Modern Engineering Thermodynamics

30 CHAPTER 1: The Beginning
b. If the earth is covered with 2000 lightning storms each
producing 100 strikes per second, determine the total
Aircraft weighs 5.00 tons
lightning electrical power available (in kW).
37. If a person reports the dimensions of a room from
measurements with a tape measure as 12 feet, 6 1 4
inches by
500. mph
14 feet, 3 1 2
inches, how many significant figures are being
used?
38. If you measure time in hours with an accuracy of five significant
figures then convert it into seconds, to how many significant
30.0 × 10 3 feet
figures should you report the answer in seconds?
39.* An engineer reports a value of 1.3695 m/s for the velocity of
conveyer system.
a. How accurate (i.e., to how many significant figures) is the
velocity measurement?
b. If this velocity is calculated from a measurement of a
distance traveled divided by the time required, how
FIGURE 1.29
Problem 45.
accurately (i.e., to how many significant figures) must the
distance and time be measured?
where W is the vehicle weight in pounds force, V is the vehicle’s
Horsepower⁡ = 53:0 hp
.h
ðWVÞ + 6:8 hp .h 3
FIGURE 1.30
lbf .mi
ft 2 . mi 3 C D AV 3 × 10 − 6 Problem 48.
40. How accurate (i.e., how many significant figures can you
measure) is (a) a bathroom scale graduated in quarter pound
increments, (b) a yardstick graduated in eighth-inch increments,
(c) a 6-inch machinist’s pocket rule graduated in 1 16th inch
increments, (d) a 1-inch micrometer graduated in 1
1000th inch
increments, and (e) an analog stopwatch graduated in one
hundredths of a second?
road speed in miles per hour, A is the vehicle’s frontal area in
square feet, and C D is a dimensionless drag coefficient. Convert
this formula into a dimensional formula that uses only the four
base units (i.e., eliminate all derived units such as horsepower
and mile) of
a. The Engineering English system (lbf, lbm, ft, s).
b. The SI system (N, kg, m, s).
41. If you are reporting a distance of less than ten miles traveled in
your car from reading the odometer, how many significant
figures do you use?
48.* You are suddenly transported through time and space to an
unknown planet, where you find yourself face to face with a
hungry giant quadroplex creature. Your finely honed survival
42. If you are calculating the potential energy of an object for which
you know its mass to three significant figures, its height to two
significant figures, and the local gravity to four significant
figures, how many significant figures do you use in your final
answer?
skills as a successful, but mild-mannered, engineering student
lead you conclude that the beast has a mass of 1.00 × 10 4 kg
(gads!). In response to its unwanted affection, you quickly pick
up a stone and throw it vertically with a carefully calibrated
launch velocity of 10.0 m/s (Figure 1.30). Your well-trained eye
43. If you are calculating the kinetic energy of an object for which
you measure the distance it travels with an instrument having
an accuracy of four significant figures, the time of travel with a
stopwatch accurate to three significant figures, and a mass
measured to an accuracy of three significant figures, how many
significant figures do you use in your answer?
determines that the stone flies to a height of 20.0 m before it
begins to drop. Knowing that the initial kinetic energy and the
final potential energy of the stone must be equal, determine
(before the creature reaches you)
a. The value of g c on this planet (in kg·m/N· s 2 ).
b. The value of the local acceleration of gravity (in m/s 2 ).
44.* Determine the potential energy of 1.00 kg of water at a
height of 1.00 m above the ground at standard gravity in (a)
c. The local weight (in newtons) of the approaching giant
bulbous creature.
the Engineering English units system and (b) the SI units
system.
45. Compute the kinetic and potential energies of an airplane
weighing 5.00 tons flying at a height of 30.0 × 10 3 ft at
500. mph (Figure 1.29). Give your answer in both SI and
Engineering English units. Assume standard gravity.
46. Assume the binding energy per molecule for liquid water at
212.°F is about 7.00 × 10 −20 ft · lbf/molecule. Then, assuming
all the binding energy is converted into mass, determine the
percent gain in mass when 1.00 lbm (10 25 molecules) of
liquid water vaporizes. Note: E = mc 2 /g c , where, in the
Engineering English units system, g c = 32.174 lbm·ft/(lbf · s 2 )
and c = 9.84 × 10 8 ft/s.
47. The engine horsepower required to overcome rolling and air
resistance for a passenger vehicle is given by the dimensional
formula