Re: Newton's second law

[Jim Green <JMGreen@SISNA.COM>]

> 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