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Forces in Fluids

Chapter 4

Skills Warmup
page 85

What happens when you open your mouth a little?


What fluid forced your cheeks outward?


Air escapes from your mouth because the air pressure that was keeping your cheeks puffed is greater than the air pressure outside your mouth. Air is the fluid that forced your cheeks outward.


Pressure = a force exerted on a surface.

Pressure = force / area

N / m2 = N divided by m2

Official SI unit of pressure is pascal ( Pa )

Look at Figure 4.1 on page 85

What happens when you put too much air in the tire?


Too much air pressure in the tire could cause it to burst

Fluid Pressure in The Environment

Eras popping

Ear drums detect small pressure changes.

Weight of the air causes atmospheric pressure.

Figure 4.2 on page 86

Air pressure change


Sea level

Ozone layer

Edge of space

Vacuum = absence of matter


Home Economics Connection

When the surrounding air pressure is less than the air pressure at sea level, water boils at a temperature lower than 100 o C.

1. How does the boiling point of water change with altitude?


The boiling point is lower at high altitudes.

2. How does the time needed to boil a potato change with altitude?


The potato will take longer at high altitudes because the water temperatures will be less than 100 o C.

Math Connection

Barometric pressure is measured in units called millibars ( mb ).

On a mercury barometer, 1 mb = 7.50 x 10 –2 of mercury also, 1 mb = 100 Pa.

Weather reporter give barometric pressure in inches of Hg.

1. How many mb is in 30 inches of Hg?

Answer: 1013 mb

2. How many Pa?

Answer: 101, 600 Pa, or 101.6 kPa

3. What is today’s barometric pressure in mb? In Pa?

Go home and find the answer for # 3 question.

Pressure and the Ocean

Figure 4.3, page 88 – 89

Intertidal Zone

Continental Shelf



Questions and Answers

1. How do you think the pressure will change as you go to the bottom?

Answer: The pressure will increase.

2. Which area of the ocean is home to organisms that withstand pressure changes of as

Answer: Intertidal

3. How deep is the continental shelf?

Answer: 200 m

4. What is the pressure at the ocean bottom?

Answer: 2 million Pa

5. What do you think happens to tissues of the fish?

Answer: When the external pressure is less on tissues, the air pockets expand and burst, killing the fish.

Science and Technology
Putting on the Brakes "
page 90

Follow the path of force through the brake system. What happens if there is a break in the fluid line?


The brakes would fail. The lack of fluid pressure would prevent the fluid in the cylinders from pushing against the pistons, and the brake pads would not push against the wheel.



Buoyant Force

Opposes gravity

Object sinks because?

Answer: weight is greater the buoyant force.

1. Weight is equal to buoyant force, what happens to the object?

Answer: remains at any level like a fish does.

2. Is the weight of the iceberg greater, the same, or less than its buoyant force?

Answer: Less

3. Why is most of the iceberg’s mass under water?

Answer: The mass of the submerged part of the iceberg has a weight equal to the buoyant force.

Buoyant Force continued

The strength of the buoyant force depends upon the object’s displaced volume.

Look at figure 4.6 and study them.

1. The volume of each of these cubes is the same. Why is each cube at a different level in the water?


The downward force of gravity is different for each cube, because each cube has a different density.

Archimedes’ Principle

2000 years ago

Observed a relationship between buoyant force and fluid displaced by an object.

Bathtub water rose

There’s an alligator at the bottom of a pond.

What is the alligator’s weight on the bottom?

Let’s say 200 N.

It displaces 100 N of water.

1. Does the alligator weighs less in the water than it does on land?



2. What is the alligator’s weight out of the water?


200 N + 100 N = 300 N

Density and Buoyancy

To predict whether an object will sink or float, you need to consider its density.

Figure 4.7, page 93

1. Which substance is the least dense?

Answer: balsa wood

Which substance is the most dense?

Answer: steel

Making Steel Float

Figure 4.9, page 95

Why is the ship on the right higher in the water than the ship on the left?


The ship on the right has a high volume of trapped air in its hull, so it has a lower density than the water. The ship on the left has a loaded hull, so it is more dense than the ship on the right. However, the ship on the left is less dense than water, so it floats.


Forces in Moving Fluids

Pressure Differences in Moving Fluids

Figure 4.10 on page 97 – Skills Warmup.

Discuss what you have read.

Blowing across the paper creates a low pressure area above it. Why does the paper rise upward?


The blowing creates a pressure imbalance.

Using Pressure Differences for Flight

Airfoil – figure 4.11, page 98.

How an airplane flies:





Questions and Answers

1. Describe the air pressure above and below an airplane’s wings.

2. What force on an airplane opposes the thrust?

3. For an airplane to fly, which force must be greater, the weight or the lift?

4. Would the plane move more slowly or quickly if it were shaped more square?

Math Connection

A plane’s air-speed indicator is affected by air temperature and pressure, which change with altitude. True air speed is computed by adding 2 % to the indicated air speed for every 300 meters of altitude.

Calculate the true air speed of a plane flying at 9 km at 250 km / h.


9 km / 300 m = 30

2 % x 30 = 60 %

0.60 x 250 km / h = 150 km / h

250 km / h + 150 km / h = 400 km / h


Jet stream - a band of fast-moving air

Important effects on an airplane’s speed and handling.

Airplane pilots need information about the elevation and latitude of the jet stream in order to set and correct their course.

Find Out: How do jet streams cause turbulence?

Research Achievements
( writing connection )

Orville and Wilbur Wright

Richard E. Byrd

Charles Lindberg

Amelia Earhart

Richard Rutan

Jeana Yeager

Bird Flight

Figure 4.12 page 100

1. How is the way a bird flies similar to the way a plane flies? How is it different?

Hawks, figure 4.13, page 100

2. How does soaring help hawks and other soaring birds catch their prey?

Hummingbirds, figure 4.14, page 101

Rapid wing movement. Why?

Gulls, figure 4.15, page 101

3. How does their flying style make this possible?

Kite Flying

China - 2,500 years ago

Airfoil-shaped wings

Egypt – 2,200 years ago

Working gliders found

Central America, Mayas

built giant, round kites for festivals

Kite flying still popular around the world

Kite construction

How do kites stay aloft?

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