Chapter 9
Solids and Fluids
Quick Quizzes
1.
(c). The mass that you have of each element is:
(
)(
)
3
3
3
3
19.3
10
kg/m
1 m
19.3
10
kg
gold
gold
gold
m
V
ρ
=
=
×
=
×
(
)(
)
3
3
3
10.5
10
kg/m
2 m
21.0
10
k
silver
silver
silver
m
V
ρ
=
=
×
=
×
(
3
g
)(
)
3
3
3
2.70
10
kg/m
6 m
16.
aluminum
aluminum
aluminum
m
V
ρ
=
=
×
3
10
kg
=
×
2
2.
(a). At a fixed depth, the pressure in a fluid is directly proportional to the density of the
fluid. Since ethyl alcohol is less dense than water, the pressure is smaller than
P
when the
glass is filled with alcohol.
3.
(c). For a fixed pressure, the height of the fluid in a barometer is inversely proportional to
the density of the fluid. Of the fluids listed in the selection, ethyl alcohol is the least dense.
4.
(b).
The blood pressure measured at the calf would be larger than that measured at the
arm. If we imagine the vascular system of the body to be a vessel containing a liquid
(blood), the pressure in the liquid will increase with depth. The blood at the calf is deeper
in the liquid than that at the arm and is at a higher pressure.
Blood pressures are normally taken at the arm because that is approximately the same
height as the heart. If blood pressures at the calf were used as a standard, adjustments
would need to be made for the height of the person, and the blood pressure would be
different if the person were lying down.
5.
(c). The level of floating of a ship is unaffected by the atmospheric pressure. The buoyant
force results from the pressure differential in the fluid. On a highpressure day, the
pressure at all points in the water is higher than on a lowpressure day. Because water is
almost incompressible, however, the rate of change of pressure with depth is the same,
resulting in no change in the buoyant force.
6.
(b). Since both lead and iron are denser than water, both objects will be fully submerged
and (since they have the same dimensions) will displace equal volumes of water. Hence,
the buoyant forces acting on the two objects will be equal.
7.
(a). When there is a moving air stream in the region between the balloons, the pressure in
this region will be less than on the opposite sides of the balloons where the air is not
moving. The pressure differential will cause the balloons to move toward each other. This
is demonstration of Bernoulli’s principle in action.
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CHAPTER 9
Answers to Even Numbered Conceptual Questions
2.
We approximate the thickness of the atmosphere by using
0
P
P
gh
ρ
=
+
with
P
at the
top of the atmosphere and
P
at sea level. This gives an approximation of
0
0
=
1 atm
=
(
)
(
)
5
4
0
10
Pa
0
1 kg m
1
P
P
h
g
ρ
−
3
1
2
0
~
1
0 m s
−
=
=
m
~1
h
or
Because both the density of the air,
0 km
ρ
, and the acceleration of gravity,
g
, decrease with
altitude, the actual thickness of the atmosphere will be greater than our estimate.
4.
Both must have the same strength. The force on the back of each dam is the average
pressure of the water times the area of the dam. If both reservoirs are equally deep, the
force is the same.
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 Spring '08
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 Buoyancy, Force, Mass, kg

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