What is the maximum static

friction force? The coefficient of

static friction for these materials

is 0.31.

fs,max = μsFN

fs,max = (0.31)(27 N)

fs,max = 8.4 N

5.18 - Kinetic friction

Kinetic friction: Friction when an object slides

along another.

Kinetic friction occurs when two objects slide past each other. The magnitude of kinetic

friction is less than the maximum amount of static friction for the same objects. Some

values for coefficients of kinetic friction are shown in Equation 2 to the right. These are

calculated empirically and do not vary greatly over a reasonable range of velocities.

Like static friction, kinetic friction always opposes the direction of motion. It has a

constant value, the product of the normal force and the coefficient of kinetic friction.

In Example 1, we state the normal force. Note that the normal force in this case does

not equal the weight; instead, it equals a component of the weight. The other

component of the weight is pulling the block down the plane.

Kinetic friction

Friction opposing sliding in motion

Force constant as object slides

Proportional to:

·coefficient of kinetic friction

·normal force

(^102) Copyright 2007 Kinetic Books Co. Chapter 05