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]

Re: [Phys-L] feeler-dealer, third law, et cetera



JD, if you want to create a new version of Newton's 3rd law and still give
it the same name, then you can make any conclusion you want to, and you can
distort anything I say. The plain fact is that Newton's 3rd law is a
historical statement made by Newton about F and -F on objects. There's
nothing in that statement about dp/dt or fields or momentum conservation.
When I say that Newton's 3rd law is not general (because magnetic forces do
not in general have F and -F on objects), I'm referring necessarily to the
actual statement of Newton's 3rd law, which is repeated in intro textbooks,
not to some other statement that you choose to invent. I stick to my guns
that this historical law does not apply in general to magnetic forces
(Newton didn't know about magnetic forces between two moving charged
particles). There could be a Denker's law that might well be a much better
and fully general law, but to try to claim that Newton's 3rd law says
something different from what it actually says, or give some updated
statement the same name, makes rationale discourse impossible.

You object surprisingly strenuously to our choosing different language, to
speak modestly of some but not all forces having the property of
reciprocity (F/-F), language created by Fred Reif, which we embrace. We do
this precisely in order to achieve precision of language, because we cannot
in good conscience claim that the historical statement of Newton's 3rd law
is a general law when it is not.

A summary:

(1) Fact: Newton's 3rd law, as stated by Newton and repeated in all intro
textbooks and elsewhere, states that forces come in F/-F pairs.

(2) Fact: Magnetic forces involving two moving charges particles do not
always come in F/-F pairs.

(3) Fact: Therefore magnetic forces do not always obey Newton's 3rd law
(and in consequence the momentum of two isolated particles need not remain
constant).

(4) Fact: Therefore Newton's 3rd law is not general.

(5) Fact: Momentum conservation IS general.

(6) Fact: Therefore we conclude correctly that there must be momentum in
the field, and that the particle momentum and the field momentum added
together will remain constant.

Bruce