Lecture 2
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Lecture 2. http://www.physics.usyd.edu.au/teach_res/jp/fluids09. Can you crush a steel can by pumping the air from the inside?. Explain. Solution Setup Force to collapse can F = p A Assume atmospheric pressure acts only on outside surface of can p = p atm = 1.013  10 5 Pa

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Lecture 2

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


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

Explain


Solution

Setup

Force to collapse can

F = p A

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?

p V = n R T

http://www.lung.ca/children/grades4_6/respiratory/how_we_breathe.html


How do we breath?


How can you measure the pressure of a gas?


D

h

A

C

B

How can you measure the pressure of a gas?

Manometer measures gauge pressure g h


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

r


How do you measure blood pressure?

p = r 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.


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

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

p = F / A

W = F x


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