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

268 Chapter 10 Force and Force-Related Parameters

to push against a rigid wall in a building, regardless of how hard you were to push with your

hands, you wouldn’t be able to move the wall, and thus, by definition, you are not doing any

mechanical work.*

Example 10.7 Determine the work required to lift a box that weighs 100 N, 1.5 m above the ground, as shown

in Figure 10.18.

The SI unit for work is Nm, which is calledjoule; thus the unit joule ( J) is defined as the

work done by a 1-N force through a distance of 1 m.

Students often confuse the concept of work with power. Power represents how fast you want

to do the work. It is the time rate of doing work, or said another way, it is the work done divided

by the time that it took to perform it. For instance, in Example 10.7, if we want to lift the box

in 3 seconds, then the power required is

Note that 1 J/s is called 1 watt (W). If you wanted to lift the box in 1.5 seconds, then the

required power to do this task is: 150 J/1.5 s or 100 W. Thus, twice as much power is required.

It is important to note that the work done in each case is the same; however, the power require-

ment is different. Remember if you want to do work in a shorter time, then you are going to

need more power. The shorter the time, the more power required to do that work. We will dis-

cuss power and its units in much more detail in Chapter 13.

power

work

time



150 J

3 s

50 J/s50 watts

1 joule 1 1 N 21 1 m 2

W 1 100 N 21 1.5 m 2 150 N#m

1.5 m

Box

Box

■Figure 10.18

The box in Example 10.7.

* Of course, your effort goes into deforming the wall on a very, very small scale that cannot be detected by the naked

eye. The forces created by your push deform the wall material, and your effort is stored in the material in the form of

what is called thestrain energy.

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