IDS 101
Name___________________
Pressure End of Module Answers
Please answer the following questions on a separate sheet of paper.
1. Imagine two tall glass tubes filled with water. Both tubes are 60 inches high, but one
tube has a diameter of 3 in, while the other has a diameter of 9 in. Compare the pressure
exerted by the water in each of these tubes. Clearly explain your reasoning.
force (density of water) ( the volume of the tube) density X area X height


area
area
area
the areas cancel, so as we demonstrat ed in class, the height is the only factor tha t counts!
pressure 
pressure in both tubes  (0.036 lbs/cubic in) (60 inches)  2.16 lb/square inch
2. A 2-inch by 2-inch by 6-inch block of wood is resting on a table. The density of the
wood is 0.025 pounds per cubic inch.
a) If the block is resting on its side, as shown in the diagram at right, what is the
pressure (in pounds per square inch) that the block exerts on the table?
(0.025 lb/cubic in)(2 in X 2 in X 6 in)
0.6 lb

 0.05 psi
2 in X 6 in
12 sq in
b) If the block is standing on end, as shown in the diagram at right, what is the
pressure (in pounds per square inch) that the block exerts on the table?
pressure 
since the weight of the block is the same, but the area of the base is different we
can use:
0.6 lb/ 4 sq in = 0.15 psi
c) Atmospheric pressure at sea level is about 15 pounds per square inch. How tall
would the two-inch by two-inch block of wood need to be to exert a pressure of
15 pounds per square inch on the table (when the block is standing on end)?
There are at least two ways to solve this problem. The first is to establish a proportion
with the previous problem because the area at the base is the same.
0.15 psi 6 in

15 psi
X
X  600 inches
The second way is to use the pressure = force/ area equation. Recall that the area of
the base will not matter, so our equation becomes: (see problem 1 for details)
15 psi  (density)( height)
15 psi  (0.025 lb/cubic in) (h)
15 psi
h
0.025 lb/cubic in
h  600 inches - - the same as above!!!
3. Three rectangular containers are full of water. Each container has a square base. The
container on the left has a base that is 8 inches by 8 inches and it is 4 inches high. The
middle container is 8 inches by 8 inches at the base and two inches high. The container
on the right is 4 inches by 4 inches and also 4 inches high. The situation is shown below.
4 in
A
B
8 in
4 in
8 in
2 in
A
A
C
D
8 in
E
A
8 in
A
F
4 in
4 in
The points labeled A, B, C, D, E, and F are all inside of the containers and below the
surface of the water. Each of the points labeled B, D, and F is at the bottom of a
container. Each of the points labeled A, C, and E is halfway between the bottom and
the top of a container. Note: to answer these questions you do not need to
numerically compute the water pressure at any of the points if you do not want to.
a) List all of the points (if any) where the water pressure will be greater than it is at
point A Explain your reasoning.
Points B and F – since the material is water in all of the boxes, the depth is the
only factor to consider.
b) List all of the points (if any) where the water pressure will be the same as it is at
point A. Explain your reasoning.
Points D and E- both of these points are at a depth of 2 inches so the pressure
would be the same as A
d) List all of the points (if any) where the water pressure will be less than it is at
point A. Explain your reasoning.
Point C – pt C is at a depth of 1 inch, so its pressure would be less than A
4. Three containers of liquid are shown in the diagram below. All three containers
are open to the atmosphere. The containers on the left contain water and the
container on the right contains mercury (which is about 13 times more dense than
water). There are seven points labeled inside the liquid in the containers. The heights
of these points are shown in inches at the right, as is the height of the surface of each
liquid
Water
Mercury
A
B
D
C
E
4 inches
F
3 inches
G
2 inches
1 inch
a) List all of the points where you would expect the pressure to be greater than it is
at point D. Explain your reasoning.
This very similar to the previous problem, however the mercury makes it a little
different. Since the pressure is the force/area, the force of the mercury is greater
than an equivalent depth of water by 13 times.
Points B, C, E, F, and G would have a greater pressure than pt. D—C and E are
in water but the width of the container does not matter, it is just the depth of the
water.
b) List all of the points where you would expect the pressure to be greater than it is
at point E. Explain your reasoning.
Points C, F and G. The mercury is 13 times greater in density, so the force for
the same depth of mercury is 13 times greater.
c) List a pair of points where you would expect the pressure to be just about equal.
Explain your reasoning.
Points A and D or B and E would be at the same pressure because they are at the
same depth.
5. We have seen that the density of air at sea level is about 1 kg/m3. The density of water
(at sea level and elsewhere) is about 1,000 kg/m3. The amount of pressure we call "one
atmosphere" (an "atmosphere" is a unit of pressure) is the same as the pressure that would
be created by a blanket of air with the same density as the air at sea level,
10,000 meters thick.
a) How thick would a blanket of water need to be to exert a pressure of one
atmosphere?
Let’s go back to earlier this quarter and think about the expression “factor
of”—We could say that the density of air is 1/1000 of that of seawater. If the
blanket of air is 10,000 meters thick, then an equivalent depth of seawater would
be 10,000 m times 1/1000 = 10 meters. So, 10 meters of seawater exerts the same
pressure as 10,000 meters of uniform air.
b) How deep below the surface of the ocean would you need to dive to experience a
pressure of two atmospheres? (Hint: you experience a pressure of one atmosphere
before you get into the water.)
From the previous part to this problem, we have to descend 10 meters into the
ocean to have 2 atmospheres, because there is already one atmosphere at the
surface of the ocean from the pressure of the atmosphere.