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Re: Rolling, Static, and Kinetic Friction



Lowell Herr asks a telling question which makes my point:

Why distinguish? So the question can be answered.

You have hit the nail on the head! We obfuscate by assigning unnecessary
names to indistinguishable concepts and then test our students to see if
they have learned those names. It does not help the student one tiny bit
to know the name we ascribe to a particular frictional force if his sole
purpose is to predict the effect of that force on the particle. If it is
his pupose to answer a multiple choice question, why then this will help
him, but I would like to think that none of us thinks that is physics.

The frictional force is tangential to the common surface of contact. We
need to know its magnitude and direction to reckon its effect; we have
no need for its name. All frictional forces have the property that they
lie in that plane and after that has been said we can squeeze no more
out of it.

I am writing a paper on a related topic which I will submit to AJP.
There the topic is the "distinction" between real and imaginary images,
another difference that does not make a difference.

Leigh