Re: [Phys-l] authoritative gravitational potential energy

[Carl Mungan <mungan@usna.edu>]

>  > .... define gravitational potential energy as the work done to
> >  move an object from a very large distance away to a
>  > point in a gravitational field

The usual definition is the *negative* of the work done *by the 
gravitational force*. Using this corrected definition, it doesn't 
matter whether the object is at rest before and after.

Furthermore, there should be some mention of the idea that PE is only 
defined for a system that includes both interacting objects (and 
implicitly the field) and that in general, we need to sum up the 
negative gravitational work that each object does on the other. By 
summing up both, we get a result that is frame invariant, whereas the 
work done on just one object is not. (One therefore needs to be 
careful when computing escape speeds in moving frames, as for example 
when you simultaneously want to escape from both the sun *and* the 
earth. I have a recent article on this topic if anyone's interested: 
<http://usna.edu/Users/physics/mungan/Publications/WAS2.pdf>.)
--
Carl E Mungan, Assoc Prof of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-5002
mailto:mungan@usna.edu     http://usna.edu/Users/physics/mungan/