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Re: Rolling, Static, and Kinetic Friction




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?

Leigh

The reason I see to introduce the terms static and kinetic friction is to
account for the differences in their respective coefficients. It is true

Howdy,

If you use Mr. Leigh's argument then we shouldn't distinguish any force
from any other force since they all give rise to acceleration when combined.

There are fundamental differences between static and kinetic friction. The
static friction force must be treated as an unknown in a dynamics relation
since the coefficient of static friction and the magnitude of the Normal
force determine the MAXIMUM value of the static friction force. As a side
comment then, since you must treat the static friction force as an unknown
you may choose its direction arbitrarily (but parallel to the surface at
which it acts) since the sign of the force will tell you which way ti
points after you solve for it. On the other hand, the kinetic friction
force has a magnitude given by the coefficient of kinetic friction and the
magnitude of the Normal force and ALWAYS has the direction opposite to the
direction of slip (if it isn't slipping, it isn't kinetic friction).

This was all said before in terms of whether the force was constrained or
not; maybe this will clear it up further.

Good Luck,

Herb Schulz
(herbs@interaccess.com)