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Re: "acceleration due to gravity"



Ludwik Kowalski wrote:

[snip]

But
unless the sequence of teaching physics changes I will
continue using m/s^2 before I start using N/kg. Should
we teach dynamics before kinematics? I do not think so.

Well, Ludwik, I think I might disagree with you here. We have been
teaching conservation of momentum before we get to accelerated motion
for some years now, and it seems to make sense to the students. The
idea of acceleration is rather abstract, and we felt that introducing
it before the students have a pretty good idea of what motion is and
how it changes didn't make all that much sense. So we start with
uniform motion using the slope of the distance curve and the area
under the velocity curve, and then sneak in uniformly accelerated
motion (without using the word acceleration) so they can see how the
area under the velocity curve generalizes. That is followed by a bit
of one-dimensional relative motion, and the concept of center of
mass. And then we go into momentum, all the way through
one-dimensional collisions. We do the collisions by looking at the
center of mass reference frame, which they know something about
relative motion and centers of mass, and we can even do partially
elastic collisions using this method. It is really empowering to the
students because by the time they are finished with this part of the
course, they can solve some really interesting problems.

This leads us to the concept of impulse, force, and Newton's laws, in
the context of changing momentum--still no acceleration. Finally
after all this, we break a changing momentum down into mass and--at
last!--acceleration. Now when we start talking about *g* in terms of
N/kg they know about Newtons, they have done a simple lab where they
converted Newtons to kilograms and they see that the conversion
factor for this was just our old friend *g.*

There's more, but this shows, I think, that teaching momentum and
forces before acceleration is a viable strategy, and pedagogically
sound.

Hugh

--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

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