13 Liquids
• Demos:
• Surface tension (pepper, soap)
• Homework:
• RQ: 2, 4, 7, 10, 12, 14, 21.
• Problems: 2, 10.
1
Pressure
• pressure = force/area
• Example: 500N applied to 0.10m2.
Pressure = 500/0.10 = 5,000N/m2.
• Example: atmospheric pressure is
100,000 N/m2.
2
Depth and Fluid Pressure
• pressure = weight density x depth
• depth only (not area), pressure equal at
bottom of all shapes below
3
Buoyancy and Archimedes’
Principle
4
sink or float?
• object more dense than fluid sinks
• object less dense than fluid floats
• object same density as fluid is neutral
5
Water Density & Volumes
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“L” = liter (about 33oz)
1mL = 0.001L = 1cc
1mL water = 1gram
water density = 1gram/mL
6
Example Volumes
• An 800 gram object displaces 400mL
(submerged). What is its density?
• Density = mass/volume
• = 800grams/400mL
• = 800grams/400cm3
• = 2g/cm3
7
Example 1 Arch.
• A 1kg object displaces 200mL of water
(submerged). Does it sink or float?
• Density = 1000g/200mL
• = 5g/mL = 5g/cm3
• Density > 1 Sinks
8
Example 2 Arch.
• A 1kg object displaces 1200mL of water
(submerged). Does it sink or float?
• Density = 1000g/1200mL
• = 0.83g/mL = 0.83g/cm3
• Density < 1g/cm3 Floats
9
Pascal’s Principle
• pressure increases on an enclosed fluid are
transmitted undiminished to entire fluid.
• Hydraulic Application:
• output force = (area ratio)x(input force)
10
surface tension
• cohesive nature of surface of a fluid, e.g.
waterbug, pin floating on water
• hot water has less
surface tension,
as does soapy water
11
Videos
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Metal boats
Effect of Screen
Soap in water, floating
Soap in milk, food coloring
Water, pepper
12
surface area
• sphere has smallest ratio of surface
area/volume of any shape
• surface tension causes fluids to form
‘drops’ (water has more than 3 times the
surface tension of alcohol)
13
Summary
• pressure = force/area
• pressure ~ depth
• buoyant force equals weight of displaced
fluid
• pressure changes are distributed
throughout enclosed fluids
14
Summary 2
• surface tension phenomena
• surface area/volume ratio low for sphere,
high for sheet
15
Ex: Sphere 12m3.
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•
•
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area/volume = 4pR2/(4/3)pR3 = 3/R.
Example: volume = 12 cubic meters.
R = 1.42 meters
area/volume = 3/1.42meter = 2.11/meter
16
12m3 rectangular plates
• 1) height = 3m, width = 2m, depth = 2m
• 2) height = 0.75m, width = 4m, depth = 4m
• 3) height = 0.12m, width = 10m, depth = 10m
• calculate area/volume ratio for these three
objects.
• How do these ratios compare to the sphere?
17