What does negative potential energy mean?

[Remain]

In the case of lifting a box, I know potential energy is negative because negative work is done by gravity. What does the negative part of -PE denote? Does it denote the direction of the energy or does it mean that energy is being lost? But how is energy being lost if the box gains more gravitational potential energy the higher it's lifted? So what does negative potential energy mean?

 

[DrStupid]

But how is energy being lost if the box gains more gravitational potential energy the higher it's lifted?

Nothing gets lost. If the potential energy increases it is less negative than before.

So what does negative potential energy mean?

It means that the potential energy is negative. What else?

 

[Bandersnatch]

The only thing the minus sign in front of PE means is that you've chosen the reference level of 0 PE to be somewhere higher in the gravitational well.

The 0 PE level is arbitrary. You can choose any convention you like.
If you choose 0 to coincide with the surface of the Earth, then any object above it will have positive PE, and any object below the surface will have negative PE.
If you choose 0 to coincide with infinite distance from the source of the gravitational field, then all objects closer than infinity will have negative PE.

However, in both cases (and in any other case) the difference between PE at some two points in the gravitational field will be the same - it's independent of the convention you pick, and that's what makes it meaningful. That difference is the amount of work needed in order to lift an object, or the amount of energy released when it falls.

Does it denote the direction of the energy

Energy is not a vector. It is a scalar - it doesn't have direction.

 

[sophiecentaur]

A good reason for choosing an arbitrary reference as 'Infinity' is that it's the same for anyone, anywhere in the Universe (at least, in a classical mechanical sense). The familiar model of the rubber sheet with masses sitting on it, to represent a solar system works well and we can all picture an observer at the outer rim (approximating to infinity), with everything 'below' that potential level. The forces are always attractive.
However, if you consider the potential energy situation existing between nearby atoms, the spacing between the atoms is governed by where the Potential Energy is at a minimum. Closer than this and the forces become repulsive and the Potential energy becomes Positive. See this wikipedia link. The mean spacing between atoms will end up at value where the Kinetic Energy of Vibration balances the mean Potential Energy (not the minimum of the curve).

This diagram appears in the link.

 

[Remain]

Thanks guys! But in the equation W = Wnc + -mgh, aren't you kind of like subtracting away energy or something?