258 Chapter 10 Force and Force-Related Parameters
side by side on an inclined surface, which of the two liquids will flow easier? You know the
answer: The water will flow down the inclined surface faster because it has less viscosity. Fluids
with relatively smaller viscosity require less energy to be transported in pipelines. For example,
it would require less energy to transport water in a pipe than it does to transport motor oil or
glycerin. The viscosity of fluids is a function of temperature. In general, the viscosity of
gases increases with increasing temperature, and the viscosity of liquids generally decreases with
increasing temperature. Knowledge of the viscosity of a liquid and how it may change
with temperature also helps when selecting lubricants to reduce wear and friction between mov-
ing parts.Example 10.2 The coefficient of static friction between a book and a desk surface is 0.6. The book weighs
20 N. If a horizontal force of 10 N is applied to the book, would the book move? And if not,
what is the magnitude of the friction force? What should be the magnitude of the horizontal
force to set the book in motion?
The maximum friction force is given bySince the magnitude of the horizontal force (10 N) is less than the maximum friction force
(12 N), the book will not move, and hence the friction force will equal the applied force. A
horizontal force having a magnitude greater than 12 N will be required to set the book in
motion.FmaxmN 1 0.6 21202 12 newtons
Applied force
Maximum static
friction forceFriction
forceDynamic
friction forceApplied
forceWeight
Friction
forceNormal force
A
MotionAt rest
■Figure 10.8 The relationship between applied force and the friction force.
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