Page 82 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 6

Friction

STATIC AND KINETIC FRICTION

Frictional forces act to oppose relative motion between surfaces that are in contact. Such forces act parallel to
the surfaces.
Static friction occurs between surfaces at rest relative to each other. When an increasing force is applied to
a book resting on a table, for instance, the force of static friction at first increases as well to prevent motion. In
a given situation, static friction has a certain maximum value called starting friction. When the force applied to
the book is greater than the starting friction, the book begins to move across the table. The kinetic friction (or
sliding friction) that occurs afterward is usually less than the starting friction, so less force is needed to keep the
book moving than to start it moving (Fig. 6-1).

Fig. 6-1

COEFFICIENT OF FRICTION
The frictional force between two surfaces depends on the normal (perpendicular) force N pressing them together
and on the natures of the surfaces. The latter factor is expressed quantitatively in the coefficient offriction µ (Greek
letter mu) whose value depends on the materials in contact. The frictional force is experimentally found to be

F f ≤ µ s N Static friction
F f = µ k N Kinetic friction
where the symbol ≤ means “less than or equal to.” In the case of static friction, F f increases as the applied
force increases until the limiting value of µ s N is reached. Thus when there is no motion, µ s N gives the starting
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