|Chronology||Current Month||Current Thread||Current Date|
|[Year List] [Month List (current year)]||[Date Index] [Thread Index]||[Thread Prev] [Thread Next]||[Date Prev] [Date Next]|
Date: Tue, 3 Feb 2004 09:40:47 -0500
From: Bob Sciamanda <trebor@VELOCITY.NET>
Subject: Re: PHYS-L Digest - 1 Feb 2004 to 3 Feb 2004 (#2004-34)
I would comment that this is an instance of the weakness (incompleteness?)
of attempts to bypass the force concept and begin with (and be confined to)
energy/momentum concepts (lifted out of the air). To me, the force concept
(embodied in all three N laws) is the indispensible root basis of Newtonian
Even if you lift out of the air the conservation of gratuitously defined
energy (K + P) and momentum, how do you arrive at the existence of an
orbital trajectory without (gratuitously) adding the stipulation that the
momentum exchange is central?
It bothers me to write that the KE =3D 1/2 PE. Wouldn't it be better=
write that that KE =3D 1/2 |PE|, where PE is set to zero at infinite
From: John Denker <jsd@AV8N.COM>
Update: Now I know there isn't one.
Consider two potentials, marked with ' and ".
' ' ' ' ' ' '
" " " " " " " " " " "
' <-- X
That is, at point X, the two potentials agree as to the slope
of the potential (i.e force), but one of the potentials has
only half the binding energy of the other. I repeat:
same force, different energy. This situation arises naturally
e.g. if you compare the Coulomb potential to the Yukawa potential.
It can be demonstrated in the classroom by rolling marbles in
I know Brother Blais wants to restrict attention the gravitational
1/r potential ... but the energy principles don't know that. To
make the result stick, at some point you will have to bring in
the *local* shape of the potential (not just its total depth)
and that is tantamount to bringing in the force.