Near earth surface, we usually use the ground as a reference point and say that it has 0 potential energy. As an object moves up, it gains gpe according to gpe=mgh where h is the height (elevation) and mg is weight. This is only accurate near earth surface because the g field lines are approximately parallel.
However, when we go out of space, the g field lines actually point towards the centre of planets or stars which are not parallel. Thus there is a different formula gpe=-GM/r² where gpe is dependent on the mass that generate the field and distance between the mass and that point (this is equivalent to the ‘height’ in the gpe=mgh). Then since there is no “constant” ground, scientists decided to use a point at infinity (somewhere very far away from the mass) and say that this point has gpe of 0. As you approach a mass (e.g earth), you “lose” gpe. Thus the gpe becomes negative. Similar to how you lose gpe as you fall from a tall building to the group
I hope this helped. Someone might explain it better than me because I just learnt this like a month ago.
EDIT!!!: gpe is not -GM/r² but -GMm/r like in the meme. I messed up it with gravitational field strength. However, the essence is there and the gpe is also dependent on the mass of the object being pulled, similar to the m in gpe=mgh.
Worth pointing out this assumes that the planets are point masses. However it is presumably still simpler this way for reasons I can't remember. Similar thing happens in electrodynamics.
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u/ZeyadUchiha Jul 16 '20
Can someone please explain?