Re: [Phys-L] point particles puzzle

[Carl Mungan <mungan@usna.edu>]

> > 4) Since there is no net force, there is no F•dx.
> >  Therefore no work is being done on any of the points.

That conclusion doesn't follow because F is an individual force, not the
sum of all forces. (The sum of forces can be used to compute pseudowork but
not real work.)

The person pulling does positive work on the spring. The spring does equal
and opposite negative work on the person. As a result the person loses
chemical energy and the spring gains elastic potential energy. If one
subdivides the spring into portions (rather than considering the whole
spring at once as I just did) then one end moves farther than the other end
and so the equal and opposite forces at their two ends do not do equal and
opposite work, and each portion of the spring thus gains a portion of the
elastic PE. -Carl

-- 
Carl E Mungan, Assoc Prof of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu     http://usna.edu/Users/physics/mungan/