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1 LIFE IN A FLUID

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3 Ian Cooper Lectures Tutorials Labs Duty Tutor Assignments Exam Textbook Worked examples

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5 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 = m 3 1 g = kg 1 g.mL -1 = 10 3 kg.m -3

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6 Density values (kg.m -3 ) Interstellar space Best laboratory vacuum Air (20 °C and 1 atm) 1.21 Styrofoam 100 Ice 917 Water (20 °C and 1 atm) 998 SaltWater 1024 Whole blood1060 Earth (average)~5500 Iron7900 Mercury Osmium (most dense found on earth) Uranium nucleus Neutron star (core) 10 18

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7 … and all the king's horses … What force is required to separate the hemispheres? Is this force significant? ? What physical quantities do we need to consider?

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8 Pressure P Is this pressure? What pressure is applied to the ground if a person stood on one heel? ? What physical quantities do we need to consider?

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

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10 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

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

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

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

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

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18 F = ? N

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19 m = 70 kg g = 10 m.s -2 F G = 700 N A = 10 mm x 10 mm A = (10x10 -3 )(10x10 -3 ) m 2 A = m 2 p = F / A p = (9700 / ) Pa p = 700 x 10 4 Pa p = 7 x 10 6 Pa p / p atm = 7x10 6 / 1x10 5 atm = 70 atm

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20 Why is it better is use a sharp knife?

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21 FLUID PRESSURE p

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22 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

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23 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?

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24 Pressure increases with depth in a fluid

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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

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

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

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31 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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