# Definition of Gravitational Potential Energy

#### Bertknight

Sorry if I'm a bit blur at this topic...

The definition of GPE for a mass at a point is 'the work done in bringing a mass from infinity to the point'. But when you bring a mass from infinity towards the gravitational body, isn't the work done always positive?

Actually, we can start from the infinite or the center of the gravitatioal body. It's only the difference between negative and positive values. If we start from the infinite, we can get an approximate feasible way to set out. If we start form the center of the gravitational body, we set out from a nearly impossible mission. In another hand, in the infinite the gravity is zero, if we take potential energy is also zero, then we feel more natural in logic.
Sorry but I really don't understand why starting from the centre of the gravitational body is inapplicable.

#### benit13

Work done can be considered negative if the work is being done by something (the gravitational field) on something else (the test mass).

I wouldn't worry too much about the details regarding certain conventions. They probably exist just to make the energy book-keeping tasks more convenient. The GPE convention, although rather odd, allows for the mechanics of objects in free-fall to simply convert GPE to KE (one decreases, the other increases), making this set of mathematics useful and meaningful.

Last edited:

#### Pelh

Gravitational potential energy can be thought of as the work done on an object to raise it to a certain position in space, or can be thought of as the quantity of energy that, if released, would be transformed into kinetic energy (and heat and sound etc). Say you raise an object 1 m above the surface of the earth. If you let it go, it would convert all its potential energy to kinetic energy to allow it to accelerate downwards. If you raised it 2 m then it would need to have more kinetic energy to fall a greater distance and therefore it must have a higher potential energy.

So what happens when you’re a very very large distance away and you no longer feel the pull of the earth? Gravitational potential energy is zero at that point. So how can potential energy be increasing as you get further away from an object but also be approaching zero? This only makes sense if we talk about gravity potential energy in negative numbers. As negative numbers get bigger, they approach zero.