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

13.4 Watts and Horsepower 385

Example 13.8 Determine the power required to move an object that weighs 800 N (179.85 lbf), a vertical

distance of 4 m (13.12 ft) in 2 s.

The power requirement expressed in SI units is given by

And using U.S. Customary units, the power requirement is

The power expressed in horsepower is

In this example, think about the relationships among various units of power needed to per-

form the same amount of work in the same amount of time.

Example 13.9 Most of you have seen car advertisements where the car manufacturer brags about how fast one

of its cars can go from 0 to 60 mph. According to the manufacturer, the BMW 750iL 2001

model can go from 0 to 60 mph in 6.7 seconds. This performance is usually measured on a test

track. The engine in the car is rated at 326 hp at 5000 rpm. The car has a reported weight of

4597 lbf. Is the claim by the manufacturer justifiable?

Well, to answer this question you may agree it would be more fun to drive to a BMW

dealership and take the car for a test run on a racing track. But let us answer this question

with the knowledge you have gained so far in this course. We need to make some assump-

tions first; we can assume that the driver weighs 180 lbfand the reported weight of the car

includes enough gasoline for this test. Next, we convert the speed and the mass values into

appropriate units.

m

weight

g



14597  1802 lbf

32.2

ft

s

2

148 slugs

V 2 Vfinala 60

mi

h

ba

1 h

3600 s

ba

5280 ft

1 mi

b 88

ft

s

V 1 Vinitial 0

ft

s

power 1180 a

lbf#ft

s

b°

1 hp

(^550)
lbf#ft
s
¢2.14 hp
power
work
time

1 179.85 lbf 21 13.12 ft 2
2s
 1180
lbf#ft
s
power
work
time

1 800 N 21 4 m 2
2s
1600 W
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