http://www.physics.usyd.edu.au/teach_res/jp/fluids09

Lecture 2

Can you crush a steel can by pumping the air from the inside?

Explain

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

p V = n R T

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)

pre

ssu

re p

(kP

a)

100 K

200 K

300 K

400 K

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