Gravitational Potential Energy. (1 Viewer)

[CHUDYMASTER]

I just don't get how Ep can be negative. So far I understand that as you move further away from the source of gravity, tending to infinite, the Ep will tend towards 0.

But in addition to my misunderstandings, I can't quite grasp how an object gains gravitational energy as you displace it up to a certain level, but when it drifts off into space, you lose this Ep.

Have I totally missed the point? Any help would be appreciated.

 

[kaseita]

say in the earths atmosphere, the object has an Ep of -1000
if the object is now say a million kms away from the source, then its Ep would be say, -100 (I'm just making up random numbers here)
It gets closer to 0, because as you get further away from the source towards infinity, the Ep will become 0.
but notice something. from going from one point near the surface, to further away, its gained GPE. it now has a smaller negative number. its gone from -1000, to -100, so in fact, its gained GPE.

 

[CHUDYMASTER]

But...

i don't understand, isn't energy purely magnitude? in which case, -1000 joules (negative or not) SHOULD be more in quantity than -100 joules.

I mean, how can it gain GPE anyway, sure the distance has increased, but if the acceleration due to gravity gets more and more smaller with distance, (say 0.00000001), should this not DECREASE the GPE???

 

[BlackJack]

Well, in earlier years GPE is also defined as the total kinetic energy it would gain if the object falls to the centre of the earth, and takes mgh. Since this starts from 0 & rose to infinity when it gets further.... it's more useful in astrophysics to change the definition slightly (to something like the energy it takes to move the object from infinity to its current distance), don't know the exact def....

But being pulled by gravity, the object would gain energy instead of using it up. Hence negative.

That explains why it gains GPE as it moves further away.

 

[kaseita]

oh no, energy is a scalar quantity, not vector. therefore when your saying a negative value, your not saying what direction its in.
plus we're talking about potential energy, not energy done to do work.

and blackjack explains the rest.

 

[CHUDYMASTER]

Thanks.

Yeah it all makes sense now, you were right the first time.

Thanks again guys.