Friction Brainstorm - October 24, 2001

A. A friction force only occurs when 2 objects are in contact (friction is a contact force).

Commentary:
1. Kinetic friction only occurs when 2 objects are in contact and at least one is in motion. 2. Static friction only occurs when the friction force is preventing motion (true??)

B. Friction force always opposes motion. (Is this always true?).

C. Magnitude of friction depends on the type of surfaces in contact.

D. No friction force implies constant velocity motion

E. 2 kinds of friction forces (static friction-not moving/kinetic friction-moving)

Commentary:
Depends of whether the 2 surfaces are moving with respect to one another
If the surfaces are moving relative to one another => kinetic friction
If the surfaces are not moving relative to each other => static friction

F. magnitude of frictional force = (coefficient of friction) * (magnitude of Normal force)

Commentary:
1. for kinetic friction,
(magnitude of frictional force)
= (coefficient of kinetic friction) * (Magnitude of Normal force)

For static friction,
0 < (magnitude of frictional force) < or = (magnitude of static friction) * (mangnitude of Normal force)

G. Friction force generate heat

H. The friction forces are perpendicular to the normal forces for any 2 surfaces in contact.

I. Friction is a force. Frictional force is a vector.

J. Amount of surface area affects friction

K. (coefficient of static friction) > or = (coefficient of kinetic friction)

Commentary:
(student comment) Textbook argues that the coefficient of static friction must always be greater than the coefficient of kinetic friction or else nothing could move
(Need to check on this.)

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