Fluids

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Fluids
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Definition

• Has no fixed shape
• Ability to flow

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Density (?)

• Mass per unit volume
• ? 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?

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Specific Gravity

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

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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

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Equation

• F mg
• m ?V
• V Ah
• F ?Ahg
• P F / A
• P ?Ahg/A
• P ?hg (for liquids)
• Called Gauge Pressure

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

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

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

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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 ?gh

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Example

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

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Pascals Principle

• Pressure applied to a confined fluid increases
  the pressure throughout by the same amount
• Pout Pin
• F1 F2
• A1 A2

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

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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

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Archimedes Principle

• Weight of fluid displaced is equal to the volume
  of the object

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• FB ?fluidgVsubmerged

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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?