fluids and pressure
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Fluids and Pressure. PHYS 1090 Unit 5. Balloon Mash. Greater force makes larger contact area. Pressure. Force applied per unit area p = F / A If pressure is constant, force and area change together. Pressure within Fluids. - PowerPoint PPT PresentationTRANSCRIPT
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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
• Pascal’s Principle: fluids exert pressure evenly in all directions
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Balloon Mash
• Upward force (pressure area) on plate exactly cancelled plate’s 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 (Pascal’s principle)
Supports weight above
• Pressure increases with depth
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Liquid Pressure Formula
p = hg
• p = pressure
• = density of liquid
• h = depth under top of liquid
h
p = pressure here
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Pressure within a Liquid
• Shape of the container does not matter!
• All that matters are depth h, fluid density , and gravitational field g.
p = hg
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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 = Vg
= 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 > weight, object rises to
surface and floats (so buoyancy = weight)
• if buoyancy < 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
• p = pressure
• V = volume
• N = number of gas molecules
• kB = 1.3806610–23 J/K
• T = absolute (Kelvin) temperature
pV = NkBT
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Gas Pressure and Volume
• At a constant temperature, pV is constant• Increasing p decreases V and vice versa
p
V
pV = NkBT
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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
• Diver’s 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 doesn’t weigh much
• Glass weight doesn’t change
• At neutral buoyancy F = 0, buoyancy↑ = glass weight↓
• Neutral buoyancy air volume is the same for all initial bubble sizes