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Fluids

Fluids. Definition. Has no fixed shape Ability to flow. Density ( ρ). Mass per unit volume ρ = m/V SI unit – kg/m 3 Specific to different materials (pg. 301) What is the mass of a solid iron wrecking ball of radius 18cm?. Specific Gravity.

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Fluids

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  1. Fluids

  2. Definition • Has no fixed shape • Ability to flow

  3. Density (ρ) • Mass per unit volume ρ = m/V • SI unit – kg/m3 • Specific to different materials (pg. 301) • What is the mass of a solid iron wrecking ball of radius 18cm?

  4. Specific Gravity • Ratio of density of a substance to the density of water (1000 kg/m3) • What is the specific gravity of whole blood?

  5. Pressure • Force per unit area: P = F/A • Unit N/m2 = pascal (Pa) • Fluids exert pressure in all directions • Pressure due to liquid at any depth, h, is due to the weight of the liquid above it

  6. Equation • F = mg • m = ρV • V = Ah • F = ρAhg • P = F / A • P = ρAhg/A • P = ρhg (for liquids) • Called Gauge Pressure

  7. Pressure is directly proportional to density and depth • Pressure at equal depths in the same liquids is the same

  8. At the same height, which experiences a greater water pressure at the bottom?

  9. Example • The surface of a water storage tank is 30m above the faucet in the kitchen. Calculate the water gauge pressure in the faucet.

  10. Atmospheric Pressure • Unit – atmosphere (atm) 1atm = 101.3 kPa • If the surface of a liquid is open, the total pressure is the sum of the atmospheric and gauge pressures P = Pa + ρgh

  11. Example • What is the total pressure on a scuba diver in a lake at a depth of 8.00? (standard atmospheric pressure)

  12. Pascal’s Principle • Pressure applied to a confined fluid increases the pressure throughout by the same amount Pout = Pin F1 = F2 A1 A2

  13. Small force over small area is the same as a large force over a large area - Hydraulics

  14. Example • The area of the output piston is 20x that of the input cylinder. What force would need to be applied to lift a 1800kg car?

  15. Buoyancy • Upward net force on objects within a liquid • Occurs because pressure in a fluid increases with depth • Since the bottom is deeper, more force is pushing up than force on the bottom pushing down • Net upward force pushes objects up

  16. Archimedes Principle • Weight of fluid displaced is equal to the volume of the object

  17. FB = ρfluidgVsubmerged

  18. Example • A 70kg statue lies at the bottom of the sea. Its volume is 3.0 x 10-2 m3. How much force is needed to lift it? • When a crown of mass 14.7kg is submerged in water, an accurate scale reads only 13.4kg. Is the crown gold?

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