mistake in proof of gravitational potential equation??? Hi, I have a problem in deriving an equation for the gravitational potential. Well, I think that the graviational potential is the work done by external forces in moving a unit mass Mp from infinity to that point through distance d, and so I converted the equation in this way: when M is the mass of a planet creating the gravitational field and r is the distance between M and Mp, V= (work done) / (mass) =F.d / Mp =(G*M*Mp/r^2)*d/ Mp ... and I got nowhere. However, my text states that the distance from the infinity to the point mass Mp should be r as well. So the text says that V= (work done) / (mass) =F.r / Mp =(G*M*Mp/r^2)*r/ Mp and in this way the correct equation, V=-GM/r (negative sign is added afterwards) shows up. But I don't understand why the text can set both the distance from the infinity to the point mass Mp and the distance between M and Mp as r. To make the problem more complicated, my teacher has said that there might be a typo or two in the text, which makes the understanfing very difficult. So it might be included in this context, I don't know... If anyone could explain me this, please help me by doing so.......... I would really really appreciate your advise!