Re: Newton's second law

[ALVIN BACHMAN <bachman2@PRODIGY.NET>]

Just a reminder: The second law, whether  F = M a
or a version of Impulse-Momentum, F dt = Pf - Pi,
are VECTOR equations.

--- Jim Green <JMGreen@SISNA.COM> wrote:
> > It is very dangerous to expand N2 into F = mdv/dt +
> v dm dt.
>
> In my opinion there is nothing wrong with F=dp/dt
>
> > 1) If part of an object merely "drops off" of a
> mother object, with no "push
> > off" force,  there is no interactive force, and no
> exchange of momentum
> > between the two system parts.
>
> The difficulty comes in choosing the system and
> _sticking with it_  In one
> choose either the "part" or the "mother" or both ie
> the cm of both, but
> don't change stories during the solution.
>
> SumFi=dp/dt is valid in each case.
>
>
> > 2) Rockets, conveyor belts, etc (where there is an
> interaction force) are
> > best handled by the Momentum-Impulse theorem.  F*dt
> = Pf - Pi
>
> This is not really a theorem - it is a derivation
> from N#2:
>
> IntFdt ~= F deltat = Int(dp/dt)dt = Pf -Pi
>
> We also sometimes forget that Young's _invention_ of
> the idea of "energy"
> comes from a similar integral.
>
> Jim
>
>
> Jim Green
> mailto:JMGreen@sisna.com
> http://users.sisna.com/jmgreen