Ch. 8 Review questions
The greatest obstacle to learning anything is thinking you already know everything.
1. The highest natural concentration of salts in water are found in the evaporating
remnants of old oceans, such as the Dead Sea in Israel. Suppose a swimmer with a
volume of 0.75 m^3 is able to float just beneath the surface of water with a density of
1.02 x 10^3 kg/m^3. How much extra mass can the swimmer carry and be able to
float just beneath the surface of the Dead Sea, which has a density of 1.22 x 10^3
kg/m^3?
Ans: 150 kg
2. The heaviest pig ever raised had a mass of 1158 kg. Suppose you placed this pig on a
raft made of dry wood. The raft completely submerged in water so that the raft’s top
surface was just level with the surface of the lake. If the raft’s volume was 3.40 m^3,
what was the mass of the raft without the pig? The density of fresh water is 1.00 x
10^3 kg/m^3
Ans: 2.24 x 10^3 kg
3. La Belle, one of four ships that Robert La Salle used to establish a French colony late
in the seventeenth century, sank off of the coast of Texas. The ship’s well-preserved
remains were discovered and excavated in the 1990s. Among those remains was a
small bronze cannon, called a minion. Suppose the minion’s total volume is 4.14 x
10^-2 m^3. What is the minion’s mass if its apparent weight in sea water is 3.115 x
10^3 N? The density of sea water is 1.025 x 10^3 kg/m^3?
Ans: 360 kg
4. Dandy Rauffenberg built a small submarine capable of diving as deep as 30.0 m. The
submarine’s volume can be approximated by that of a cylinder with a length of 3.00
m and a cross-sectional area of 0.500 m^2. Suppose this submarine dives in a fresh
water river and then moves out to sea, which naturally consists of salt water. What
mass of fresh water must be added to the ballast to keep the submarine submerged?
The density of fresh water is 1.00 x 10^3 kg/m^3 and the density of sea water is 1.025
x 10^3 kg/m^3
Ans: 38 kg
5. The largest iceberg ever observed had an area of 3.10 x 10^4 km^2, which is larger
than the country of Belgium. If the top and bottom surfaces of the iceberg were flat
and the thickness of submerged part was 0.84 km, how large was the buoyant force
acting on the iceberg? The density of sea water is 1.025 x 10^3 kg/m^3.
Ans: 2.6 x 10^17 N
6. A cannon built in 1868 in Russia could fire a cannonball with a mass of 4.80 x 10^2
kg and a radius of 0.250 m. When suspended from a scale and submerged in water, a
cannonball of this type has an apparent weight of 4.07 x 10^3 N. How large is the
buoyant for acting on the cannonball? The density of fresh water is 1.00 x 10^3
kg/m^3
Ans: 640 N
7. The tallest iceberg ever measured stood 167 m above the water. Suppose that both
the top and the bottom of this iceberg were flat and the thickness of the submerged
part was estimated to be 1.50 km. Calculate the density of the ice. The density of sea
water is 1.025 x 10^3 kg/m^3.
Ans: 921 kg/m^3
8. The Russian submarines of the “Typhoon” class are the largest submarines in the
world. They have a length of 1.70 x 10^2 m and an average diameter of 13.9 m.
When submerged, they displace 2.65 x 10^7 kg of sea water. Assume that these
submarines have a simple cylindrical shape. If a “Typhoon” class submarine has
taken on enough ballast that it descends with a net acceleration of 2.00m/s^2, what is
the submarine’s density during its descent?
Ans: 1.29 x 10^3 kg/m^3
9. To keep Robert La Salle’s ship La Belle well preserved, shipbuilders are
reconstructing the ship in a large tank filled with fresh water. Polyethylene glycol, or
PEG, will then be slowly added to the water until a 30 percent PEG solution is
formed. Suppose La Belle displaces 6.00 m^3 of liquid when submerged, If the
ship’s apparent weight decreases by 800. N as the PEG concentration increases from
0 to 30 percent, what is the density of the final PEG solution? The density of fresh
water is 1.00 x 10^3 kg/m^3
Ans: 1.01 x 10^3 kg/m^3
10. A cube of volume 1.00m^3 floats in gasoline, which has a density of 675 kg/m^3.
How large of a buoyant force acts on the cube?
Ans: 6.62 x 10^3 N
11. The largest helicopter in the world, which was built in Russia, has a mass of 1.03 x
10^3 kg. If you placed this helicopter on a large piston of a hydraulic lift, what force
would need to be applied to the small piston in order to slowly lift the helicopter?
Assume the weight of the helicopter is distributed evenly of the large piston’s area,
which is 1.40 x 10^2 m^2. The area of the small piston is 0.80 m^2.
Ans: 5.8 x 10^3 N
12. Astronauts and cosmonauts have used pressurized spacesuits to explore the lowpressure regions of space. The pressure inside one of these suits must be close to that
of Earth’s atmosphere at sea level so that the space explorer may be safe and
comfortable. The pressure on the outside of the suit is a fraction of 1.0 Pa. If the
average interior surface area of a pressurized spacesuit is 3.3 m^2, what is the force
exerted on the suits material if the pressure outside of the suit is zero and the pressure
inside of the suit is 1.01 x 10^5 Pa?
Ans: 3.3 x 10^5 N
13. A strange idea to control volcanic eruptions is developed by a daydreaming engineer.
The engineer imagines a giant piston that fits into the volcano’s shaft, which leads
from the Earth’s surface down to the magma chamber. The piston controls an
eruption by exerting pressure that is equal to or greater than the pressure of the hot
gases, ash, and magma that rise from the magma chamber through the shaft. The
engineer assumes that the pressure of the volcanic material is 4.0 x 10^11 Pa, which
is the pressure in Earth’s interior. If the material rises into a cylindrical shaft, with a
radius of 50.0 m, what force is needed on the other side of the piston to balance the
pressure of the volcanic material?
Ans: 3.1 x 10^15 N
14. The largest goat ever grown on a farm had a mass of 181 kg; on the other hand, the
smallest pygmy goats have a mass of only 16 kg. Imagine an agricultural show in
which a large goat with a mass of 181 kg exerts a pressure on a hydraulic lift piston
that is equal to the pressure exerted by three pygmy goats. The area of the piston the
large goat is standing on is 1.8 m^2, what is the area of the piston on which the
pygmy goats stand?
Ans: 0.48 m^2
15. The greatest load ever raised was the offshore Ekofisk complex in the North Sea. The
complex, which had a mass of 4.0 x 10^7 kg, was raised 6.5 m by over 100 hydraulic
jacks. Imagine that his load could have been raised using a single huge hydraulic lift.
If the load had been placed on the large piston and a force of 1.2 x 10^4 N had been
applied to the small piston, which had an area of 5.0 m^2, what must the large
piston’s area have been?
Ans: 1.6 x 10^5 m^2
16. The pressure that can exist in the interior of a star due to the weight of the outer layers
of hot gas is typically several hundred billion times greater than the pressure exerted
on Earth’s surface by Earth’s atmosphere. Suppose a pressure equal to that estimated
for the sun’s interior (2.0 x 10^16 Pa) acts on a spherical surface within a star. If a
force of 1.02 x 10^31 N produces this pressure, what is the area of the surface? What
is the sphere’s radius?
Ans: 6.4 x 10^6 m
17. The eye of a giant squid can be more than 35 cm in diameter – the largest eye of any
animal. Giant squid also live at depths greater than a mile below the ocean’s surface.
At a depth of 2.0 km, the outer half of a giant squid’s eye is acted on by an external
force of 4.6 x 10^6 N. Assuming the squid’s eye has a diameter of 38 cm, what is the
pressure on the eye? Treat the eye as a sphere.
Ans: 2.0 x 10^7 Pa
18. In 1969, a whale dove and remained underwater for nearly two hours. Evidence
indicates that the whale reached a depth of 3.00 km, where the whale sustained an
actual pressure of 3.03 x 10^7 Pa. What the density of the sea water?
Ans: 1.03 x 10^3 kg/m^3
19. In 1994, a 16.8 m tall oil-filled barometer was constructed in Belgium. Suppose the
barometer column was 80.0 percent filled with oil. What was the density of the oil if
the gauge pressure at the top of the oil column was 1.01 x 10^5 Pa? The pressure at
the bottom of the column of oil is 2.22x10^5 Pa
Ans: 918 kg/m^3
20. One of the lowest atmospheric pressures ever measured at sea level was 8.88 x 10^4
Pa, which existed in hurricane Gilbert in 1988. This same pressure can be found at a
height 950 m above sea level. Use this information to estimate the density of air.
Ans: 9.53 kg/m^3
21. In 1993, Fransisco Ferreras of Cuba held his breath and took a dive that lasted more
than 2 minutes. The maximum pressure Ferreras experienced was 13.6 times greater
than atmospheric pressure. To what depth did Ferreras dive? The density of sea
water is 1.025 x 10^3 kg/m^3.
Ans: 137 m
22. A penguin can endure pressures as great as 4.90 x 10^6 Pa. What is the maximum
depth to which a penguin can dive in sea water? The density of sea water is 1.025 x
10^3 kg/m^3.
Ans: 487m
23. In 1942, the British ship Edinburgh, which was carrying a load of 460 gold ingots,
sank off the coast of Norway. In 1981, all of the gold was recovered from a depth of
245 m by a team of 12 divers. What was the pressure exerted by the ocean’s water at
that depth? The density of sea water is 1.025 x 10^3 kg/m^3.
Ans: 2.56 x 10^6 Pa
24. In 1960, a bathyscaph descended 10,916 m below the ocean’s surface. What was the
pressure exerted on the bathyscaph at that depth? The density of sea water is 1.025 x
10^3 kg/m^3.
Ans: 1.10 x 10^8 Pa
25. What size force will open a door of area 1.54 m^2 if the net pressure on the door is
1.013 x 10^3 Pa?
Ans: 1.56 x 10^3 N
26. Gas at a pressure of 1.50 x 10^6 Pa exerts a force of 1.22 x 10^4 N of the upper
surface of a piston. What is the piston’s upper surface area?
Ans: 8.13 x 10^-13 m^2
27. A cube of osmium with a volume of 166 cm^3 is placed in fresh water. If the cube’s
apparent weight is 35.0 N, what is its density in kg/m^3? The density of fresh water
is 1.00 x 10^3 kg/m^3
Ans: 2.25 x 10^4 kg/m^3
28. A platinum flute with a density of 21.5 g/cm^3 is submerged in fresh water. If its
apparent weight is 40.2 N, what is the flute’s mass? The density of fresh water is 1.00
x 10^3 kg/m^3
Ans: 4.30 kg.
29. Dentist’s chairs are examples of hydraulic-lift systems. If a chair weight 1600 N and
rests on a piston with a cross-sectional area of 1440 cm^2, what force must be applied
to a smaller piston with a cross-sectional area of 72 cm^2 to lift the chair?
Ans: 80. N
30. A mechanic exerts a force of 55 N on a 0.015 m^2 hydraulic piston to lift a small
automobile. The piston that the automobile sits on has an area of 2.4 m^2. What is
the weight of the automobile?
Ans: 8.8 x 10^3 N
31. and 32. In a machine shop, a hydraulic lift is used to raise heavy equipment for
repairs. The system has a small piston with a cross-sectional area of 7.0 x 10^-2 m^2
and the large piston has a cross sectional area of 0.21 m^2. An engine weighing 2700
N rests on the large piston. What force must be applied to the small piston to lift the
engine?
32. If the engine rises 0.20 m, how far does the smaller piston move?
Ans: 31. 900 N 32. 0.60 m
33. And 34. A cube of granite building block with a volume of 1.00 x 10^-3 m^3 is
submerged in water. The density of granite is 2.70 x 10^3 kg/m^3. The density of
fresh water is 1.00 x 10^3 kg/m^3 What is the magnitude of the buoyant force acting
on the block?
34. What is the apparent weight of the block?
Ans: 9.80 N
34. 16.7 N
35. And 36. A child folds a piece of aluminum foil having a mass of 0.012 kg into a boat
that floats on the water. What volume of water does the foil boat displace? The
density of fresh water is 1.00 x 10^3 kg/m^3
36. The child crumples the same boat into a tight ball that sinks in the water. How much
volume does the foil displace, assuming that no air is trapped? What is the apparent
weight of the foil ball under the water? The density of aluminum is 2.70 x 10^3
kg/m^3.
Ans to 35.: 1.2 x 10^-5 m^3
Ans to 36. : 4.4 x 10^-6 m^3, 0.074 N
37. Canoes often have plastic foam blocks mounted under the seats for flotation in case
the canoe fills with water. What is the approximate minimum volume of foam
needed for flotation for a 480 N canoe?
Ans: 4.9 x 10^-2 m^3
38. Plastic foam is about 0.10 times as dense as water. What weight of bricks could you
stack on a 1.0 m x 1.0 m x 0.10 m slab of foam so that the slab of foam floats in water
and is barely submerged, leaving the bricks dry? The density of fresh water is 1.00 x
10^3 kg/m^3
Ans: 880 N
39. A toy rocket launcher is designed so that a child stomps on a rubber cylinder, which
increases the air pressure in a launching tube and pushes a foam rocket into the sky.
If the child stomps with a force of 150 N on a 2.5 x 10^-3 m^2 area piston, what is
the additional force transmitted to the 4.0 x 10^-2 m^2 launch tube?
Ans: 24 N
40. The dimensions of a brick that weighs 25 N are 0.19 m  0.070 m  m. What
pressure does the brick exert on the ground if it is resting on its largest face?
Ans: 1.9 x 10^3 N/m^2
41. And 42. A reservoir behind a dam is 17 m deep. What is the pressure at the base of
the dam? The density of fresh water is 1.00 x 10^3 kg/m^3
42. What is the pressure 4.0 m below the surface?
Ans 41 : 1.7 x 10^5 Pa
42: 3.9 x 10^4 Pa
43. And 44. An antique yellow metal statuette of a bird is suspended from a spring scale.
The scale reads 11.81 N when the statuette is in the air, and it reads 11.19 N when it
is in the water. What is the volume of the statuette? The density of fresh water is 1.00
x 10^3 kg/m^3
44. Is the bird gold (19.3 x 10^3 kg/m^3) or gold plated aluminum (2.7 x 10^3 kg/m^3)?
Ans 43: 6.33 x 10^-5 m^3
44: 19.0 x 10^3, so gold
45. What is the apparent weight of a rock submerged in water if the rock weighs 45 N in
the air and has a volume of 2.1 x 10^-3 m^3?
Ans: 24 N
46. What is the maximum weight that a balloon filled with 1.00 m^3 of helium can lift in
the air? The density of air is 1.20 kg/m^3 and the density of helium is 0.177 kg/m^3
Ans: 10.0 N
47. If a rock weighs 54 N in the air, and has an apparent weight of 46 N in a fluid twice
as dense as water, what is its apparent weight in water?
Ans: 50. N
48. A large buoy used to support an oceanographic research instrument is made of a
cylindrical, hollow iron tank. The tank is 2.1 m in height and 0.33 m in diameter.
The total mass of the buoy and research instrument is 120 kg. How much of the buoy
will be above the waterline when it is floating? The density of sea water is 1.025 x
10^3 kg/m^3
Ans: 0.70 m
49. A nurse administers medication in a saline solution to a patient by infusion into a vein
in the patient’s arm. The density of the solution is 1.0 x 10^3 kg/m^3, and the gauge
pressure inside of the vein is 2.4 x 10^3 Pa. How high above the insertion point must
the container be hung so that there is sufficient pressure to force the fluid into the
patient?
Ans: 0.24 m
50. A scuba diver searches for treasure at a depth of 20.0 m below the surface of the sea.
What pressure must the scuba device deliver air to the diver? HINT: use absolute
pressure! The density of sea water is 1.03 x 10^3 kg/m^3
Ans: 303 kPa
51. Icebergs are made of freshwater ice, which has a density of 0.92 x 10^3 kg/m^3 at
zero degrees Celsius. Ocean water has a density of 1.03 x 10^3 kg/m^3, what
fraction of the iceberg is below the surface?
Ans: 89%
52. A horizontal pipe of 25 cm^2 cross section carries water at a velocity of 3 m/s. The
pipe feeds into a smaller pipe with a cross section of 15 cm^2. What is the velocity of
the water in the smaller pipe?
Ans: 5.0 m/s
53. A 1350 kg car is supported by four tires inflated to a gauge pressure of 220 kPa.
Ignoring the effects of tread thickness, calculate the area of contact between each tire
and the road.
Ans: 0.0150m ^2
54. A 1.000 m tall pipe is filled to the halfway level with glycerin (1.26 x 10^3 kg/m^3)
then to the top with water (0.998 x 10^3 kg/m^3). What is the gauge pressure at the
bottom?
Ans: 11.1 kPa
55. A hydraulic jack is made with a small piston 1.2 cm in diameter that is used to move
a large piston 5.4 cm in diameter. If a man can exert a force of 280 N on the small
piston, how heave a load can he lift with the jack?
Ans: 5.7 x 10^3
56. A Goodyear blimp has a volume of 5750 m^3 and a mass of 4300 kg when empty.
What additional load is it able to lift when the entire volume is filled with helium
(0.18 kg/m^3)?
Ans: 1650 kg
57. A plastic bag is filled with helium (0.180 kg/m^3). How large of a volume of helium
would be required to lift a 50.0 kg girl off her feet?
Ans: 48.5 m^3
58. A 0.0132 kg seashell of density 3.54 x 10^3 kg/m^3 is suspended by a thread from a
spring scale. The seashell is then lowered into seawater until it is completely
submerged. What is the apparent weight of the seashell?
Ans: 0.0918 N
59. A king’s crown is said to be solid gold, but may be made of lead and covered with
gold. When it is weighed in air, the scale reads 0.475 kg. When it is submerged in
water (1.00 x 10^3 kg/m^3), the scale reads 0.437 kg. Is it solid gold (19.3 x 10^3
kg/m^3).
Ans: 12.5 x 10^3 kg/m^3
60. What is the velocity of water in a 4.0 cm diameter pipe that flows into a 2.0 cm
diameter pipe if the velocity in the smaller pipe is 3.0 m/s?
Ans: 0.75 m/s
61. A block of wood has a density of 0.820 g/cm and dimensions of 19.0 cm, 9.0 cm,
and 4.0 cm. How large a force will just submerge the block in a vat of oil with a
density 0.920 g/cm ?
Ans: 0.67 N
62. The absolute pressure below the surface of a freshwater lake is 2.43 x 10^5 Pa. At
what depth does this pressure occur? Assume that atmospheric pressure is 1.01 x
10^5 Pa and that the density of the water is 1.00 x 10^3 kg/m .
Ans: 14.5 m
63. Water in a hypodermic syringe moves from the barrel of the syringe, which has a
cross-sectional area of 0.35 cm^2, to the needle of the syringe, which has a crosssectional area of 1.3 x 10^-4 cm^2. If the water moves through the barrel at a speed of
0.32 cm/s, what is the speed with which the water leaves the needle?
Ans: 860 cm/s