Density and Pressure
Lesson 1
Announcements
HW #1 due Friday
Lesson 1: Density & Pressure
AP Physics B Objectives
II.A.1. Hydrostatic pressure
Students should understand the concept of pressure as it applies to fluids,
so they can:
a) Apply the relationship between pressure, force, and area.
b) Apply the principle that a fluid exerts pressure in all directions.
c) Apply the principle that a fluid at rest exerts pressure perpendicular to any surface that it contacts.
d) Determine locations of equal pressure in a fluid.
e) Determine the values of absolute and gauge pressure for a particular situation.
f) Apply the relationship between pressure and depth in a liquid.
Student Objectives
 Students will be able to
1.
2.
3.
calculate and distinguish between gauge pressure and atmospheric
pressure.
determine hydrostatic pressure for various applications.
apply principles of density and pressure to hurricanes.
 Challenge Problem:
The density of gasoline is 680 kg/m3. If the tank of a jeep holds
25 gallons of gasoline, how much mass does the jeep gain when
someone fills an empty tank?
Density
Formula:  = m/V
: density (kg/m3)
m: mass (kg)
V: volume (m3)
Units: kg/m3
What is pressure?
What is normal air pressure in your car tires?
What do the units of pressure tell you about
a likely formula for pressure?
Pressure
 P = F/A
P : pressure (Pa)
F: force (N)
A: area (m2)
 Pressure unit:
Pascal ( 1 Pa = 1 N/m2)
 The force on a surface caused by pressure is
always normal (or perpendicular) to the
surface. This means that the pressure of a
fluid is exerted in all directions, and is
perpendicular to the surface at every location.
Atmospheric Pressure
 Atmospheric pressure is
normally about 100,000
Pascals.
 Differences in atmospheric
pressure cause winds to
blow.
http://www.youtube.com/watch?v=z1ONNM_73-8&feature=related
Low atmospheric pressure
inside a hurricane’s eye
contributes to the severe
winds and the development of
the storm surge.
 Sample Problem:
Calculate the net force on an airplane window if cabin pressure is 90% of
the pressure at sea level, and the external pressure is only 50% of that at
sea level. Assume the window is 0.43 m tall and 0.30 m wide.
The Pressure of a Liquid
 If you have a column of liquid, how does the pressure
vary from top to bottom? How might you calculate the
pressure at a given depth?
The Pressure of a Liquid
 Formula:
P = gh
 P: pressure (Pa)
 : density (kg/m3)
 g: acceleration constant (9.8 m/s2)
 h: height of liquid column (m)
 This type of pressure is sometimes called gauge pressure.
Why?
 If the liquid is water, this is referred to as hydrostatic pressure.
Why?
Absolute Pressure
Absolute pressure is obtained by adding
the atmospheric pressure to the
hydrostatic pressure.
pabs = patm + pliquid
pabs = patm + gh
http://www.youtube.com/watch?v=HUmZrtiXDik 10.3 m!!!
Drinking straw
Juice box
Hydrostatic Pressure in Dam
Design
The depth of Lake Mead at the Hoover Dam is
180 m. What is the hydrostatic pressure and what
is the absolute pressure at the base of the dam?

http://www.iit.edu/~karagian/smart00/physics.html
Hydrostatic Pressure in
Levee Design
Hurricane Katrina, August 2005
A hurricane’s storm
surge can overtop levees,
but a bigger problem can
be increasing the
hydrostatic pressure at
the base of the levee.
New Orleans Elevation Map
New Orleans is largely below sea level, and relies upon
a system of levees to keep the lake and the river at bay
Sample Problem:
Calculate the increase in hydrostatic pressure experienced by
the levee base for an expected (SPH Design) storm surge.
How does this compare to the increase that occurred during
Hurricane Katrina, where the water rose to the top of the levee?