Forces in Fluids Pressure Floating and Sinking Pascal’s Principle Bernoulli’s Principle Table of Contents
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Forces in Fluids
Pressure
Floating and Sinking
Pascal’s Principle
Bernoulli’s Principle
Table of Contents
Forces in Fluids - Pressure
What Is Pressure?
Pressure decreases as the area over which a force is distributed increases.
Forces in Fluids
Area The area of a surface is the number of square units that it covers. To find the area of a rectangle, multiply its length by its width. The area of the rectangle below is 2 cm X 3 cm, or 6 cm2.
- Pressure
Forces in Fluids
Area
Practice Problem
Which has a greater area: a rectangle that is 5 cm X 20 cm or a square that is 10 cm X 10 cm?
Both have the same area, 100 cm2.
5 cm X 20 cm = 100 cm2
10 cm X 10 cm = 100 cm2
- Pressure
Forces in Fluids - Pressure
Fluid Pressure
All of the forces exerted by the individual particles in a fluid combine to make up the pressure exerted by the fluid.
Forces in Fluids - Pressure
Variations in Fluid Pressure
As your elevation increases, atmospheric pressure decreases.
Forces in Fluids - Pressure
Variations in Fluid Pressure
Water pressure increases as depth increases.
Forces in Fluids
Previewing VisualsBefore you read, preview Figure 5. Then write two questions that you have about the diagram in a graphic organizer like the one below. As you read, answer your questions.
Pressure Variations
Q. Why does the pressure change with elevation and depth?
A. Air and water exert pressure, so pressure varies depending on how much air or water is above you.
Q. How much greater is water pressure at a depth of 6,500 m than it is at sea level?
A. It is about 650 times greater.
- Pressure
Forces in Fluids
Links on Fluids and Pressure
Click the SciLinks button for links on fluids and pressure.
- Pressure
Forces in Fluids
End of Section:Pressure
Forces in Fluids - Floating and Sinking
Buoyancy
The pressure on the bottom of a submerged object is greater than the pressure on the top. The result is a net force in the upward direction.
Forces in Fluids - Floating and Sinking
Buoyancy
The buoyant force acts in the direction opposite to the force of gravity, so it makes an object feel lighter.
Forces in Fluids - Floating and Sinking
Buoyancy
Achimedes’ principle states that the buoyant force acting on a submerged object is equal to the weight of the fluid the object displaces.
Forces in Fluids - Floating and Sinking
Buoyancy
A solid block of steel sinks when placed in water. A steel ship with the same weight floats.
Forces in Fluids - Floating and Sinking
Density
Changes in density cause a submarine to dive, rise, or float.
Forces in Fluids
Relating Cause and EffectAs you read, identify the reasons why an object sinks. Write them down in a graphic organizer like the one below.
Object sinks.
Weight is greater than buoyant force.
Object is denser than fluid.
Object takes on mass and becomes denser than fluid.
Object is compressed and becomes denser than fluid.
- Floating and Sinking
Effect
Causes
Forces in Fluids
Density
Click the Video button to watch a movie about density.
- Floating and Sinking
Forces in Fluids
End of Section:Floating and
Sinking
Forces in Fluids - Pascal’s Principle
Transmitting Pressure in a Fluid
When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid.
Forces in Fluids - Pascal’s Principle
Hydraulic Devices
In a hydraulic device, a force applied to one piston increases the fluid pressure equally throughout the fluid.
Forces in Fluids - Pascal’s Principle
Hydraulic Devices
By changing the size of the pistons, the force can be multiplied.
Forces in Fluids
Hydraulic Systems Activity
Click the Active Art button to open a browser window and access Active Art about hydraulic systems.
- Pascal’s Principle
Forces in Fluids
Comparing Hydraulic Lifts
In the hydraulic device in Figure 15, a force applied to the piston on the left produces a lifting force in the piston on the right. The graph shows the relationship between the applied force and the lifting force for two hydraulic lifts.
- Pascal’s Principle
Forces in Fluids
Comparing Hydraulic Lifts
Lift A: 4,000 N; lift B: 2,000 N
Reading Graphs:
Suppose a force of 1,000 N is applied to both lifts. Use the graph to determine the lifting force of each lift.
- Pascal’s Principle
Forces in Fluids
Comparing Hydraulic Lifts
3,000 N
Reading Graphs:
For Lift A, how much force must be applied to lift a 12,000-N object?
- Pascal’s Principle
Forces in Fluids
Comparing Hydraulic Lifts
Lift A: applied force is multiplied by four; lift B: applied force is multiplied by two.
Interpreting Data:
By how much is the applied force multiplied for each lift?
- Pascal’s Principle
Forces in Fluids
Comparing Hydraulic Lifts
The slope gives the ratio of the lifting force to the applied force. The greater the slope, the more the lift multiplies force.
Interpreting Data:
What can you learn from the slope of the line for each lift?
- Pascal’s Principle
Forces in Fluids
Comparing Hydraulic Lifts
Lift A, because it multiplies force more than lift B.
Drawing Conclusions:
Which lift would you choose if you wanted to produce the greater lifting force?
- Pascal’s Principle
Forces in Fluids - Pascal’s Principle
Hydraulic Brakes
The hydraulic brake system of a car multiplies the force exerted on the brake pedal.
Forces in Fluids
Asking QuestionsBefore you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions.
How is pressure transmitted in a fluid?
Pressure is transmitted equally to all parts of the fluid.
What is a hydraulic system?
A hydraulic system uses a confined fluid to transmit pressure.
Question Answer
- Pascal’s Principle
Forces in Fluids
End of Section:Pascal’s Principle
Forces in Fluids - Bernoulli’s Principle
Bernoulli’s Principle
Bernoulli’s principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases.
Forces in Fluids - Bernoulli’s Principle
Applying Bernoulli’s Principle
Bernoulli’s principle helps explain how planes fly.
Forces in Fluids - Bernoulli’s Principle
Applying Bernoulli’s Principle
An atomizer is an application of Bernoulli’s principle.
Forces in Fluids - Bernoulli’s Principle
Applying Bernoulli’s Principle
Thanks in part to Bernoulli's principle, you can enjoy an evening by a warm fireplace without the room filling up with smoke.
Forces in Fluids - Bernoulli’s Principle
Applying Bernoulli’s Principle
Like an airplane wing, a flying disk uses a curved upper surface to create lift.
Forces in Fluids
Main Idea
Detail Detail Detail
Identifying Main IdeasAs you read the section “Applying Bernoulli’s Principle,” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.
Bernoulli’s principle is a factor that helps explain…
how airplanes fly
why smoke rises up a chimney
how an atomizer works
- Bernoulli’s Principle
Forces in Fluids
Links on Bernoulli’s Principle
Click the SciLinks button for links on Bernoulli’s principle.
- Bernoulli’s Principle
Forces in Fluids
End of Section:Bernoulli’s Principle
Forces in Fluids
Graphic Organizer
How a Hydraulic Device Works
Pressure in a confined fluid is increased.
Force is applied to a small piston.
The pressure is transmitted equally throughout the fluid.
The confined fluid presses on a piston with a larger surface area.
The original force is multiplied.
Forces in Fluids
End of Section:Graphic Organizer
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