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Apr 20, 2023
Liquids
Apr 20, 2023
Pressure
Pressure is defined as
Pressure =
Metric unit of pressure is Pascal.
1 Pascal = 1 Newtons per square meter
Atmospheric pressure is about 100,000 Pascals
( Force )
( Area )
Apr 20, 2023
Sample Problem
“Gold” brick is roughly:12 kilograms [Mass]120 Newtons [Weight](0.2 m)x(0.1 m) =
(0.02 m2) [Area]Pressure on surface is(120)/(0.02)
= 6000 Pascals
Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the “gold” brick.
Apr 20, 2023
Check Yourself
In which case is the pressure greatest? A B
A B
Apr 20, 2023
Check Yourself
In which case is the pressure greatest? A B
AB
Apr 20, 2023
Demo: Bed of Nails
One may safely lay or sit on a bed of nails, as long as there are enough nails so that the pressure, measured as force per nail, is small.
Weight of 150 pounds is distributed over 300 nails. Force per nail is ½ lb. Need 5 lb per nail to pierce skin.
Apr 20, 2023
Pressure in Liquids
Pressure in a liquid depends on depth.
As with bricks, weight of what’s above determines pressure.
High
Medium
Low
High
Medium
Low
Apr 20, 2023
Demo: Pressure & Weight
A can full of water has holes in the sides through which water comes out.
What happens when you drop the can?
The can is now in freefall and weightless. Water stops flowing as the can falls since the pressure was due to the water’s weight.
Extensive and Intensive
Extensive properties – depends on how much of the substance is present. Ex. – mass, length, volume
Intensive properties – does not matter how much you have, the property is the same. Ex. – density, specific heat, freezing point, boiling point
Apr 20, 2023
Buoyancy
Since pressure depends on depth, a submerged object has more force due to pressure below it than above it.
Net effect is to have a net upward force, which we call buoyancy.
Buoyancy
Weight
If weight exceeds buoyancy force then object sinks, otherwise it floats.
Pressure Pressure
Pressure
Apr 20, 2023
Buoyancy & Depth
For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth.
1
2
3
4
5
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Buoyancy
Buoyancy
Buoyancy
Apr 20, 2023
Archimedes’ PrincipleWeight of liquid displaced by floating or submerged
object equals the buoyant force on the object.
Apr 20, 2023
Demo: Take a Dip
A 1 kg mass (1000 g) submerged in water displaces 150 cm3.
Water has density of 1 g/cm3 so displaced water has mass of 150 grams.
Buoyant force equals weight of 150 grams.
1000 g
1000 g
850 g
150 cc displaced
Beaker ofWater
Apr 20, 2023
Demo: Floating in the Tub
Buoyant force depends only on the volume of fluid displaced, not on the volume of fluid in which the object is immersed.
Rubber duck floats the same in a little water or in a full beaker of water
Apr 20, 2023
Check Yourself
?
Place block of wood in the water. Scale reading goes up, down, or stays the same?
50 N
10 N Block
10 N Water
40 N Water
Apr 20, 2023
Check YourselfUse similar block of petrified wood. Scale reading goes up, down, or stays the same?
?
50 N
30 N Block
10 N Water
40 N Water
Apr 20, 2023 Physics 1 (Garcia) SJSU
Check Yourself
Did the designers of this “water bridge” have to account for the weight of ships or just the water?
Apr 20, 2023
Density & Floating
By Archimede’s principle, a solid object will float if the density of the object is less than the density of the liquid.
Billiard ball (4.0 g/cm3) floating in a cup of mercury (13.6 g/cm3)
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Diet or Regular?
What can you say about the density of diet cola as compared with regular cola?
Apr 20, 2023 Physics 1 (Garcia) SJSU
Ships
Density of wood is about 0.5 to 0.8 g/cm3 so not surprising that wooden ships float.
Density of iron 7.9 g/cm3. How is it that a battleship can float?
Apr 20, 2023 Physics 1 (Garcia) SJSU
Density & Floating, RevisitedWhen an object is not solid then it floats if the average density, (total mass)/(total volume), is less than the density of the liquid.
Solid iron ballVolume: 100 ccMass: 790 gDensity: 7.9 g/cc
Iron ball, 90% hollowVolume: 1000 ccMass: 790 gDensity: 0.79 g/cc
Apr 20, 2023 Physics 1 (Garcia) SJSU
Iron Ships, Floating & Sinking
Iron ship floats since it is hollow inside.
If water floods the inside then it sinks.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Floating & Liquid Density
The greater the density of a liquid, the greater the buoyant force on objects floating or immersed in the liquid. Floating in Great Salt Lake, Utah is
easy because the lake water is dense due to high concentration of salt.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Hydrometer
Hydrometer is a calibrated float; denser the liquid, the higher the hydrometer floats.
What is Viscosity?
• Resistance of a fluid to a change in shape, or movement of neighboring portions relative to one another. Viscosity denotes opposition to flow. It may also be thought of as internal friction between the molecules.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Viscosity
• Viscosity is a major factor in determining the forces that must be overcome when fluids are used in lubrication or transported in pipelines. It also determines the liquid flow in spraying, injection molding, and surface coating.
Apr 20, 2023 Physics 1 (Garcia) SJSU
• Fluids with low viscosity flow quickly
• Fluids with high viscosity flow slowly
• heating a fluid decreases viscosity; cooling a fluid increases viscosity
– Ex: Warm honey flows more quickly than cold honey.
Viscosity Notes
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