The Student Room Group
Reply 1
rezarf
I understand everything else about them, just not why they're the wrong way up!!

Gravitational potential is negative.

edit: If you don't understand why this is, this forum post has a good discussion of it: http://www.physicsforums.com/archive/index.php/t-163171.html
Isn't it negative because it is the energy REQUIRED to take an object to a very large distance where the potential is 0?
Reply 3
GPE at infinity is defined as zero. In other words, zero is your maximum GPE therefore anything less than maximum is negative.
rezarf
I understand everything else about them, just not why they're the wrong way up!!

This is more of a rationalisation than a true explanation, but anyway...
We know that gravity is an attractive force, and presumably you also know that a system will tend to move towards the lowest energy state? Given this it follows that the potential will be lower as the distance between two particles decreases.

That's a very non-technical explanation, but hopefully it helps a little; if you want something more detailed, just follow M_E_X's link. :smile:
Reply 5
Illusionary

We know that gravity is an attractive force, and presumably you also know that a system will tend to move towards the lowest energy state? Given this it follows that the potential will be lower as the distance between two particles decreases.

That's a very non-technical explanation, but hopefully it helps a little; if you want something more detailed, just follow M_E_X's link. :smile:

That's right, exactly :smile:. We say bodies can fall in to a "potential well", and this makes sense with potential being negative.
If potential was positive and a body was on the top of a "potential hill", it would just roll down and hence escape from the potential - this wouldn't make sense.
Reply 6
Is it not something to do with gravity being a negative vector quantity?
Reply 7
Its to do with how we define zero

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