Physics 101: Lecture 23
Fluids: Gases and Liquids

Today’s lecture will cover Textbook Sections 11.1-11.4
Density
Pressure
Physics 101: Lecture 23, Pg 1
Density

Density = Mass/Volume
  = M/V

SI unit: [kg/m3]
Densities of some common things (in kg/m3):
Water (at 4 degrees Celsius) 1000
ice
917 (floats on water)
blood
1060 (sinks in water)
lead
11,300
Copper
8890
Mercury 13,600
Aluminum 2700
Wood (yellow pine) 550
air
1.29
Helium
0.179
For comparisons of densities we use:
Specific gravity = density of substance/density of water
Physics 101: Lecture 23, Pg 2
Pressure
The magnitude of a force acting perpendicular to
a surface per unit area is called pressure:

P=F/A
SI Unit: [N/m2 ]
N/m2 is also called Pascal [Pa]. 105 Pa = 1 bar of pressure
Because of pressure any gas or liquid applies a force
perpendicular to the surface with which it comes in contact.
The air above the surface of the earth creates a pressure
at sea level of (=atmospheric pressure)
1.013 x 105 Pa = 1 atmosphere
Physics 101: Lecture 23, Pg 3
Pressure and Depth in a Static Fluid
In a static fluid when knowing the pressure at a certain
depth, P1 at d1, the pressure at a larger depth, P2 at d2,
can be calculated from the increase in pressure due to the
weight of the fluid that is located between d1 and d2:

P2 A = P1 A + m g = P1 A +  V g
P2 = P1 +  h g
(h=d2-d1)
Here we assumed that the fluid is not compressible, i.e. its
density is the same at any depth.
Physics 101: Lecture 23, Pg 4
Pressure and Depth
Barometer: a way to measure atmospheric pressure
P2 = P1 +  g h
Patm =  g h  Measure h, determine Patm or
P1=0
Example: Mercury
 = 13,600 kg/m3
Patm = 1.05 x
105
Pa
P2=Patm
h
h = 0.757 m = 757 mm = 29.80” (for 1 atm)
Open tube manometer: P1 = Patm  P2-Patm = ρ g h : gauge pressure
P2 : absolute pressure
Physics 101: Lecture 23, Pg 5
Concept Question
Suppose you have a barometer with mercury and a barometer with
water. How does the height hwater compare with the height hmercury?
1. hwater is much larger than hmercury
2. hwater is a little larger than hmercury
3. hwater is a little smaller than hmercury
4. hwater is much smaller than hmercury
CORRECT
water is much less dense than
mercury, so the same amount of
pressure will move the water
farther up the column.
P1=0
P2=Patm
Pa  gh
h
h
Pa
g
 mercury  13.6  water
so,
h water  13.6 h mercury
Physics 101: Lecture 23, Pg 6
Concept Question
P=0
Pa
Pa  gh
h
Pa
g
h
Evacuate the straw by sucking.
How high will the water rise?
no more than h = Pa/g = 10.3m = 33.8 feet
no matter how hard you suck!
Physics 101: Lecture 23, Pg 7
Concept Question
Is it possible to stand on the roof of a five story (50 foot) tall house
and drink, using a straw, from a glass on the ground?
1. No
CORRECT
2. Yes
Even if a person could completely remove all of the air
from the straw, the height to which the outside air
pressure moves the water up the straw would not be high
enough for the person to drink the water.
Physics 101: Lecture 23, Pg 8
Summary
• Density:  = M/V
• Pressure: P = F/A
P2 = P1 +  g h
Physics 101: Lecture 23, Pg 9