Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

10.1 What We Mean By Force 257

Friction Forces—Dry Friction and Viscous Friction

There are basically two types of frictional forces that are important in engineering design: dry

frictional forcesand viscous friction(or the fluid friction). Let us first take a closer look at dry

friction, which allows us to walk or to drive our cars. Remember what happens when frictional

forces get relatively small, as is the case when you try to walk on ice or drive your car on a sheet

of ice. Stated in a simple way, dry friction exists because of irregularities between surfaces in con-

tact. To better understand how frictional forces build up, imagine you were to perform the fol-

lowing experiment. Place a book on a table and begin by pushing the book gently. You will

notice that the book will not move. The applied force is balanced by the friction force gener-

ated at the contact surface. Now push on the book harder, you will notice that the book will

not move until the pushing force is greater than the frictional force that prevents the book from

moving. The results of your experiment may be plotted in a diagram similar to the one shown

in Figure 10.8. Note that thefriction force is not constantand reaches a maximum value (see

pointAin Figure 10.8), which is given by

(10.4)

whereNis the normal force — exerted by the table on the book to prevent it from falling — and,

in the case of a book resting on the table,Nis equal to the weight of the book, and mis the

coefficient of static friction for the two surfaces involved, the book and the desk surface.

Figure 10.8 also shows that once the book is set in motion, the magnitude of the friction force

drops to a value called thedynamic(or kinetic)friction.

The other form of friction which must be accounted for in engineering analysis is fluid fric-

tion, which is quantified by the property of a fluid calledviscosity. The value of viscosity of a

fluid represents a measure of how easily the given fluid can flow. The higher the viscosity value

is, the more resistance the fluid offers to flow. For example, if you were to pour water and honey

FmaxmN

25

20

15

10

5

0

0 102030

Deflection (mm)

Force-deflection curve

40

Force (load) (N)

■Figure 10.7

A good fit to a set of force-deflection data points.

TABLE 10.1(b) A Set of Force-deflection

Data Points

The Deflection

Weight (N) of the Spring (mm)

5.0 9

10.0 17

15.0 29

20.0 35

62080_10_ch10_p251-302.qxd 5/22/10 12:32 AM Page 257

Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀