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On 12/14/2010 05:19 PM, William Robertson wrote:
I believe that the concept of system is given short shrift in
too many physics or other science texts. There exists research showing
that understanding or not understanding a choice of system can
dramatically affect one's problem solving ability in physics.
Well, that cuts both ways.
Yes, physics is about principles. But physics is also about applications
and approximations.
In the physics course, I want students to learn the principles. But
just as importantly, I want them to learn how to make well-controlled
approximations.
If we are talking about the earth/moon system, the gravitational energy
is clearly in the system, not "in" the moon. The principles of the thing
are clear, and the same principles apply -- in principle -- to every other
gravitating system.
On the other hand, in a very wide range of practical applications,
including soccer balls, planes, trains, and automobiles, we find "m g h"
is an exceedingly good approximation, and is significantly simpler than
"G M m / r". Treating the earth (and the earth's gravitational field)
as imperturbable is an approximation. Like all approximations, sometimes
it is appropriate and sometimes it is not.
Deciding what approximation to use in this-or-that situation requires
judgment and skill.
It is ultra-super-important that students understand we are not peeved
about the approximation but rather about certain _inappropriate uses_
of the approximation.
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