Page 66 - Fisika Terapan for Engineers and Scientists

266 CHAPTER 8 Conservation of Energy

rate (in gallons per hour) will this engine consume gasoline? 84. A 27000-kg truck has a 550-hp engine. What is the maxi-
See Table 8.1 for the energy content in gasoline. mum speed with which this truck can move up a 10 slope?
77. The takeoff speed of a DC-3 airplane is 100 km/h. Starting *85. Consider a “windmill ship,” which extracts mechanical energy
from rest, the airplane takes 10 s to reach this speed. The mass from the wind by means of a large windmill mounted on the
of the (loaded) airplane is 11000 kg. What is the average deck (see Fig. 8.22). The windmill generates electric power,
power delivered by the engines to the airplane during takeoff ? which is fed into a large electric motor, which propels the ship.
78. The Sun emits energy in the form of radiant heat and light at The mechanical efficiency of the windmill is 70% (that is, it
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the rate of 3.9 10 W. At what rate does this energy carry removes 70% of the kinetic energy of the wind and transforms
away mass from the Sun? How much mass does this amount it into rotational energy of its blades). The efficiency of the
to in 1 year? electric generator attached to the windmill is 90%, and the
efficiency of the electric motor connected to the generator is
79. The energy of sunlight arriving at the surface of the Earth
also 90%. We want the electric motor to deliver 20000 hp in a
amounts to about 1.0 kW per square meter of surface (facing
(relative) wind of 40 km/h. What size windmill do we need?
the Sun). If all of the energy incident on a collector of sunlight 3
The density of air is 1.29 kg/m .
could be converted into useful energy, how many square
meters of collector area would we need to satisfy all of the
energy demands in the United States? See Table 8.1 for the
energy expenditure of the United States.
80. Equations (2.11) and (2.16) give the velocity and the acceler-
ation of an accelerating Maserati sports car as a function of
time. The mass of this automobile is 1770 kg. What is the
instantaneous power delivered by the engine to the automo-
bile? Plot the instantaneous power as a function of time in
the time interval from 0 to 10 s. At what time is the power FIGURE 8.22
maximum? A “windmill ship.”
81. The ship Globtik Tokyo, a supertanker, has a mass of 650000
metric tons when fully loaded.
*86. An electric water pump is rated at 15 hp. If this water pump is
(a) What is the kinetic energy of the ship when her speed is
to lift water to a height of 30 m, how many kilograms of water
26 km/h?
can it lift per second? How many liters? Neglect the kinetic
(b) The engines of the ship deliver a power of 44000 hp. energy of the water.
According to the energy requirements, how long a time
*87. The engines of the Sikorski Blackhawk helicopter generate
does it take the ship to reach a speed of 26 km/h, starting
3080 hp of mechanical power, and the maximum takeoff mass
from rest? Make the assumption that 50% of the engine
of this helicopter is 7400 kg. Suppose that this helicopter is
power goes into friction or into stirring up the water and
climbing vertically at a steady rate of 5.0 m/s.
50% remains available for the translational motion of the
ship. (a) What is the power that the engines deliver to the body of
the helicopter?
(c) How long a time does it take the ship to stop from an ini-
tial speed of 26 km/h if her engines are put in reverse? (b) What is the power that the engines deliver to the air (by
Estimate roughly how far the ship will travel during this friction and by the work that the rotors of the helicopter
time. perform on the air)?
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82. At Niagara Falls, 6200 m per second of water falls down a *88. In order to overcome air friction and other mechanical fric-
height of 49 m. tion, an automobile of mass 1500 kg requires a power of 20 hp
from its engine to travel at 64 km/h on a level road. Assuming
(a) What is the rate (in watts) at which gravitational potential
the friction remains the same, what power does the same auto-
energy is dissipated by the falling water?
mobile require to travel uphill on an incline of slope 1:10 at
(b) What is the amount of energy (in kilowatt-hours) wasted the same speed? Downhill on the same incline at the same
in 1 year? speed?
(c) Power companies get paid about 5 cents per kilowatt-hour *89. With the gears in neutral, an automobile rolling down a long
of electric energy. If all the gravitational potential energy incline of slope 1:10 reaches a terminal speed of 95 km/h. At
wasted in Niagara Falls could be converted into electric this speed the rate of decrease of the gravitational potential
energy, how much money would this be worth? energy matches the power required to overcome air friction
83. The movement of a grandfather clock is driven by a 5.0-kg and other mechanical friction. What power (in horsepower)
weight which drops a distance of 1.5 m in the course of a week. must the engine of this automobile deliver to drive it at 95 km/h
What is the power delivered by the weight to the movement? on a level road? The mass of the automobile is 1500 kg.