382 Chapter 13 Energy and Power
(13.9)
or
From Equation (13.9), the definition of power, it should be clear that the value of power required
to perform a task represents how fast you want the task done. If you want a task done in a
shorter time, then more power is required. For the sake of demonstrating this point better,
imagine that in order to perform a task 3600 J are required. The next question then becomes,
how fast do we want this task done? If we want the task done in 1 second, 3600 J/s power is
required; if we want the task done in 1 minute, then 60 J/s power is needed; and if we want the
task done in 1 hour, then the required power is 1 J/s. From this simple example, you should
see clearly that to perform the same task in a shorter period of time, more power is required.
More power means more energy expenditure per second. Another example that you have a
direct experience with is the following situation: Which requires more power, to walk up a
flight of stairs or to run up the stairs? Of course, as you already know, it requires more power
to run up the stairs, because as compared to walking, when running up the stairs, you perform
the same amount of work in a shorter time period. Many engineering managers understand the
concept of power well, for they understand the benefit of teamwork. In order to finish a proj-
ect in a shorter period of time, instead of assigning a task to an individual, the task is divided
among several team members. More useful energy expenditure per day is expected from a team
than from a single person, thus the project or the task can be done in less time.13.4 Watts and Horsepower
As we explained in the previous section, power is defined as the time rate of doing work, or
stated another way, work or energy divided by time. The units of power in SI units are defined
in the following manner:(13.10)
Note the following: 1 N#m is called 1 joule ( J), and 1 J/s is called 1 watt (W). In U.S. Custo-
mary units, the units of power are expressed in lbf#ft /s and horsepower (hp), in the following
manner:(13.11)
and
1 hp 550 (13.12)
lbf#ft
s
power
work
time
1 force 21 distance 2
time
lbf#ft
s
power
work
time
1 force 21 distance 2
time
N#m
s
J
s
W
power
energy
time
power
work
time
1 force 21 distance 2
time
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