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Distance Force in
in meters newtons
or feet or pounds

W = Fd W = Fd
= (pounds) (feet) = (newtons) (meters)
= ft • lb = N • m
= joule
FIGURE 3.3 Work is done against gravity when an object is
lifted. Work is measured in joules or in foot-pounds.

FIGURE 3.1 This is Glen Canyon Dam on the Colorado River
between Utah and Arizona. The 216 m (about 710 ft) wall of
concrete holds Lake Powell, which is 170 m (about 560 ft) deep at
the dam when full. Water falls through 5 m (about 15 ft) diameter
penstocks to generators at the bottom of the dam.

EXAMPLE 3.1
How much work is needed to lift a 5.0 kg backpack to a shelf 1.0 m
above the floor?

SOLUTION
The backpack has a mass (m) of 5.0 kg, and the distance (d ) is 1.0 m.
To lift the backpack requires a vertically upward force equal to the
weight of the backpack. Weight can be calculated from w = mg:
m = 5.0 kg w = mg
g = 9.8 m/s 2 m _
(
2)
= (5.0 kg) 9.8
w = ? s
Distance m _

Force (d) = (5.0 × 9.8) kg ×
2
(F ) s
kg⋅m
_

= 49 2
s
= 49 N
W = Fd The definition of work is found in equation 3.1,
FIGURE 3.2 The force on the book moves it through a vertical F = 49 N W = Fd
distance from the second shelf to the fifth shelf, and work is done, d = 1.0 m = (49 N)(1.0 m)
W = Fd.
W = ? = (49 × 1.0) (N·m)
= 49 N·m
= 49 J

In the English system, the force is measured in pounds (lb), EXAMPLE 3.2
and the distance is measured in feet (ft). The unit of work in the How much work is required to lift a 50 lb box vertically a distance of
English system is therefore the ft∙lb. The ft∙lb does not have a 2 ft? (Answer: 100 ft·lb)
name of its own as the N∙m does (Figure 3.3).

3-3 CHAPTER 3 Energy 63