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Chapter 3: Technical ProblemSolving and Communication Skills
19
Chapter 3: Technical ProblemSolving and Communication Skills
20
: Express your weight in the units of pounds and newtons, and your mass in the units
of slugs and kilograms.
Solution:
Calculate mass using gravitational acceleration in USCS:
ft
lb 180
== g
w
m
Convert to SI using the factor from Table 3.6:
kg
Chapter 3: Technical ProblemSolving and Communication Skills
21
: Express your height in the units of inches, feet, and meters.
Solution:
Chapter 3: Technical ProblemSolving and Communication Skills
22
: For a wind turbine that is used to produce electricity for a utility company, the power
output per unit area swept by the blades is 2.4 kW/m
2
. Convert this quantity to the
dimensions of hp/ft
2
.
Solution:
2
ft
Chapter 3: Technical ProblemSolving and Communication Skills
23
: A worldclass runner can run half a mile in a time of 1 min and 45 s. What is the
runner’s average speed in m/s?
Solution:
1000 L = l m
Convert mi to m using those factors:
ft 1
m 0.3048
2
1
2
1
Express the runner’s speed in m/s:
m
7.66
m 804.7 =
s
Alternatively, use the conversion factors:
s
Chapter 3: Technical ProblemSolving and Communication Skills
: One U.S. gallon is equivalent to 0.1337 ft
3
, 1 ft is equivalent to 0.3048 m, and 1000 L
are equivalent to 1 m
3
. By using those definitions, determine the conversion factor
between gallons and liters.
Solution:
Use the definitions of derived units and the conversion factors listed in Tables 3.2, 3.5, and
Chapter 3: Technical ProblemSolving and Communication Skills
25
: A passenger automobile is advertised as having a fuel economy rating of 29 mi/gal
for highway driving. Express the rating in the units of km/L.
Solution:
Convert using factors from Table 3.6:
km
12.33
km
1.609
gal
0.2642
mi
29 =
L
Chapter 3: Technical ProblemSolving and Communication Skills
26
: (a) How much horsepower does a 100 W household lightbulb consume? (b) How
many kW does a 5 hp lawn mower engine produce?
Solution:
(a) Convert W to kW using the prefix definition in Table 3.3. Convert from kW to hp using
the factor from Table 3.6:
hp
kW
Chapter 3: Technical ProblemSolving and Communication Skills
27
: The estimates for the amount of oil spilled into the Gulf of Mexico during the 2010
Deepwater Horizon disaster were 120180 million gal. Express this range in L, m
3
, and
ft
3
.
Solution:
Convert from gal to L:
L
Convert from gal to m
3
:
3
3
m
Convert from gal to ft
3
:
3
3
ft
Chapter 3: Technical ProblemSolving and Communication Skills
28
: In 1925, the TriState Tornado ripped a 219 mi path of destruction through Missouri,
Illinois, and Indiana, killing a record 695 people. The maximum winds in the tornado
were 318 mph. Express the wind speed in km/hr and in ft/s.
Solution:
Convert from mph to km/hr:
km
511.66
km
1.609
mi
318 =
hr
s
Chapter 3: Technical ProblemSolving and Communication Skills
29
: Uphill water slides are becoming more popular at large water parks. Uphill speeds
of riders can reach 19 ft/s. Express this speed in mph.
Solution:
Chapter 3: Technical ProblemSolving and Communication Skills
30
: A brandnew engineering hire is late for her first product development team
meeting. She gets out of her car and starts running 8 mph. It is exactly 7:58
A.M.,
and
the meeting starts at exactly 8:00
A.M
. Her meeting is 500 yd away. Will she make it on
time to the meeting? If so, with how much time to spare? If not, how late will she be?
Chapter 3: Technical ProblemSolving and Communication Skills
31
: A robotic wheeled vehicle that contains science instruments is used to study the
geology of Mars. The rover weighs 408 lb on Earth. (a) In the USCS dimensions of
slugs and lbm, what is the rover's mass? (b) What is the weight of the rover as it rolls
off the lander's platform (the lander is the protective shell that houses the rover during
landing)? The gravitational acceleration on the surface of Mars is 12.3 ft/s
2
.
Solution:
lbm
(b) Since the gravitational acceleration on Mars is only 12.3 ft/s
2
, the weight becomes
ftslug
155.9
ft
12.3)slugs(12.67 ⋅
=
=w
Chapter 3: Technical ProblemSolving and Communication Skills
32
: Calculate various fuel quantities for Flight 143. The plane already had 7682 L of
fuel on board prior to the flight, and the tanks were to be filled so that a total of 22,300
kg were present at takeoff. (a) Using the incorrect conversion factor of 1.77 kg/L,
calculate in units of kg the amount of fuel that was added to the plane. (b) Using the
correct factor of 1.77 lb/L, calculate in units of kg the amount of fuel that should have
been added. (c) By what percentage would the plane have been underfueled for its
journey? Be sure to distinguish between weight and mass quantities in your
calculations.
Solution:
(a) Amount of fuel that was added (kg):
kg
(c) Percent that the plane was under–fueled:
kg 8,703 kg 16,139 =
Chapter 3: Technical ProblemSolving and Communication Skills
33
: Printed on the side of a tire on an all–wheel–drive sport utility wagon is the warning
“Do not inflate above 44 psi,” where psi is the abbreviation for the pressure unit pounds
per square inch (lb/in
2
). Express the tire's maximum pressure rating in (a) the USCS
unit of lb/ft
2
(psf) and (b) the SI unit of kPa.
Solution:
(a) Convert the area measure from in. to ft:
2
lb
3366
in
12
lb
44 ,
.=
Chapter 3: Technical ProblemSolving and Communication Skills
34
: The amount of power transmitted by sunlight depends on latitude and the surface
area of the solar collector. On a clear day at a certain northern latitude, 0.6 kW/m
2
of
solar power strikes the ground. Express that value in the alternative USCS unit of (ft
⋅
lb/s)/ft
2
.
2
ft
Chapter 3: Technical ProblemSolving and Communication Skills
: The property of a fluid called viscosity is related to its internal friction and
resistance to being deformed. The viscosity of water, for instance, is less than that of
molasses and honey, just as the viscosity of a light motor oil is less than that of grease.
A unit used in mechanical engineering to describe viscosity is called the poise, named
after the physiologist Jean Louis Poiseuille, who performed early experiments in fluid
mechanics. The unit is defined by 1 poise = 0.1 (N
⋅
s)/m
2
. Show that 1 poise is also
equivalent to 1 g/(cm
⋅
s).
Solution:
Chapter 3: Technical ProblemSolving and Communication Skills
36
: Referring to the description in P3.16, and given that the viscosity of a certain engine
oil is 0.25 kg/(m
⋅
s), determine the value in the units (a) poise and (b) slug/(ft
⋅
s).
Solution:
2
(b) Use conversion factors from Table 3.6:
slug
0.0052
m
0.3048
slug
0.0685
kg
0.25 ⋅
=
s
ft
Chapter 3: Technical ProblemSolving and Communication Skills
37
Referring to the description in P3.16, if the viscosity of water is 0.01 poise,
determine the value in terms of the units (a) slug/(ft
⋅
s) and (b) kg/(m
⋅
s).
Solution:
s
ft
⋅
(b)
Express 0.01 P in terms of SI base units:
(
)
kg
sN
msN
2
⋅
⋅/
Chapter 3: Technical ProblemSolving and Communication Skills
38
The fuel efficiency of an aircraft's jet engines is described by the thrustspecific fuel
consumption (TSFC). The TSFC measures the rate of fuel consumption (mass of fuel
burned per unit time) relative to the thrust (force) that the engine produces. In that
manner, even if an engine consumes more fuel per unit time than a second engine, it is
not necessarily more inefficient if it also produces more thrust to power the plane. The
TSFC for an early hydrogenfueled jet engine was 0.082 (kg/h)/N. Express that value in
the USCS units of (slug/s)/lb.
Solution:
Use SI to USCS conversion factors from Table 3.6:
sslug
10946
N
4.448
hr
102.778
slug
0.0685
hrkg
0.082
64 −
×=
×
.
