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4.18. A stiff 10.0 g ball is held directly above and in contact with a 600.0 g basketball and both
are dropped from a height of 1.00 m. What is the maximum theoretical height to which the small
ball can bounce?
Need: Height, H = ______ m
Know: Together, the two balls begin with a combined GPE of (m1 + m2) gh =
about 6 J of energy relative to the ground if h is one meter.
In falling, that GPE is converted to TKE. In hitting the ground, the TKE is
converted first to elastic energy (which is another form of potential energy) then
back to TKE again. In the general case, some fraction of this TKE is absorbed by
the heavier ball, some fraction by the lighter ball.
4.19. What would be the power required by the lightbulb in Example 4.6 if it sustains a voltage
drop of 120. V?
Need: Power to light a bulb in W if current is 0.1 A and voltage drop is 120. V
4.20. What would be the current in the lightbulb in Example 4.6 if it sustains a voltage drop of
120. V and requires a power of 100. W?
Need: Current to light a bulb in A if power is 100. W and voltage drop is 120. V
4.21. An electric oven is heated by a circuit that consists of a heating element connected to a
voltage source. The voltage source supplies a voltage of 110. V, which appears as a voltage drop
of across the heating element. The resulting current through the heating element is 1.0 A. If the
heating element is perfectly efficient at converting electric power into thermal power, what is the
thermal power produced by the heating element? (A: 1.1 × 102 W to two significant figures.)
Need: Power = ____ W
Know: V = 110. V and I = 1.0 A
4.22. A truck starter motor must deliver 15 kW of power for a brief period. If the voltage to the
starter motor is 12 V, what is the current through the starter motor while it is delivering that level
of power?
Need: Current = ____ A
Know: Power P = 15 kW = 1.5 104 W and V = 12 V.
4.23. A hybrid car is an automobile that achieves high fuel efficiency by using a combination of
thermal energy and electrical energy for propulsion. One of the ways it achieves high fuel
efficiency is by regenerative braking. That is, every time the car stops, the regenerative braking
system converts part of the TKE of the car into electrical energy, which is stored in a battery. That
stored energy can later be used to propel the car. The remaining part of the TKE is lost as heat.
Draw a control surface diagram showing the energy conversions that take place when the hybrid
car stops. (A: See the following diagram.)
Need: Control surface diagram
4.24. Suppose the car in Exercise 4.23 has a mass of 1000. kg and is traveling at 33.5 miles per
hour. As it comes to a stop, the regenerative braking system operates with 75% efficiency. How
much energy per stop can the regenerative braking system store in the battery? Illustrate with a
control boundary showing the energy flows.
Need: Energy stored in battery per stop = _____ J.
Know: Mass of vehicle, m = 1000. kg, speed, v = 33.5 mph = 15.0 m/s. Breaking
efficiency = 75%.
4.25. Suppose the car in Exercises 4.23 and 4.24 stored 1.00 × 102 megajoules (MJ) of energy in
its battery. Suppose the electric propulsion system of the car can convert 90% of that energy into
mechanical power. Suppose the car requires 30. kW of mechanical power to travel at 33.5 miles
per hour. How many miles can the car travel using the energy in its battery? (A: 28 miles.)
Need: Distance traveled using 100. MJ of energy = _____ miles.
Know: Mass of car, m = 1000. kg, speed, v = 15.0 m/s, engine power, P = 30. kW,
and battery propulsion efficiency = 90.%.
4.26. Determine the amount of gasoline consumed per second by the automobile in Example 4.8
2.6 10-2 miles/s)
4.27. To maintain a speed v on a horizontal road, a car must supply enough power to overcome air
resistance. That required power goes up with increasing speed according to the formula
P = Power in kW = K V3
where v is the speed measured in miles/hour and K is a constant of proportionality. Suppose it
takes a measured 7.7 kW for a car to overcome air resistance alone at 30.0 mph.
a. What is the value of K in its appropriate units?
b. Using a spreadsheet, prepare a graph of power (kW on the y axis) as a function of speed
(mph on the x axis) for speeds from 0 mph to 100 mph.
Need: Value of K and a graph of power vs speed.
Know: P = 7.7 kW @ 30. mph and P = K V 3 kW
Kosky, Balmer, Keat and Wise: Exploring Engineering, Fourth Edition, Solution Manual, Chapter 4
0
50
100
150
200
250
300
020 40 60 80 100
mph
kW to overcome air resistance
Calibration point
3
4
5
6
7
8
9
10
11
12
13
14
15
16
A B C
K = =0.000286 kW/mph3
V, mph P, kW
0 =$B$3*B6^3
=B6+10 =$B$3*B7^3
=B7+10 =$B$3*B8^3
=B8+10 =$B$3*B9^3
=B9+10 =$B$3*B10^3
=B10+10 =$B$3*B11^3
=B11+10 =$B$3*B12^3
=B12+10 =$B$3*B13^3
=B13+10 =$B$3*B14^3
=B14+10 =$B$3*B15^3
=B15+10 =$B$3*B16^3
4.28. Review Exercises 21–24 in Chapter 2 concerning the dynamics (and consequent fate) of
bungee jumpers. Draw a control surface around the jumper and cord. Show the various forms of
energy possessed by the jumper and cord, along with arrows showing the directions of energy
conversion inside and across the control surface: (a) when the jumper is standing on the cliff top,
(b) when the jumper is halfway down, and (c) when the cord brings the jumper to a safe stop.
Draw a control surface around the jumper and cord. Show the various forms of energy possessed
by the jumper and cord, along with arrows showing the directions of energy conversion inside and
across the control surface: a) when the jumper is standing on the cliff top, b) when the jumper is
halfway down and, c) when the cord brings the jumper to a safe stop.
Need: Control surface for bungee jumpers
4.29. After working for a company for several years, you feel you have discovered a more
efficient energy conversion method that would save your company millions of dollars annually.
Since you made this discovery as part of your daily job, you take your idea to your supervisor, but
she claims it is impractical and refuses to consider it further. You still feel it has merit and want to
proceed. What do you do?
a. You take your idea to another company to see if it will buy it.
b. You contact a patent lawyer to initiate a patent search on your idea.
c. You go over your boss’s head and talk to her supervisor about your idea.
d. You complain to your company’s human resources office about having poor supervision.
Apply the Fundamental Canons and the Engineering Ethics Matrix: Engineers, in the
fulfillment of their professional duties, shall:
Kosky, Balmer, Keat and Wise: Exploring Engineering, Fourth Edition, Solution Manual, Chapter 4
Options
Canons
a) Take to
another
company
b) Initiate
patent search
c) Talk to
boss’
supervisor
d) Complain
to human
resources
Hold
paramount the
safety, health
and welfare of
the public.
Does not
apply
Does not
apply
Does not
apply
Does not
apply
Perform
services only
in the area of
your
competence
Does not
apply
Does not
apply
Does not
apply
Does not
apply
Issue public
statements
only in an
objective and
truthful
manner
Does not
apply
Does not
apply
Does not
apply
Does not
apply
Act for each
employer or
client as
faithful agents
or trustees
No
No
Yes
No, unless
there is other
evidence for
poor
performance
by your boss
Avoid
deceptive acts
No, without
your
company’s
permission
No, without
company’s
permission
Yes
Yes, unless
you lie about
the boss’
performance
Conduct
themselves
honorably …
No
No
Yes
No, you are
probably
reacting
personally
instead of
professionally
Solution: There is ethical tension here. Canons 1 and 6 imply that you should pursue your
idea for the public benefit. Canons 4 and 5 imply that you should not pursue your idea in a
way that violates the legal and ethical obligations you have assumed as a condition of
employment. The four options presented offer no way out of this impasse. This suggests
that the correct answer is “none of the above.” You should explore other options that
fulfill your duties to both your employer and the public. These might range, for example,
from informing your supervisor that you are going to pursue the idea through company
channels to quitting your job, waiting the required legal interval, and then pursuing the
idea on your own.
4.30. Your course instructor claims that energy is not really conserved. He uses the example of a
spring that is compressed then tied with a nylon string. When the compressed spring is put into a
jar of acid, the spring dissolves and the energy it contained is lost. How do you react?
a. Ignore him and follow the established theories in the text.
b. Go to the department chairperson and complain that the instructor is incompetent.
c. Say nothing, but make detailed statements about the quality of the instructor on the course
evaluation at the end of the term.
d. Respectfully suggest that the energy in his spring example really is conserved.
Apply the Fundamental Canons and the Engineering Ethics Matrix:
Options
Canons
a) Ignore
b) Complain
c) Say it in
course
evaluation
d)
Respectfully
suggest
Hold
paramount
the safety,
health and
welfare of the
public.
Does not
apply
Does not
apply
Does not
apply
Does not
apply
Perform
services only
in the area of
your
competence
Does not
apply
No- you
probably are
not
competent to
make a
general
judgment
about the
professor’s
competence
Does not
apply
Does not
apply
Issue public
statements
only in an
objective and
truthful
manner
Does not
apply
Does not
apply
Does not
apply
Does not
apply
Act for each
employer or
client as
faithful
agents or
trustees
Does not
apply
Does not
apply
Does not
apply
Does not
apply
Kosky, Balmer, Keat and Wise: Exploring Engineering, Fourth Edition, Solution Manual, Chapter 4
Copyright, Elsevier, 2014
