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Re: momentum before force (was: friction)



In a message dated 96-07-15 20:15:17 EDT, James Mcleans writes:

<< Raacc@aol.com says:
...
It seems to me that velocity should be introduced followed immediately by
momentum. Then acceleration and net force can be defined as the slopes of
the velocity and momentum graphs respectively.)
...

James Mcleans writes:

When you teach this sequence, how do you justify momentum before force has
been introduced? Using a qualitative idea of force?

I can maybe imagine doing this, but I can't see how to justify that
momentum is (m*v) (rather than, say, (m^2 *v)), since I don't know how to
test it. Maybe you do conservation of momentum?
>>

First, I haven't used this approach yet, but I am seriously considering it.
With most physics texts I've seen, force is introduced after acceleration
and before momentum. So, it would probably be a nightmare for students.
However, I am thinking of chucking the text, at least for mechanics.

Using video analysis, I stress observation of the motion, observation of the
graphs of the motion, finding equations (models) that describe the motion,
and checking the models. So, this approach emphasizes graphs heavily. In
doing so, I define velocity as the slope of a position (or displacement)
graph versus time. At this point, it would be just as easy to define
momentum. Then when finding velocity, it is just one more step to find
momentum.

As far as justification, I don't see why you need any. If you were teaching
someone English, and showed them an orange, you would say this is an orange.
What would be your justification? Why do we call an orange and orange? Of
course, you could go into the lingual history of the word, but is this
important? Physics has a terminology and momentum is one of the terms. Why
is momentum mv and not m^2v? Because we don't define it to be m^2v, we
define it to be mv.

When introducing acceleration, I define it as the slope of a velocity versus
time graph. If momentum had already been introduced it would then be just as
easy to define force as the slope of a momentum versus time graph. From a
graphical view, wouldn't discussing force before momentum be the same as
discussing acceleration before velocity?

rac

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Robert A. Carlson raacc@aol.com

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