Fluids and Pressure

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Fluids and Pressure

• PHYS 1090 Unit 5

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Balloon Mash

• Greater force makes larger contact area

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Pressure

• Force applied per unit area
• p F/A
• If pressure is constant, force and area change
  together

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Pressure within Fluids

• Pascals Principle fluids exert pressure
  evenly in all directions

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Balloon Mash

• Upward force (pressure ? area) on plate exactly
  cancelled plates weight
• As weight increased, contact area did too
• (pressure may have increased as well)

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Fountain

• Water shoots farther from lower holes
• Streams weaken as water drains

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Static Fluids

• Pressure counteracts weight of fluid above
  (Pascals principle)

• Pressure increases with depth

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Liquid Pressure Formula

• p rhg

• p pressure
• r density of liquid
• h depth under top of liquid

p pressure here
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Pressure within a Liquid
p rhg

• Shape of the container does not matter!
• All that matters are depth h, fluid density r,
  and gravitational field g.

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Fountain

• Stream velocity depends on pressure
• Pressure depends on depth
• Rocks in the can have no effect

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Sinking and Floating

• Objects displace a volume of water equal to their
  submerged volume
• A floating boat displaces an additional volume of
  air

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What forces are present?
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What forces are present?
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What forces are present?
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What forces are present?
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Pressure in a fluid

• Pressure increases with depth
• Greater pressure at bottom than top of an
  immersed object
• Results in upward buoyancy force that is the
  (vector) sum of all pA forces

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Buoyancy Force

• Buoyancy force weight of fluid
  displaced(Principle of Archimedes)
• F rVg
• r density of fluid
• V volume of fluid displaced volume of object
  submerged
• g 9.8 N/kg

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Sinking and Floating

• All objects are lighter under water
• Difference is buoyancy force
• If buoyancy gt weight, object rises to surface and
  floats (so buoyancy weight)
• if buoyancy lt weight, object sinks

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Clay Lump

• Weight of the lump was constant
• Making a boat increased the volume of water
  displaced
• That increased the buoyancy force
• A great enough buoyancy floated the boat

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Expanding and Contracting

• The same amount of gas occupies more volume at a
  higher temperature.

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Convection

• Warm fluids expand, becoming less dense
• Circulation is driven by buoyancy forces
• Much faster than conduction

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Ideal Gas Law
pV NkBT

• p pressure
• V volume
• N number of gas molecules
• kB 1.38066?1023 J/K
• T absolute (Kelvin) temperature

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Gas Pressure and Volume
pV NkBT

• At a constant temperature, pV is constant
• Increasing p decreases V and vice versa

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Balloon Mash

• Pressure may have increased with greater force
  because air was compressed (volume became less)

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Diver

• Divers weight weight of (glass air)
• Buoyancy weight of excluded water
• Increasing pressure decreases air volume
• Buoyancy decreases
• Weight is unchanged

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Neutral Buoyancy

• Air doesnt weigh much
• Glass weight doesnt change
• At neutral buoyancy SF 0, buoyancy? glass
  weight?
• Neutral buoyancy air volume is the same for all
  initial bubble sizes