Re: [Phys-l] Newton and Snell (was Global evolution as fact)

[ludwik kowalski <kowalskil@mail.montclair.edu>]

On Jan 11, 2011, at 2:30 PM, John Mallinckrodt wrote:

> On Jan 11, 2011, at 10:58 AM, chuck britton wrote:
>
> > It's time to start teaching N2 the way Newton wrote it.
>
> > F = dp/dt
>
>
> Newton "wrote" N2 (and thought about it) as
>
>  change in momentum = impulse
>
> where
>
>  impulse = integral of force over time.
>
> The constructions
>
>  rate of change of momentum = force
>
> and
>
>  acceleration = force/mass
>
> may seem to be obvious implications of the above, but they suffer  
> from significantly larger conceptual pitfalls.
>
> I have argued for some time that Newton himself was at least  
> uncomfortable with "acceleration," generally preferring to talk  
> about and construct proofs in terms of "changES in velocity (or  
> momentum) rather than "changING velocity (or momentum)."  I believe  
> this has to do with the fact that integration is a conceptually  
> simpler idea than differentiation.  Thus, I advocate avoiding any  
> mention of "acceleration" until after having dealt with 1) forces in  
> static equilibrium, 2) the first law (in the context of galilean  
> relativity), 3) the second and third laws (in the context of impulse  
> and conservation of momentum), and 4) work and conservation of  
> energy, none of which require the concept of acceleration.


I would do this, if I were to teach again. The reason I am replying  
has to do with the principle of causality, that was discussed here  
recently. The mathematical statement

change in momentum = impulse

does not address the issue "what causes what." But I suspect that  
Newton did address the issue. He probably wrote that change in  
momentum results from an impulse. Can someone to either confirm or  
refute this? I do not have an easy access to his Principia.

Ludwik

http://csam.montclair.edu/~kowalski/life/intro.html