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?

SolutionSetupForce to collapse canF = p AAssume atmospheric pressure acts only on outside surface of canp = patm = 1.013105 PaNeed 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 mA = 2( R2) + 2 R h

ActionF = (1.013105){(2)(0.15)2 + (2)(0.15)(0.30)} NF = 4.3105 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 = N k T / V = n R T / 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

0

20

40

60

80

100

120

140

160

180

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

volume V (m3)

pres

sure

p (k

Pa)

100 K200 K300 K400 K

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

divergence convergenceHIGH - more uniform conditions - inhibits cloud formation

LOW - less uniform conditions - encourages cloud formation

sunshine

How can you measure the pressure of a gas?

How can you measure the pressure of a gas?

A

D

CB

h

Manometer measures gauge pressure g h

What is a simple way to measure atmospheric pressure?

h

patmpatm

B

A

C

What is a simple way to measure atmospheric pressure?

Mercury Barometer

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?

tumor

Increased pressure transmitted down spinal cord

Pascal's Principle 1653 Blaise Pascal (1623 – 1662)

Pressure applied to an enclosed fluid is transmitted undiminishedto every portion of the fluid and walls of the containing vessel.

(0,0) h

ph

p0

(0,0) h

ph

p0

p0’

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?

h2

h1

F1

F2

A1 A2

oil

p = F / A

W = F x

p1 = p2

F1 / A1 = F2 / A2

F2 = (A2 / A1) F1

A2 >> A1 F1 << F2

W1 = W2

F1 h1 = F2 h2

F1 h1 = (A2 / A1) F1 h1

h1 = (A2 / A1) h2

A2 >> A1 h1 >> h2