Transcript Fluids - Lompoc Unified School District

Fluids
Definition
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Has no fixed shape
Ability to flow
Density (ρ)
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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?
Specific Gravity
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Ratio of density of a substance to the
density of water (1000 kg/m3)
What is the specific gravity of whole
blood?
Pressure
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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
Equation
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F = mg
m = ρV
V = Ah
F = ρAhg
P=F/A
P = ρAhg/A
P = ρhg (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
At the same height, which experiences a
greater water pressure at the bottom?
Example
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The surface of a water storage tank is
30m above the faucet in the kitchen.
Calculate the water gauge pressure in
the faucet.
Atmospheric Pressure
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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
Example
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What is the total pressure on a scuba
diver in a lake at a depth of 8.00?
(standard atmospheric pressure)
Pascal’s Principle
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Pressure applied to a confined fluid
increases the pressure throughout by
the same amount
Pout = Pin
F1 = F2
A1
A2
Small force over small area is the same as
a large force over a large area - Hydraulics
Example
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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?
Buoyancy
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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
Archimedes Principle
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Weight of fluid displaced is equal to the
volume of the object
FB = ρfluidgVsubmerged
Example
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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?