4.1.4—Friction

Friction acts along the contact surface and opposes relative motion or its tendency. In limiting equilibrium F=μR; otherwise F≤μR.

Draw the normal reaction and friction separately, identify the possible direction of motion, and solve force/moment equations before checking the friction limit.

A block on a rough incline has friction up the slope if it would otherwise slide down; the limiting value μR determines the largest angle for rest.

Friction is not always μR: that equality applies at limiting friction, not every static situation.