lupus

2017-Jan-05, 07:56 PM

Someone like grapes probably could, but not me.... :)

As an example, take two magnets. Put the south poles facing each other and press the magnets together. When you let go, they will fly apart. Where did the energy to make them fly apart come from???

It came from you pushing the magnets together. The concepts of work and energy are inseparable. The work you do with your fingers on the magnets is stored in the magnetic field. That energy can then make the magnets fly apart.

Gravity is the same concept. The only difference is that instead of trying to make two south poles fly apart, it wants to pull everything in. Like grapes said, this reversing of the direction of the force in terms of the work done is what gives us a minus sign for gravitational potential. Again, like he said, the closest you'll get to a positive number is when the masses are infinitely far apart, and then the potential is just zero(not positive). [Important edit: the potential energy being 0 when the masses are infinitely far apart is not an arbitrary choice. Since the force at that point is zero for any mass, then the potential at that point must be a constant. This is because the potential is obtained by integration of the force. Since it must be the same number for any mass infinitely far apart, zero is the only choice.]

Imagine that we put a minus sign on every dollar bill. The money in your wallet is a negative, and the balance of your loans are positive. What would this change? Nothing. When you apply a negative bill to a positive loan, the balance does down. As it should, because you made a payment.

The total energy of the universe is zero. The universe is flat. These are the same thing. That was the whole point of saying gravitational potential is negative. Since we know the net sum is zero to begin with, then now it really makes no difference if you call a $20 bill a +$20 bill or a -$20 bill. Once you count them all, you'll get zero.

Brilliant explanation - thanks to you and Grapes.

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