Question

Gravitation potential

Answer 1

If we are bringing a particle from infinity to a point in gravitational field "without acceleration" then the amount of WD is said GP right?

Answer 2

yep

Answer 3

but the particle will come itself won't it? :O

Answer 4

the work done by us?is what?

Answer 5

Well you don't bring the particle, the work is done by gravitational field

Answer 6

so what is the WD by us?

Answer 7

Negative of that work which the field does

Answer 8

then why are we creating problems in its way o.O

Answer 9

see it wants to come itself and we are stopping it from coming O.o by applying back force

Answer 10

Well we are humans, we don;t care what others want ! We'll do what we want to do, if we want to be an obstacle for some of our purpose, we will be! B|

Answer 11

wtf

Answer 12

@Vincent-Lyon.Fr
Why will the particle from infinity come itself?
Why do we have to bring it with constant velocity(no change in KE)
We are applying equal and opposite force to that of gravitational field to maintain 0 force i.e constant velocity isn't it?

Answer 13

ahh

ok .. well the - sign itself indicates that.. it indicates that we need to do negative work .. which means work is done on the particle by the field itself.. so the potential energy goes down.. think of it this way

imagine you are a worker.. and your work is to push a car.. your boss will pay you for pushing the car.. if you push the car by 1 m... you get 10 bucks..

so if you push you spend energy.. you get 10 bucks for it.. thats like the spent energy gets stored up as P.E.

this is what happens if for example.. you have two positive charges.. and you bring one positive charge close to the other, the work you did on it, stores up as P.E of that charge..


now imagine this.. what if instead you pushing the car, you just sit in the car and the car drives you itself.. your boss not only NOT pays you.. but also asks you to pay up.. cause you used the car.. similarly when a particle is in gravitational field.. the work is done ON the particle itself... hence the particle LOSES P.E.. so we declare that the particle has zero p.E. at infinity.. so in the finite world particle will always have negative p.e

This is just like to start with you had no money and you used the car.. so you ll OWE your boss.. thats like negative money!.. hope that helps :D.. my analogy may need some betterment.. anyone please help :P

Answer 14

I do not like this way of presenting how to calculate Potential energy. It is too complicated to invent a counter-force and to work out the work done by this force to define potential energy.

Change in potential energy (associated to a conservative force) is simply the opposite of the work done by the force itself from A to B.
If you want an absolute PE, you must then choose an origin where, by convention PE is zero.

In the case of gravitation and Coulomb's forces, the origin is usually chosen at infinity. But it is not necessary.