Page 65 - Fisika Terapan for Engineers and Scientists

Problems 265

56. The ancient Egyptians and Romans relied on slaves as a 66. Nineteenth-century English engineers reckoned that a laborer
source of mechanical power. One slave, working desperately by turning a crank can do steady work at the rate of 5000
turning a crank, could deliver about 200 W of mechanical ft . lbf/min. Suppose that four laborers working a manual crane
power (at this power the slave would not last long). How many attempt to lift a load of 9.0 short tons (1 short ton 2000 lb).
slaves would be needed to match the output of a modern auto- If there is no friction, what is the rate at which they can lift
mobile engine (150 hp)? How many slaves would an ancient this load? How long will it take them to lift the load 15 ft?
Egyptian have to own in order to command the same amount 67. The driver of an automobile traveling on a straight road at
of power as the average per capita power used by residents of 80 km/h pushes forward with his hands on the steering wheel
the United States (14 kW)? with a force of 50 N. What is the rate at which his hands do
57. An electric clock uses 2.0 W of electric power. How much work on the steering wheel in the reference frame of the
electric energy (in kilowatt-hours) does this clock use in 1 year? ground? In the reference frame of the automobile?
What happens to this electric energy? 68. An automobile with a 100-hp engine has a top speed of
58. While an automobile is cruising at a steady speed of 65 km/h, 160 km/h. When at this top speed, what is the friction force
its engine delivers a mechanical power of 20 hp. How much (from air and road) acting on the automobile?
energy does the engine deliver per hour? 69. A horse walks along the bank of a canal and pulls a barge by
59. A large windmill delivers 10 kW of mechanical power. How means of a long horizontal towrope making an angle of 35
much energy does the windmill deliver in a working day of 8 with the bank. The horse walks at the rate of 5.0 km/h, and
hours? the tension in the rope is 400 N. What horsepower does the
60. The heating unit of a medium-sized house produces horse deliver?
170000 Btu/h. Is this larger or smaller than the power 70. A 900-kg automobile accelerates from 0 to 80 km/h in 7.6 s.
produced by a typical automobile engine of 150 hp? What are the initial and the final translational kinetic energies
61. The heart of a resting person delivers a mechanical power of of the automobile? What is the average power delivered by the
about 1.1 W for pumping blood. Express this power in hp. engine in this time interval? Express your answer in horsepower.
How much work does the heart do on the blood per day? 71. A six-cylinder internal combustion engine, such as used in an
Express this work in kcal. automobile, delivers an average power of 150 hp while running
62. The lasers to be used for controlled fusion experiments at at 3000 rev/min. Each of the cylinders fires once every two
the National Ignition Facility at the Lawrence Livermore revolutions. How much energy does each cylinder deliver each
15
Laboratory will deliver a power of 2.0 10 W, a thousand time it fires?
times the output of all the power stations in the United States, 72. In Chapter 6, we saw that an automobile must overcome the
2
1
in a brief pulse lasting 1.0 10 9 s. What is the energy in this force of air resistance, f CAv . For the automobile of
2
air
2
laser pulse? How does it compare with the energy output of all Example 6 of Chapter 6 (C 0.30,A 2.8 m , and
3
the power stations in the United States in one day? 1.3 kg/m ), calculate the power dissipation due to air resist-
63. During the seven months of the cold season in the Northeastern ance when traveling at 30 km/h and when traveling at
United States, a medium-sized house requires about 1.0 10 8 90 km/h. What is the difference in the total energy supplied
Btu of heat to keep warm. A typical furnace delivers 1.3 10 5 to overcome air friction for a 300-km trip at 30km/h? For a
Btu of heat per gallon of fuel oil. 300-km trip at 90 km/h?
(a) How many gallons of fuel oil does the house consume 73. A constant force of 40 N is applied to a body as the body
during the cold season? moves uniformly at a speed of 3.5 m/s. The force does work
on the body at a rate of 90 W. What is the angle between the
(b) What is the average power delivered by the furnace?
force and the direction of motion of the body?
64. Experiments on animal muscle tissue indicate that it can pro-
74. An electric motor takes 1.0 s to get up to speed; during this
duce up to 100 watts of power per kilogram. A 600-kg horse
time, the power supplied by the motor varies with time
has about 180 kg of muscle tissue attached to the legs in such 2
according to P P (P P )(t 1) , where t is in
a way that it contributes to the external work the horse per- 1 0 1
seconds, P 1.50 kW, and P 0.75 kW. What is the total
forms while pulling a load. Accordingly, what is the theoretical 0 1
energy supplied for the time period 0 t 1 s?
prediction for the maximum power delivered by a horse? In
trials, the actual maximum power that a horse can deliver in a 75. A constant force F (6.0 N)i (8.0 N)j acts on a particle.
short spurt was found to be about 12 hp. How does this com- At what instantaneous rate is this force doing work on a parti-
pare with the theoretical prediction? cle with velocity v (3.0 m/s)i (2.5 m/s)j?
65. If a 60-W light bulb is left on for 24 hours each day, how 76. An automobile engine typically has an efficiency of about
many kilowatt-hours of electric power does it use in one year? 25%; i.e., it converts about 25% of the chemical energy avail-
If the electric energy costs you 15 cents per kilowatt-hour, able in gasoline into mechanical energy. Suppose that an auto-
what is your cost for one year? mobile engine has a mechanical output of 110 hp. At what