http://www.physics.usyd.edu.au/teach_res/jp/fluids/wfluids.htm
http://www.physics.usyd.edu.au/teach_res/jp/fluids/
web notes: lect2.ppt
pressure3.pdf
pascal.pdf
Can you crush a steel can by pumping the air from the inside?
Solution
Setup
Force to collapse can
F=pA
Assume atmospheric pressure acts only on outside surface
of can
p = patm = 1.013105 Pa
Need to know outside surface area A of can  need to
know dimension of can. Consider a cylinder of radius R =
0.15 m and height h = 0.30 m
A = 2( R2) + 2 R h
Action
F = (1.013105){(2)(0.15)2 + (2)(0.15)(0.30)} N
F = 4.3105 N
How big is this force?
Consider a 70 kg person  weight = 700 N  force
equivalent to the weight of > 61 people.
How do we breath
– get the air in and
out of our lungs?
http://www.lung.ca/children/grades4_6/respiratory/how_we_breathe.html
How do we breath?
Gas pressure
p=NkT/V=nRT/V
Boltzmann constant k = 1.38x10-23 J.K-1
Universal gas constant R = 8.314 J.mol-1.K-1
k = R / NA
R = k NA
Avogadro's constant NA = 6.023x1023 mol-1
Gas laws (fixed quantity of gas)
Boyle's Law (constant temperature)
p = constant / V
Charles Law (constant pressure)
V = constant  T
(constant volume)
p = constant  T
Isothermals pV = constant
180
160
pressure p (kPa)
140
120
100 K
100
200 K
300 K
80
400 K
60
40
20
0
0.00
0.05
0.10
0.15
0.20
volume V (m3)
0.25
0.30
0.35
0.40
Falling air 
compressed
(adiabatic
process)  rise
in temperature
 inhibits
formation of
clouds  rain
unlikely (great
desert regions)
Rising air 
expands
(adiabatic
process )  fall
in temperature
 formation of
clouds  more
likely to rain
convergence
divergence
HIGH - more uniform
conditions - inhibits cloud
formation
sunshine
divergence
convergence
LOW - less uniform
conditions - encourages cloud
formation
How can you measure the pressure of a gas?
How can you measure the pressure of a gas?
D
h
A
B
C
Manometer measures gauge pressure
gh
What is a simple way to measure atmospheric pressure?
What is a simple way to measure atmospheric pressure?
A
Mercury Barometer
h
patm
patm
B
C

How do you measure blood pressure?
p=g
h
Why does a brain tumor affect the signal cord?
Why does a brain tumor affect the signal cord?
Pascal's Principle
1653 Blaise Pascal (1623 – 1662)
Pressure applied to an enclosed fluid is transmitted undiminished
to every portion of the fluid and walls of the containing vessel.
ph
ph
p0’
p0
(0,0)
p0
h
(0,0)
h
Linear relationship between pressure and depth.
If the pressure at the surface increases then the pressure at
a depth h also increases by the same amount.
Tennis Ball Impact on Eye
A blow to the eye by a tennis ball can cause more damage than
one might expect because of the transmission of the pressure to the back of the
eye
http://hyperphysics.phy-astr.gsu.edu/hbase/pasc2.html
Hydraulic brakes
http://hyperphysics.phy-astr.gsu.edu/hbase/pasc2.html
How can a person easily lift a car?
F2
F1
h1
oil
h2
A1
A2
p1 = p2
p=F/A
F1 / A1 = F2 / A2
W=Fx
F2 = (A2 / A1) F1
A2 >> A1 F1 << F2
W 1 = W2
F1 h1 = F2 h2
F1 h1 = (A2 / A1) F1 h1
h1 = (A2 / A1) h2
A2 >> A1
h1 >> h2