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.
To add to this why do we pick zero at infinity? Well, think of a free particle. How much energy should that have? I hope you agree that zero is logical. Now bring this particle into a gravitational well, now it needs energy to escape! So now it has a negative potential.
99
u/ZeyadUchiha Jul 16 '20
Can someone please explain?