Re: Newton's second law?

From: Lewis Mammel (l.mammel_at_worldnet.att.net)
Date: 12/20/04 

Date: Mon, 20 Dec 2004 05:30:43 GMT

Androcles wrote:
>
> "Andr? Michaud" <srp@microtec.net> wrote in message
> news:562f286c.0412191821.80b9cba@posting.google.com...

> > Rocket launching is a special case.
>
> Not at all. It is included in F = dp/dt.

I don't believe this can be justified. If you want to justify it, you
have to identify your terms. F is the force of what acting on what?
p is the momentum of what?

Halliday and Resnick ( 1966 ) comment, "It is important to
note that we CANNOT derive a genral expression for Newton's
second las for variable mass systems by treating the mass
in F = dP/dt = d(Mv)/dt as a variable."

Also please note, if you're following Newton, his "force" is
a finite impulse acting, in many examples, in zero time.
His Laws all pertain to finite changes of momentum.

"When a body is falling, the uniform force of its gravity
acting equally, impresses, in equal intervals of time, equal
forces on that body, and therefor generates equal velocities;
and in the whole time impresses a whole force, and generates
a whole velocity proportional to the time."

He reserves infinitesimal reasoning for the analysis of particular
problems. e.g. Proposition I. Theorem I. "...Now let the number of
those triangles be augmented, and their breadth diminished IN INFINITUM; ..."

> > Universal motion of stable
> > masses is the general case for which the 2nd law was established
> > by Newton.
>
> You are guessing.

He certainly considered systems of masses, and the rocket
is easily analysed by considering the system, rocket plus fuel,
using Newton's Corollary III, as I stated.

> > The equation that applies to rockets is a simple extension of
> > the basic 2nd law equation for stable masses.
> >
> Not at all. The second law is F = dp/dt.

See above.

> F= ma is taught in schools as a precursor to aid understanding.

It's not a precursor, it's a reformulation, and it's entirely
equivalent to Newton's Second Law.

Lew Mammel, Jr.

Relevant Pages

  • Re: OT: It made me laugh
    ... drunk and reverting to acting like a 12 year old again. ... Don't bother trying to justify it. ... others laugh too. ...
    (uk.rec.motorcycles)
  • Re: Hyphen acting like minus
    ... it's not acting like a minus sign but rather like a line break hyphen because I am trying to right justify the string in the label. ...
    (microsoft.public.windowsce.embedded.vc)
Quantcast