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



John,

I have no doubts that physics can teach these topics more effectively, especially if there is some hands-on component to the teaching. My question really goes to whether we should be teaching kinematics at all. Most accelerations in practice are not constant. Most projectile motions are not parabolic (even in the flat earth approximation) because of air resistance. There are many useful topics we choose to leave out of general physics. I just hope we are not spending an inordinate amount of time on idealized motions just to produce practice problems for F=ma. Most motion problems can be done more easily using momentum and energy - and don't require the artificial restriction of constant acceleration - although that class of problems is easily included. Take the case of dropping a ball---most of the interesting parameters of the motion come from energy balance - the kinematic equations aren't actually used at all.

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement [clement@hal-pc.org]
Sent: Wednesday, January 12, 2011 12:19 AM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] Newton and Snell (was Global evolution as fact)

The big reason for spending time on kinematics is that math does not teach
them in a manner so as to generate understanding. The research shows that
students have to use multiple representations: pictorial (motion maps),
graphs, descriptions, and lastly equations. Unfortunately most conventional
physics courses also do not generate good understanding.

The work of Jerry Epstein has shown that students come out of math courses
with very low understanding, so we have to teach them the relevant math. I
can confirm this from what I have observed in the calculus based courses.
Actually well over 30 years ago I taught a conceptual physics course at Duke
U where I stressed kinematic graphs, and a student who had taken calculus
told me that it made her understand calculus. In addition the students did
very well at relating velocity, position, and acceleration graphs.

It has been stated that the sequence of starting with impulse momentum is
superior to the conventional one. I would say that this would need to be
demonstrated by using conceptual tests such as the FCI/FMCE. At present the
Modeling program has some classes that have achieved normalized gain as high
as 90%, so it would seem to be that the conventional sequence works very
well provided research based instruction is used. So I must take exception
to the claim that impulse-momentum is superior. It has not been
demonstrated.

Actually the dividing line between math and physics is fairly broad, as are
all dividing lines between fields. Kinematics is both math and physics.

John M. Clement
Houston, TX


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.



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