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



John,

May I also chime in with a preference for your outline of topics. We spend too much time with the kinematic equations for constant acceleration - mostly because we wish to apply F=ma at some point. These are really math equations, not physics per se. It may be nice to know how long a runway must be if an aircraft with a certain acceleration needs a certain take off speed - but these are math problems. We probably only do them in our courses because we spent time developing the kinematic equations. If we covered other topics, and used impulse<-->momentum instead of F=ma, we would probably find a whole new set of problems that we would feel are important.

Bob at PC



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Mallinckrodt
Sent: Tuesday, January 11, 2011 2:30 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Newton and Snell (was Global evolution as fact)

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.

John Mallinckrodt
Cal Poly Pomona

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