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Post  Post subject: Potential Energy  |  Posted: Tue Nov 01, 2016 11:04 am

Joined: Tue Nov 01, 2016 10:53 am
Posts: 2


I need help in my understanding of potential energy. So I understand that the potential energy on points near the earth surface is mgh.
Take the surface of earth to be h = 0. Then assuming that an object does not fall below the surface of earth, the potential energy will always be positive. I have been taking potential energy to be positive for a long time while I use the formula;
Em = Ep + Ek
and it always works.

However, many sources stated that gravitational potential energy is negative (I'll provide a link if needed, otherwise just google 'is potential energy negative or positive')

If someone could help me clarify this thing, that would be great!

Thanks in advance!

Post  Post subject: Re: Potential Energy  |  Posted: Tue Nov 01, 2016 11:56 am

Joined: Wed Jun 26, 2013 10:44 am
Posts: 499
Location: Newcastle-upon-Tyne

It is negative by convention. One way to think about it is that it is because the PE at an infinite distance from the gravitational centre is given the value zero. When it falls towards the body the object gains kinetic energy, if the total energy is to be constant the change in PE must be negative to balance the gain in kinetic energy.

Another way is that the definition of potential energy is the integral of F dr (where F is the gravitational force and r is the distance the object falls). As gravity is an inverse square law you are essentially integrating 1/r2 dr which is -1/r. This is where the minus sign comes in...

You might find this page useful: http://hyperphysics.phy-astr.gsu.edu/hb ... t.html#gpi

"The big trouble with dumb bastards is that they are too dumb to believe there is such a thing as being smart"

- Kurt Vonnegut, The Sirens of Titan

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