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Re: Rolling, Static, and Kinetic Friction



Leigh Palmer wrote:
...
It is said: "A difference, to be a difference, must make a difference."
Since the dynamical effect is the same for rolling, static, and sliding
friction, why distinguish among them at all for the high school student?

Since when is the dynamical effect the same? Static friction is a force
of constraint that prevents relative motion parallel to two contacting
surfaces and which dissipates no energy. Various kinetic frictions (i.e.
rolling, sliding, etc.) provide no constraint and *do* dissipate energy. This
is a *significant* difference--even for high school students.

I concede your point, but my plea was for dynamical effects. Static
friction can certainly change the kinetic energy of a car just as
surely as can sliding friction or rolling friction. That the latter
two dissipate energy* while the first changes it conservatively
is of secondary importance in an introductory course. Once the
students know what F = ma means, and believe it, Then one can go on
to more sophisticated topics. In my experience students will view
getting the label correct as a point of equivalent importance to
getting the acceleration right. We seldom test them to see if they
understand F = ma because we don't know how to do it**. I suggest
that a student who knows the "correct" name for a particular force
category has demonstrated nothing regarding mastery of physics or
understanding.

Leigh

*sliding friction need not reduce the kinetic energy of, say, an
automobile. It could increase it. I've done it myself. Think.

**I'll make another post on this.