# web notes: lect2.ppt pressure3.pdf pascal.pdf.

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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 = p A Assume atmospheric pressure acts only on outside surface of can p = p atm = 1.013  10 5 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(  R 2 ) + 2  R h Action F = (1.013  10 5 ){(2  )(0.15) 2 + (2  )(0.15)(0.30)} N F = 4.3  10 5 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 / N A R = k N A Avogadro's constant N A = 6.023x10 23 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

How can you measure the pressure of a gas?

A D C B h Manometer measures gauge pressure  g h

What is a simple way to measure atmospheric pressure?

h p atm  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?

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?

h2h2 h1h1 F1F1 F2F2 A1A1 A2A2 oil

p = F / A W = F x p 1 = p 2 F 1 / A 1 = F 2 / A 2 F 2 = (A 2 / A 1 ) F 1 A 2 >> A 1 F 1 << F 2 W 1 = W 2 F 1 h 1 = F 2 h 2 F 1 h 1 = (A 2 / A 1 ) F 1 h 1 h 1 = (A 2 / A 1 ) h 2 A 2 >> A 1 h 1 >> h 2

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