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Fluids

Fluids. Physics 202 Professor Vogel (Professor Carkner’s notes, ed) Lecture 19. Fluids. A fluid is a substance that can flow A liquid or a gas A fluid has no internal structure Since a fluid can flow, any individual piece of the fluid can be hard to keep track of

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Fluids

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  1. Fluids Physics 202 Professor Vogel (Professor Carkner’s notes, ed) Lecture 19

  2. Fluids • A fluid is a substance that can flow • A liquid or a gas • A fluid has no internal structure • Since a fluid can flow, any individual piece of the fluid can be hard to keep track of • Mass and force are often not useful • The important quantities of a fluid are density and pressure

  3. Density • The density (r) of a fluid is the mass per unit volume for an arbitrary volume element • Density can vary with temperature or pressure • but, liquids are much less compressible than gases • The SI unit of density is kg/m3 • Air ~1.21 kg/m3 • Water ~1000 kg/m3 • Rock ~3000 kg/m3 • Metal ~8000 kg/m3

  4. Pressure • Pressure is defined as the force per unit area P=DF/DA • The SI unit of pressure is the pascal (Pa), a newton per square meter • An important practical unit of pressure is the atmosphere, the pressure of the Earth’s atmosphere at sea level 1 atm = 1.01 X 105 Pa = 14.7 psi • For solving problems, you almost always need to use pascals (not atmospheres)

  5. Pascal’s Principle • Pressure applied to an enclosed fluid is transmitted to every portion of the fluid and the container • Pascal’s principle is the basis for the hydraulic lever • Consider a U-shaped tube: • If you apply a pressure at one end, the same pressure is felt at the other end • But what if the other end of the tube is thicker?

  6. A Hydraulic Jack

  7. Hydraulic Jack • Since the pressures are the same and the areas are different, the force on the other end is larger (from P=F/A) • But energy must be conserved: • W=Fd, so if the force is greater at the other end the displacement must be less • A person can lift a car with a hydraulic jack, but ratcheting the jack 3 feet may only move the car an inch

  8. Archimedes’ Principle • What happens if you put an object in a fluid? • The fluid exerts a force on the object • Called the buoyant force • The object will also displace fluid • If you measure the buoyant force and the weight of the displaced fluid, you find: • An object in a fluid experiences an upward buoyant force equal to the weight of fluid it displaces • This is Archimedes’ principle • Applies to objects both floating and submerged

  9. Buoyancy

  10. Will it Float? • What determines if a object will sink or float? • Density • An object less dense than the fluid will float • A floating object displaces fluid equal to its weight • An object denser than the fluid will sink • A sinking object displaces fluid equal to its volume

  11. Floating • How will an object float? • The denser the object, the lower it will float, or: • The volume of fluid displaced is proportional to the ratio of the densities • Example: ice floating in water, W=rVg riVig=rwVwg Vi/Vw=rw/ri Vw=Vi (ri/rw) rw = 1024 kg/m3 and ri = 917 kg/m3 Vw=0.89 Vi

  12. Iceberg

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