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Table of Contents

- Pressure
- Floating and Sinking
- Pascal’s Principle
- Bernoulli’s Principle

What Is Pressure?

- Pressure decreases as the area over which a force is distributed increases.

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

AreaPractice 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

AreaFluid Pressure

- All of the forces exerted by the individual particles in a fluid combine to make up the pressure exerted by the fluid.

Previewing Visuals

- Before 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.

Buoyancy

- Water and other fluids exert an upward force called the buoyant force on any submerged object
- The pressure on the bottom of a submerged object is greater than the pressure on the top
- This is due to increasing pressure with increasing depth in a fluid (water)
- The result is a net force in the upward direction or opposite gravity

Buoyancy

- If an object’s weight is greater than the buoyant force acting on it then it will sink!
- An object will float if the buoyant force acting on the object is greater than the weight of the object
- A submerged object whose weight is equal to the buoyant force has no net force acting on it and will not sink either!

Archimedes’ Principle

- Archimedes’ principle states that the buoyant force acting on a submerged object is equal to the weight of the fluid the object displaces.

Buoyancy

- Displacement refers to the movement of an object from its original location.
- In this case we are referring to the displacement of fluid!

So how does a cruise ship like the Queen of the Seas stay afloat?

- Remember that the buoyant force equals the weight of the displace fluid for an object
- Larger size objects have a greater buoyant force acting on them than smaller objects do because they displace more fluid (even if they are the same weight)
- A ship floats on the surface as long as the buoyant force acting on it is equal to its weight!

Buoyancy

- A solid block of steel sinks when placed in water. A steel ship with the same weight floats.

Relating Cause and Effect

- As you read, identify the reasons why an object sinks. Write them down in a graphic organizer like the one below.

Causes

Weight is greater than buoyant force.

Effect

Object is denser than fluid.

Object sinks.

Object takes on mass and becomes denser than fluid.

Object is compressed and becomes denser than fluid.

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.

Hydraulic Devices

- In a hydraulic device, a force applied to one piston increases the fluid pressure equally throughout the fluid.

Hydraulic Devices

- By changing the size of the pistons, the force can be multiplied.

Hydraulic Systems Activity

- Click the Active Art button to open a browser window and access Active Art about hydraulic systems.

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.

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

Comparing Hydraulic Lifts3,000 N

Reading Graphs:

For Lift A, how much force must be applied to lift a 12,000-N object?

- Pascal’s Principle

Comparing Hydraulic LiftsLift 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

Comparing Hydraulic LiftsThe 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

Comparing Hydraulic LiftsLift 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

Comparing Hydraulic LiftsHydraulic Brakes

- The hydraulic brake system of a car multiplies the force exerted on the brake pedal.

Asking Questions

- Before 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.

Question

Answer

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.

Bernoulli’s Principle

- Bernoulli’s principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases.

Applying Bernoulli’s Principle

- Bernoulli’s principle helps explain how planes fly.

Applying Bernoulli’s Principle

- An atomizer is an application of 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.

Applying Bernoulli’s Principle

- Like an airplane wing, a flying disk uses a curved upper surface to create lift.

Identifying Main Ideas

- As 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.

Main Idea

Bernoulli’s principle is a factor that helps explain…

Detail

Detail

Detail

how airplanes fly

why smoke rises up a chimney

how an atomizer works

Links on Bernoulli’s Principle

- Click the SciLinks button for links on Bernoulli’s principle.

Graphic Organizer

How a Hydraulic Device Works

Force is applied to a small piston.

Pressure in a confined fluid is increased.

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.

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