 # Fluid Pressure Chapter 13.1.

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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/area Pa = 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 container Area of the bottom of a container or its surface Amount 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 What must you know to calculate pressure? Force and Area

Review What is the relationship between the depth of water and the pressure it exerts? Water pressure increases as depth increases.

Review 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 How does the pressure exerted by the atmosphere change as altitude increases? Atmospheric pressure decreases as altitude above sea level increases.

Review 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.

Review 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 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 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 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 Fluid Pascal’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 Systems Describe 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 pressure High 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 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 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 How are fluid speed and fluid pressure related?
As the speed of a fluid increases, the pressure within the fluid decreases.

Review 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 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 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.

Review 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 causes a buoyant 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 float Objects 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 float Objects 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 How does buoyancy affect the apparent weight of an object in a fluid?

Review What determines if an object will float or sink in a fluid?

Review How does Archimedes’ principle relate the buoyant force acting on an object and the volume of fluid displaced by the object?

Review How is the density of a floating object related to the density of the fluid it is floating in?

Review 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 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 A 350 N block of wood is thrown into a lake, where it floats. What is the buoyant force acting on it?