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Fluid Pressure Chapter 13.1
Fluid Pressure
Chapter 13.1
Pressure
What is the definition of pressure?The result of a force distributed over an area
The book talks about the difference in pressure between a bicycle seat and a theater seat. Give one original idea regarding this same idea:Snowshoes vs. high heels
What is the formula for finding pressure?Pressure = force/areaPa = N/m2
What is the SI unit for pressure? Why do we usually use the prefix “kilo” in front of this unit?Pascal (kilopascal)A pascal is too small for normal use
Pressure in Fluids
What is the scientific definition of fluid?A substance that assumes the shape of its
container
What phases of matter are considered fluids?Liquids & gasses
What things affect water pressure?Depth
As you get deeper in the water, pressure increases
Type of fluid
What things do not affect water pressure?Shape of containerArea of the bottom of a container or its
surfaceAmount of fluid (volume)
Air Pressure and the Atmosphere
What is air pressure at sea level on Earth?101 kPa
Relate air pressure and altitude:As you get “deeper” in the air, the pressure
increases
Why are you not affected by the 1000+ Newtons of force pushing down on you right now?The inside of your body also exerts
pressure(an equal and opposite force)The forces are balanced = net force of
zero
Review
1. What must you know to calculate pressure?
Force and Area
Review
2. What is the relationship between the depth of water and the pressure it exerts?
Water pressure increases as depth increases.
Review
3. How is pressure distributed at a given level in a fluid?
At a given depth, pressure is constant and exerted equally in all directions.
Review
4. How does the pressure exerted by the atmosphere change as altitude increases?
Atmospheric pressure decreases as altitude above sea level increases.
Review
5. Why don’t you feel the pressure exerted by the atmosphere?
You cannot feel atmospheric pressure acting on your body because the pressure inside your body balances the pressure outside.
Review6. Some deep-sea fish have been known to
explode as they are brought to the ocean’s surface. How do pressure changes cause this to happen?
Deep-sea fish have high internal body pressures in order to counteract the extreme water pressure that exists where they live. When the fish is suddenly brought to the surface, where the pressure is much less, its internal pressure can cause it to rupture or explode.
Review
7. The 500 N student stands on one foot. A 750 N student stands on two feet. If both students wear the same size shoe, which exerts the greater pressure?
Do the math!
The 500 N student.
Review
8. A circus performer on a pair of stilts exerts a pressure of 32 kPa on the ground. If the performer stands on one stilt, what pressure does the stilt exert on the ground?
Twice as much, because the area is twice as small.
Review
9. A book with a weight of 12 N rests on its back cover. If the back cover measures 21 cm by 28 cm, how much pressure does the book exert?
200 Pa
Forces and Pressure in Fluids
Chapter 13.2
Transmitting Pressure in a FluidPascal’s Principle
State Pascal’s Principle:
The change in pressure anywhere in a fluid is transmitted equally & unchanged through all the liquid (in all directions)
Hydraulic SystemsDescribe a hydraulic system:
A device that uses pressurized fluid acting on pistons of different sizes to change a force.
If Pascal’s Principle is true, how can a hydraulic system produce an increased output force?Because a constant fluid pressure is exerted
on the larger area of the output piston.Force = Pressure x Area
Bernoulli’s Principle
State Bernoulli’s Principle:As the speed of a fluid increases, the
pressure within the fluid decreases.
How is a fluid’s speed related to the pressure it exerts?Slow or no speed = great pressureHigh speed = lower pressure
Wings & Lift
Explain lift:An upward force is created by having a low
pressure over a high pressure.
How does a car’s wing create a downward force?It is an upside-down wing.
Note: Wing & Spoiler are different on a car
Spray Bottles
How do spray bottles work?
Check out figure 9 (pg. 397)
Review
1. Describe Pascal’s principle in your own words:
When there’s a change in pressure somewhere in a fluid, it will spread evenly throughout the fluid (trying to even out).
Review
2. How is a hydraulic system able to increase force?
The increased output force is produced because the fluid pressure is exerted on the larger area of the output piston.
Review
3. How are fluid speed and fluid pressure related?
As the speed of a fluid increases, the pressure within the fluid decreases.
Review
4. How does an airplane wing produce lift?
Bernoulli’s principle:
Air travels faster over the top of the wing, producing low pressure. Slower air on the bottom of the wing creates higher pressure.
Review
5. Water is nearly incompressible. Air is easily compressed. Predict what will happen when an air-filled balloon is pulled to the bottom of a swimming pool.
The air-filled balloon will be compressed.
Review
6. When cars pass one another in opposite directions on the highway, they tend to be forced together. Use Bernoulli’s principle to explain why this happens.
Review7. Two table tennis balls are suspended
from strings so they hang at the same height with a small space between them. When a hair dryer is used to blow air between them, they come together and touch. Explain why this occurs:
Buoyancy
Chapter 13.3
Buoyant Force
What is the definition of buoyancy?The ability of a fluid to exert an upward
force on an object. It is what seems to cause a “loss of weight”
of an object in a fluid.
Does every object in a fluid experience buoyancy?Yes, even if it sinks, there’s still some upward
force.
What is a buoyant force?An upward force, which acts in the opposite
direction as gravity.It makes it easier to lift things in fluid.
What causes a buoyant force?Forces from fluid pressure act on objects in
all directions, but most from the bottom, pushing the object up.See Figure 10, page 400.
The net force is up (using vector addition, of course!)
Archimedes’ Principle
The buoyant force on an object is equal to the weight of the fluid displaced by the object.
When an object is submerged, it pushes aside (displaces) a volume of fluid equal to its own volume.(the part underwater)
Density & Buoyancy
How are density and buoyancy related?
Density = mass/volume (g/cm3)
Buoyancy = weight of the volume of displaced fluid (N)
Relate an object’s density to whether or not it will float:Objects less dense than the fluid will floatObjects more dense than the fluid will sink
Relate an object’s weight and buoyant force to whether or not it will float:Objects that weigh less than the buoyant
force will floatObjects that weigh more than the buoyant
force will sink
Suspended
When an object has the same density as the fluid it’s in, it will be suspended.
Buoyant force = weight
Not sinking, not floating
Sinking
If the object’s weight is greater then the buoyant force, it will sink.
If a ship takes on water, it gets heavier and will sink.
Floating
The shape of a ship allows it to displace enough water to float.
(Increasing displacement increases buoyant force)
It’s easier to float in dense liquids, because there’s a greater buoyant force.(salt water is more dense than fresh water)
Review
1. How does buoyancy affect the apparent weight of an object in a fluid?
Review
2. What determines if an object will float or sink in a fluid?
Review
3. How does Archimedes’ principle relate the buoyant force acting on an object and the volume of fluid displaced by the object?
Review
4. How is the density of a floating object related to the density of the fluid it is floating in?
Review
5. An empty oil tanker displaces enough water to support its weight. Why doesn’t the tanker sink when laded with thousands of tons of oil?
Review
6. A small object is able to float at any level when placed in water. What does this observation tell you about the object’s density?
Review
7. A 350 N block of wood is thrown into a lake, where it floats. What is the buoyant force acting on it?
