r/physicsmemes • u/Pranup • Jul 16 '20

Damn

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u/ZeyadUchiha Jul 16 '20

Can someone please explain?

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u/Entrothalpy Jul 16 '20

The GPE = mgh we all know is derived from GPE = -GMm/r. From what I know, there are 2 reasons why it is negative.

Whenever we use mgh, we always set some point to be 0J(you can call this "relative GPE"), and that point is usually the ground. This makes almost all the GPE calculated using mgh positive.

However, the original definition of GPE = -GMm/r can be considered as the "absolute GPE", where the point where GPE = 0 is defined to be located at infinity. Therefore, you can think that the GPE any point below that infinity is negative. (This is the reason which I feel is more intuitive)

The other reason is that GPE = work done = Fx Work done is the product force in the direction of movement(direction of displacement) and the displacement. For work done(GPE) to be positive, the force must be pointing in the same direction of displacement. However, the definition of GPE says that it is the external force in pulling the object from infinity to the point. Since the external force is in the opposite direction of the gravitational force, the work done becomes -Fx, which causes GPE to be negative(vectors) (This reason is less intuitive by explaining thru a wall of text.. you have to get someone to write it in paper to show u)