# 1 LIFE IN A FLUID. 2 web notes: lect1.ppt density.pdf pressure1.pdf pressure2.pdf pressure3.pdf.

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1 LIFE IN A FLUID

2 http://www.physics.usyd.edu.au/teach_res/jp/fluids/wfluids.htm web notes: lect1.ppt density.pdf pressure1.pdf pressure2.pdf pressure3.pdf

3 Dr Manjula Sharma Lectures Tutorials Labs Assignments Exam Textbook Worked examples

4 Identify  Setup  Execute  Evaluate IDENTIFY Identify what the question asking Identify the known and unknown physical quantities (units) SETUP Visualise the physical situation Diagrams - reference frames / coordination system / origin / directions Write down key concepts, principles, equations, assumptions that may be needed to answer the question EXECUTE Answer to the question from what you know. Numerical questions - solve before calculations - manipulate equations then substitute numbers add comments. EVALUATE CHECK - answer reasonable, assumptions, units, signs, significant figures, look at limiting cases

5 LEARNING AND DOING PHYSICS Physical quantities Symbols Units Typical values Relationships Principles Laws Mathematics Equations Graphs Appreciation Deep understanding

6 ?

7 Definition (  Greek letter rho )  density of object m mass of object V volume of object Units SI unit kg.m -3 Other unitsg.mL -1 1 mL = 1 cm 3 = (10 -2 ) 3 m 3 = 10 -6 m 3 1 g = 10 -3 kg 1 g.mL -1 = 10 3 kg.m -3

8 Density values (kg.m -3 ) Interstellar space10 -20 Best laboratory vacuum10 -17 Air (20 °C and 1 atm) 1.21 Styrofoam 100 Ice 917 Water (20 °C and 1 atm) 998 SaltWater1024 Whole blood1060 Earth (average)~5500 Iron7900 Mercury13 600 Osmium (most dense found on earth) 22 500 Uranium nucleus 10 17 Neutron star (core) 10 18

9 … and all the king's horses … What force is required to separate the hemispheres? Is this force significant? ?

10 Pressure P Is this pressure? What pressure is applied to the ground if a person stood on one heel?

11 Pressure P Units pressure pPapascal1 Pa = 1 N.m -2 force FNnewton1 N = 1 kg.m.s -2 area Am 2

12 How large is atmospheric pressure? Rough calculation p = F / A p = m g / A p =  V g / A p =  A h g / A p =  h g p = (1)(10 4 )(10) Pa p = 10 5 Pa Pressure at Earth’s surface due to weight of atmosphere  air = 1.21 kg.m -3 g = 9.8 m.s -2 h ~ 10 km

13 Atmospheric Pressure p atm 1.013x10 5 Pa 101.3 kPa 1013 hPa (hecto) 1013 mb (millibar) 1.013 b (bar) 760 mmHg 14.7 lb.in 2 (14.7 psi) 10.34 mH 2 0

14 ? What does it mean when we are told to inflate a tyre to 100 kPa?

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16 ? How does a gas or liquid exert a pressure on its container walls?

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19 p atm = 1x10 5 Pa R = ??? m A = 2  R 2 p = F / A F = p A F = (1x10 5 )(2  )( ) 2 N F = ??? N 70 kg person HOW MANY PEOPLE? N = F / F person ~

20 F = ? N

21 m = 70 kg F G = 700 N A = 10 mm x 10 mm A = (100)(10 -3 )(10 -3 ) m 2 A = 10 -4 m 2 p = F / A p = 700 / 10 -4 Pa p = 700 x 10 4 Pa p = 7 x 10 6 Pa p / p atm = 7x10 6 / 1x10 5 atm = 70 atm

22 Why is it better is use a sharp knife?

23 FLUID PRESSURE p

24 parallel to the surface and the interaction is not simply described by the pressure. Pressure has no intrinsic direction of its own (pressure is a scalar). t t

25 How long can your snorkel be? Why do your ears feel different when you dive into deep water? Why does atmospheric pressure change as you go up a mountain? How does a barometer work?

26 Pressure increases with depth in a fluid

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30 200 mm p h = p atm +  g h  p = p h – p atm  p =  g h  p = (1030)(9.8)(0.2) Pa  p = 2x10 3 Pa if h > 200 mm  p too large, lungs collapse

31 A scuba diver's lungs can rupture if he doesn't breathe while surfacing? Why?

32 You dive deep down into the water. Does it matter whether it is in the ocean or a swimming pool?

33 You dive deep down into the water. Does it matter whether it is in the ocean or a swimming pool? ! The pressure within a uniform stationary fluid is the same at all points in the same horizontal plane.

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