From: pauld@exploratorium.edu
Date: Thu Nov 27 2003 - 15:57:05 PST
Message-ID: <1520.209.239.173.234.1069977425.squirrel@www.exo.net> Date: Thu, 27 Nov 2003 15:57:05 -0800 (PST) Subject: Re: pinhole 2 friction questions for the physics folks out there From: pauld@exploratorium.edu
I hope these are a start toward answering your questions:
1. Atoms attract each other when a little ways apart and repel when
pushed together. (Due to electrical interactions between electrons and
nuclei, i.e Van Der Waals interactions)
When two rough surfaces (and even fire polished glass has rough spots a
few hundred atoms high) slide across each other. The protrusions on one
surface colliode with the protrusions on the other surface (collide via
electrical forces.) When the surfaces are inspected before and after two
objects slide across each other with friction, the protrusions are shown
to have changed shape, the tops have been deformed by plastic flow and
melting.
2.
The Friction force
The part of the contact force between two objects parallel to the surfaces
between them. (The perpendicular part is called the "normal" force.)
Paul D
> 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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