From: Debbie Berlin (debbie_berlin@yahoo.com)
Date: Mon Nov 24 2003 - 13:12:37 PST
Message-ID: <20031124211237.57196.qmail@web11407.mail.yahoo.com> Date: Mon, 24 Nov 2003 13:12:37 -0800 (PST) From: Debbie Berlin <debbie_berlin@yahoo.com> Subject: 2 friction questions for the physics folks out there
Every time I teach friction, it bothers me more and
more. Although everything we teach is just a model on
some level, friction particularly bothers me because
the model often doesn't work (sufficiently describe
the real world). Right now, I'm puzzling over two
questions and I was hoping for help. Forgive any
misconceptions I may have...and thank you in advance
for your help!
1) WHAT'S REALLY GOING ON: For most purposes, I
define friction as tiny bumps in surfaces that slow
objects down or keep them where they are. But really,
it's an electrostatic force. For example, let's
imagine a foot on the ground. The electrons in the
"foot molecules" are on the outside, as are the
electrons in the "ground molecules," so the electrons
determine the force. But, what's really happening
past that? Someone explained that the electrons
repel, but that doesn't seem to sum to a resistive
force...two electrons approaching each other slow down
and moving away from each other speed up. So, what's
the real deal? What actually slows stuff down? I'm
ready for a better model. Even if the kids don't want
to hear it, it's bugging me.
2) THE DEFINITION OF FRICTION I GIVE KIDS: Every
textbook defines friction as a resistive force in
general, or else specifically defines kinetic friction
as opposite in direction to velocity and static
friction as opposite to the sum of the other forces.
Yet, there are some cases that this doesn't extend
to...for example, a table cloth is dragged slowly and
the plates on top of it start to move with it. So,
it's really the friction that accelerates the plates.
So, in this case, friction isn't really resisting
motion or outside forces...not much of a "resistive
force" at all unless you want to talk about resisting
inertia, which is just awful. Is there a more general
definition that is simple enough for kids to
understand?
Thanks--I really am bothered by this.
--Debbie
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