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Forces in Fluids
Chapter 13.1 Fluid Pressure
Pressure
What is pressure? It is the amount of force
per unit of area. What is the formula? P = F ÷ A What is the unit for
Pressure? Pascal Chart on the Board
Chapter 13.1 Fluid Pressure
Pressure in Fluids
What is a fluid?
It is a substance that assumes the shape of its container.
What are two examples of fluids?
Liquids and gases
Water Pressure
What happens to the pressure exerted on your body as you swim downward in a pool?
Water pressure increases as depth increases.
The pressure in a fluid at any given depth is constant, and it is exerted equally in all directions.
Chapter 13.1 Fluids Pressure
Atmosphere
Air is a mixture of gases that make up Earth’s atmosphere.
The weight of Earth’s atmosphere exerts a pressure of about 101 kPa at sea level.
Air pressure
What is the relationship between altitude and air pressure?
As altitude increases, air pressure decreases.
1. Venus, 9120 kPa
2. Nitrogen
3. Mercury, 10-15 bar; Earth, 1 bar; Venus, 90.0 bar; Mars, 0.0070 bar
4. Pressure = Force/area, Force = 9120 kPa x 2 m2 = 9.12 x 106 Pa x 2 m2 = 1.816 x 107 N
5. The helium-filled balloon would have the smallest volume on Venus because Venus has the greatest atmospheric pressure and the helium inside the balloon is compressible.
Data Analysis (Page 392)
1. Force and area are needed to calculate pressure.
2. Water pressure increases as depth increases.
3. At a given depth, pressure is constant and exerted equally in all directions.
4. Atmospheric pressure decreases as altitude above sea level increases.
5. You cannot feel atmospheric pressure acting on your body because the pressure inside your body balances the pressure outside.
Section 13.1 Assessment
6. 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 to rupture or explode.
7. The 500-N student exerts greater pressure on area A than a 750-N student does on area 2A. (500 N/A > 750/2A)
Section 13.1 Assessment
8. The pressure exerted when standing on one stilt is
twice the pressure exerted when on two stilts.
9. Pressure = Force/Area, Pressure = 12 N/(.21 x .28 m) = 12 N/.0588 m2 = 2.0 102 N/m2 = 2.0 x 102 Pa
Section 13.1 Assessment
Chapter 13.2 Forces and Pressure in Fluids
Transmitting Pressure in a Fluid
What is Pascal’s Principle?
A change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid.
Chapter 13.2 Forces and Pressure in Fluids
Hydraulic Systems
What is a hydraulic system?
It is a device that uses pressurized fluid acting on pistons of different sized to change a force.
Chapter 13.2 Forces and Pressure in Fluids
Hydraulic Systems
In a hydraulic lift system, an increased output force is produced because a constant fluid pressure is exerted on the larger area of the output piston
Dump Truck
Chapter 13.2 Forces and Pressure in Fluids
Pascal’s Principle
What is the formula?
F1 ÷ A1 = F2 ÷ A2
Chart on the Board
Worksheet
Chapter 13.2 Forces and Pressure in Fluids
Bernoulli’s Principle
Daniel Bernoulli Swiss Scientist, (1700-1782)
What is Bernoulli’s principle?
As the speed of a fluid increases, the pressure within the fluid decreases
Pass out Worksheet
Chapter 13.2 Forces and Pressure in Fluids
Chapter 13.2 Forces and Pressure in Fluids
Wings and Lift
The ability of birds and airplanes to fly is largely explained by Bernoulli’s principle.
The air traveling over the top of an airplane wing moves faster than the air passing underneath.
This creates a low-pressure area above the wing.
The pressure difference between the top and the bottom of the wing creates an upward force known as lift.
Chapter 13.2 Forces and Pressure in Fluids
Chapter 13.2 Forces and Pressure in Fluids
What are the four forces on an airplane?
Chapter 13.2 Forces and Pressure in Fluids
Chapter 13.2 Forces and Pressure in Fluids
Spray Bottles
As water streams through the sprayer, it passes over the top of a small tube that reaches down into the solution chamber. The pressure difference between the solution chamber and the tube forces the concentrated solution up the tube.
Venturi Tube
Chapter 13.2 Forces and Pressure in Fluids
1. A change in pressure at any point in a fluid is
transmitted equally and unchanged in all directions throughout the fluid.
2. The increased output force is produced because the fluid pressure is exerted on the larger area of the output piston.
3. As the speed of a fluid increases, the pressure within the fluid decreases.
4. Lift of a wing is largely explained by Bernoulli’s principle. Air traveling faster over the top of the wing produces an area of reduced pressure. The resulting pressure difference between the top and bottom of the wing produces lift.
Section 13.2 Assessment
5. The water pressure increases with increasing
depth in the pool, causing the air-filled balloon to be compressed.
6. Area of output piston/area of input piston = 25; Input force = output force/25 = 50N/25 = 2N; Output force = input force x 25; Output force = 40N x 25 = 1000 N
Section 13.2 Assessment
7. The moving air between the balls has a lower
pressure than the surrounding, nonmoving air. The pressure difference forces the balls together.
8. Each moving car pulls a layer of air with it as it moves. When the cars pass, these two areas of moving, lower-pressure air meet, producing a pressure difference, which forces the cars toward each other.
Section 13.2 Assessment
Chapter 13.3 Buoyancy
Buoyant Force
What is Buoyancy?
It is the ability of a fluid to exert an upward force on an object placed in it.
Buoyancy results in the apparent loss of weight of an object in a fluid
This upward force which acts in the opposite direction of gravity, is called a buoyant force.
Chapter 13.3 Buoyancy
Chapter 13.3 Buoyancy
Archimedes' Principle
Who is Archimedes? He was an ancient Greek mathematician who died in 212 B.C., is credited with an important discovery that bears his name.
Chapter 13.3 Buoyancy
Archimedes’ Principle
What is Archimedes' principle?
The buoyant force on an object is equal to the weight of the fluid displaced by the object.
Chapter 13.3 Buoyancy
Density and Buoyancy
Density and Buoyancy are closely related.
Densities are often expressed in the non-SI units of g/cc.
If an object is less dense than the fluid it is in, it will float.
If the object is more dense than the fluid it is in, it will sink.
Why does oil float on water?
Because oil is less dense than water.
Chapter 13.3 Buoyancy
Chapter 13.3 Buoyancy
Density and Buoyancy
When the buoyant force is equal to the weight, an object floats or is suspended. When the buoyant force is less than the weight, the object sinks.
Chapter 13.3 Buoyancy
Suspended
An object that has the same density as the fluid it is submerged in will be suspended (it will float at any level) in the fluid.
Chapter 13.3 Buoyancy
Sinking
An object that has a greater density than the fluid will sink.
Chapter 13.3 Buoyancy
Floating
An object that has less density than the fluid it is in will float.
Chapter 13.3 Buoyancy
Huge Steel Ships
What is the relationship between a ship’s shape and its density?
As the ship’s shape increases its volume, its density decreases
Chapter 13.3 Buoyancy
Balloons Why do some balloons float in air
whereas others do not? Helium and hot air are both less
dense than normal-temperature air.
When a balloon is filled with either one, a buoyancy force from the displaced normal temperature air acts on the balloon.
If the size of the buoyant force is large enough, the balloon rises into the air
1. Buoyancy results in the apparent loss of weight of
an object in a fluid.
2. Floating: If an object is less dense than the fluid it is in; it will float. If the buoyant force is equal to the weight of the object, the object will float or be suspended. Sinking: If an object is more dense than the fluid it is in, it will sink. If the buoyant force is less than the weight of the object, the object will sink.
13.3 Assessment
3. The buoyancy force acting on an object is equal to
the weight of fluid displaced by the object.
4. Floating objects have a density that is less than that of the fluid in which they are floating.
5. As the tanker is loaded with oil, its increased weight causes it to displace more water. This increased water displacement results in a greater buoyancy force, which supports the loaded tanker.
13.3 Assessment
6. The density of the object must be equal to the
density of the water.
7. 350 N
13.3 Assessment
Chapter 13 Assessment, pages 407 and 408
Chapter 13 Standardized Test Prep
Chapter 13 General Review
Chapter 13 Test is coming
Chapter 13 Forces in Fluids
