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



Hear, hear - totally agree.
The only need to mention acceleration is if you want to hang a name on the slope of a velocity graph.

Finding areas of v vs t graphs is VERY powerful and surprisingly easy for kids to 'understand'.
WAY better than memorizing/manipulating equations.

At 11:30 AM -0800 1/11/11, 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.

John Mallinckrodt
Cal Poly Pomona

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