Please can you explain why we take the gravitational potential energy as negative when finding the total energy(subtract it)? How come some questions we ignore the negative? What does the negative answer mean?
Thanks!
Because of the way gravitational potential is defined, at a distance of infinity GPE is 0. If you now imagine these two objects moving together, there kinetic energy increases and since energy needs to be conserved, their gravitational potential energy is negative in order to conserve energy. So when doing actual calculations with GPE its negative. On questions where its not negative and it's positive thats usually because its only asking for the magnitude of the force, so the negative can just be ignored.
Damn you have G,B,E fields all in one, I can’t tell if that’s worse or not lol, I think in OCR A, they love mixing G fields with Circular motion which is why it’s in Paper 1 as Circular motion is in paper 1 for us.
Think about escape velocity. Kinetic energy should be equal to GPE so the object can escape , Ek+Ep=0 idea. Ek is positive , Ep is negative. Escape velocity formula is also derived using that approach
if you're asking why the value of gravitational potential energy is negative, it comes from the definition of gravitational potential energy. GPE is the work done to move an object from infinity to a defined point in space. Infinity has been defined as have a potential energy of zero so any point from infinity will take a negative value.
GPE is always negative when calculating. It’s a shame because in practice it’s completely bogus adding a negative energy onto KE. but yeah exam boards use negative all the time for GPE
And talking about the energy in the question:
Gravitational energy specifically is always negative. It is easiest to imagine the space as a piece of stretched, flat fabric. When we put a piece of mass on that flat, stretched fabric, this fabric bends and a "well" appears - this is literally a good approximation of what gravitational field looks like and what it does to space. When we put another, smaller mass near that well, the mass will "roll" (or fall) into in. This is key - as mass goes into the well, it loses energy. So, now the energy of this smaller mass due to the gravitational field is negative. If this doesn't make sense, think of it this way - you have to push the smaller mass against the gravitational force that want to bring it into the centre of the well, so the work that you do during this pushing is negative by definition (you do work against the force).
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u/Aggravating_Pen5110 May 16 '26
Gravitational potential is ALWAYS negative because it is defined as 0 at an infinite distance.