Problems
21
55.0˚
Figure P1.61
estimate, in which we consider factors rather than incre-
ments. We write 500 m
# 10
3
m because 500 differs from
100 by a factor of 5 while it differs from 1 000 by only a fac-
tor of 2. We write 437 m
# 10
3
m and 305 m
# 10
2
m.
What distance differs from 100 m and from 1 000 m
by equal factors, so that we could equally well choose to
represent its order of magnitude either as
# 10
2
m or as
# 10
3
m?
57.
For many electronic applications, such as in computer
chips, it is desirable to make components as small as possi-
ble to keep the temperature of the components low and to
increase the speed of the device. Thin metallic coatings
(films) can be used instead of wires to make electrical con-
nections. Gold is especially useful because it does not oxi-
dize readily. Its atomic mass is 197 u. A gold film can be
no thinner than the size of a gold atom. Calculate the
minimum coating thickness, assuming that a gold atom oc-
cupies a cubical volume in the film that is equal to the vol-
ume it occupies in a large piece of metal. This geometric
model yields a result of the correct order of magnitude.
58.
The basic function of the carburetor of an automobile is to
“atomize” the gasoline and mix it with air to promote
rapid combustion. As an example, assume that 30.0 cm
3
of
gasoline is atomized into N spherical droplets, each with a
radius of 2.00 ! 10
"
5
m. What is the total surface area of
these N spherical droplets?
The consumption of natural gas by a company satis-
fies the empirical equation V $ 1.50t ' 0.008 00t
2
, where
V is the volume in millions of cubic feet and t the time in
months. Express this equation in units of cubic feet and
seconds. Assign proper units to the coefficients. Assume a
month is equal to 30.0 days.
60.
In physics it is important to use mathematical approxi-
mations. Demonstrate that for small angles (+ 20°)
tan ,
% sin , % , $ #,-/180°
where , is in radians and ,- is in degrees. Use a calculator
to find the largest angle for which tan , may be approxi-
mated by sin , if the error is to be less than 10.0%.
A high fountain of water is located at the center of a circu-
lar pool as in Figure P1.61. Not wishing to get his feet wet,
61.
59.
a student walks around the pool and measures its circum-
ference to be 15.0 m. Next, the student stands at the edge
of the pool and uses a protractor to gauge the angle of ele-
vation of the top of the fountain to be 55.0°. How high is
the fountain?
62. Collectible coins are sometimes plated with gold to en-
hance their beauty and value. Consider a commemorative
quarter-dollar advertised for sale at $4.98. It has a diame-
ter of 24.1 mm, a thickness of 1.78 mm, and is completely
covered with a layer of pure gold 0.180 %m thick. The vol-
ume of the plating is equal to the thickness of the layer
times the area to which it is applied. The patterns on the
faces of the coin and the grooves on its edge have a negli-
gible effect on its area. Assume that the price of gold is
$10.0 per gram. Find the cost of the gold added to the
coin. Does the cost of the gold significantly enhance the
value of the coin?
There are nearly # ! 10
7
s in one year. Find the percent-
age error in this approximation, where “percentage error’’
is defined as
64. Assume that an object covers an area A and has a uniform
height h. If its cross-sectional area is uniform over its
height, then its volume is given by V $ Ah. (a) Show that
V $ Ah is dimensionally correct. (b) Show that the vol-
umes of a cylinder and of a rectangular box can be written
in the form V $ Ah, identifying A in each case. (Note that
A, sometimes called the “footprint” of the object, can have
any shape and the height can be replaced by average
thickness in general.)
65.
A child loves to watch as you fill a transparent plastic bot-
tle with shampoo. Every horizontal cross-section is a cir-
cle, but the diameters of the circles have different values,
so that the bottle is much wider in some places than oth-
ers. You pour in bright green shampoo with constant vol-
ume flow rate 16.5 cm
3
/s. At what rate is its level in the
bottle rising (a) at a point where the diameter of the bot-
tle is 6.30 cm and (b) at a point where the diameter is
1.35 cm?
66. One cubic centimeter of water has a mass of 1.00 ! 10
"
3
kg.
(a) Determine the mass of 1.00 m
3
of water. (b) Biological
substances are 98% water. Assume that they have the same
density as water to estimate the masses of a cell that has a di-
ameter of 1.0 %m, a human kidney, and a fly. Model the kid-
ney as a sphere with a radius of 4.0 cm and the fly as a cylin-
der 4.0 mm long and 2.0 mm in diameter.
Assume there are 100 million passenger cars in the United
States and that the average fuel consumption is 20 mi/gal of
gasoline. If the average distance traveled by each car is
10 000 mi/yr, how much gasoline would be saved per year if
average fuel consumption could be increased to 25 mi/gal?
68. A creature moves at a speed of 5.00 furlongs per fortnight
(not a very common unit of speed). Given that
1 furlong $ 220 yards and 1 fortnight $ 14 days, deter-
mine the speed of the creature in m/s. What kind of crea-
ture do you think it might be?
67.
Percentage error $
&
assumed value " true value&
true value
!
100%
63.