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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Physicsyd au teach res jp fluids09

Lecture 2

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


Physicsyd au teach res jp fluids09

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

Explain


Physicsyd au teach res jp fluids09

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.


Physicsyd au teach res jp fluids09

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


Physicsyd au teach res jp fluids09

How do we breath?


Physicsyd au teach res jp fluids09

How can you measure the pressure of a gas?


Physicsyd au teach res jp fluids09

D

h

A

C

B

How can you measure the pressure of a gas?

Manometer measures gauge pressure g h


Physicsyd au teach res jp fluids09

What is a simple way to measure atmospheric pressure?


Physicsyd au teach res jp fluids09

What is a simple way to measure atmospheric pressure?

A

Mercury Barometer

h

patm

patm

B

C

r


Physicsyd au teach res jp fluids09

How do you measure blood pressure?

p = r g h


Physicsyd au teach res jp fluids09

Why does a brain tumor affect the signal cord?


Physicsyd au teach res jp fluids09

Why does a brain tumor affect the signal cord?


Physicsyd au teach res jp fluids09

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.


Physicsyd au teach res jp fluids09

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


Physicsyd au teach res jp fluids09

Hydraulic brakes

http://hyperphysics.phy-astr.gsu.edu/hbase/pasc2.html


Physicsyd au teach res jp fluids09

How can a person easily lift a car?


Physicsyd au teach res jp fluids09

F2

F1

h1

oil

h2

A1

A2


Physicsyd au teach res jp fluids09

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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