Students will:
describe how fluids exert pressure
Analyze how atmospheric pressure varies with depth
Give examples of fluids flowing from high to low pressure
Vocabulary:
 Fluid
 Pressure
 Pascal
 Atmospheric Pressure
Fluids
 ALL can flow
 ALL can take the shape of its container
 Fluids include liquid and gases.
 Remember all matter is either: a solid, liquid, or a gas
 Gases are what makes up the air we breath.

For Example: Oxygen is a gas
 Particles move easily past each other
 Solids are NOT a fluid because they can NOT flow, can
NOT take the shape of its container , and their
particles DO NOT move easily past each other
Pressure
 The amount of force exerted on a given area
 The SI Unit for pressure is pascal (symbol, Pa)
 Fluids exert pressure evenly in all directions
 Example:

The air you blow into a bubble exerts pressure evenly in all
directions. So, the bubble expands in all directions.
 Pressure = force
area
REMEMBER THIS??
 A force is a push or a pull exerted on an object in order
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to change the motion of the object.
Pressure is a FORCE
Inertia is NOT a FORCE
Weight is a FORCE
Mass is NOT a FORCE
Momentum is NOT a FORCE
Friction is a FORCE
Gravitational Force is a FORCE
Air Resistance is a FORCE
Acceleration is NOT a FORCE
Atmospheric pressure
 Look at figure 3 on page 182
 Atmospheric pressure changes as you travel through
the atmosphere.
 The further DOWN through the atmosphere you go,
the GREATER the pressure is.
 Pressure varies depending on depth
 Sea level has the greatest atmospheric pressure in the
figure 3
 For example,
 As you go from the top of a mountain to sea level, the
pressure increases.
Water Pressure
 Increases as depth increases
 A diver feels more pressure the deeper he swims because
more water above the diver is being pulled by Earth’s
gravitational force AND the atmospheric pressure presses
down on the water, so the total pressure on the diver
includes water pressure and atmospheric pressure
 Water exerts more pressure than air, because water is more
dense than air.
 Remember:


Density is the amount of matter in a given volume.
Density=Mass/Volume
D=m/v
 Example:

A diver 10 m underwater would feel twice as much pressure than if
he was just standing on the beach’s surface.
Pressure Difference and Fluid Flow
 ***Fluids flow from areas of HIGH pressure to LOW
pressure***
 Example:
When a fluid flows from “Area A” to “Area B”, that means “Area
A” has a HIGHER PRESSURE than “Area B”
 With tornadoes, the air pressure outside of
the tornado is higher than the pressure
inside the tornado. This pressure difference
causes air to enter into the tornado.
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Now Do:
 Chapter 7 Section 1 Review in your notebook
 Do numbers 3-8 in your SNB
 No, you do not have to write the questions.
Answers to Chapter 7 Section 1
 3. B- Fluids include liquids and gases
 4. Particles in the fluid collide with the side of the
container. The force of the collisions creates
pressure on the container.
 5. You aren’t crushed by atmospheric pressure
because the fluids inside your body exert pressure
that works against atmospheric pressure.
 6. Atmospheric pressure increases as depth
increases because at lower levels of the
atmosphere, there is more air above that is being
pulled down by gravitational force
Answers Continued
 7. Examples of fluids flowing from high pressure
to low pressure are drinking through a straw,
breathing, and squeezing toothpaste from a tube.
 8. Pressure = force/area
Pressure= 2.4 N/0.012 m2
Pressure = 200 Pa
Chapter 7 Section 2
Buoyant Force
 Students will:
 Explain the relationship between fluid pressure and
buoyant force
 Predict whether an object will float or sink in a fluid
 Analyze the role of density in an object’s ability to float
 Explain how the overall density of an object can be
changed
Vocabulary
Buoyant force
Archimedes’ principle
Buoyant Force
 Is the upward force that fluids exert on all matter.
 A liquid exerts a buoyant force on an object that
increases as the density of the fluid increases.
 In a fluid, buoyant forces exists because the pressure is
greater at the BOTTOM of an object than the pressure
at the top
 Look at figure 1 on page 186
Archimedes’ principle
 The principle that states that the buoyant force on an
object in a fluid is an upward force equal to the weight
of the volume of fluid that the object displaces.
 ONLY the weight of the displaced fluid determines the
buoyant force on an object.
 The weight of the object DOES NOT affect buoyant
force.
Weight vs. Buoyant force
 Refer to figure 2 page 187
 If the weight of the water an object displaces is equal
to the weight of the object, the object FLOATS
 Example: A fish is suspended in the water
 If the weight of the water an object displaces is less
than the weight of the object, the object SINKS
 Example: a rock sinks
 If the weight of the water an object displaces is more
than to the weight of the object, the object BUOYED
UP
 Example: a duck would be buoyed up after a dive
 Buoyed up means “pushed up in water”
Floating, Sinking, and Density
 A rock has more mass per volume than water has.
 Mass per unit of volume is density
 The rock sinks because it is more dense than the water
is.
 The duck floats because it is less dense than the water
is.
 Most substances don’t float in air because most
substances are denser than air.
 There are only a FEW substances that are LESS dense
than air.
 Example: Helium is 7 times less dense than air, thus
helium is used in balloons to make them “float in air”
Changing overall density
 Changing Shape
 Ships float because of their shape- see fig.5, pg.189
 Shaping the steel into a hollow form increases the
volume occupied be the same mass. The overall density
of the ship is reduced.
 Changing Mass
 Ballast tanks are devices used by submarines to control
density.
 Changing Volume
 Most fishes have an organ called a swim bladder that
allows them to adjust their overall density
Chapter 7 Section 3
Fluids and Motion
 Students will:
 Describe the relationship between pressure
and fluid speed.
 Analyze the roles of lift, thrust, and wing
size in flight.
 Explain Pascal’s principle.
 Describe drag, and explain how it affects lift.
Vocabulary
 Bernoulli’s Principle
 Lift
 Thrust
 Drag
 Pascal’s Principle
Fluid Speed and Pressure
Bernoulli’s Principle
States that the pressure in a
fluid decreases as the fluid’s
velocity increases
So, the faster the fluid’s speed
is, the lower the pressure.
Factors that Affect Flight
 According to Bernoulli’s principle, the fast-moving air
above the wing exerts less pressure than the slow-moving
air below the wing.
 The greater pressure below the wing exerts an upward force
(LIFT: the upward force on an airplane wing from air
flow)
 Wing size, speed, and turbulence affect lift!
 High-performance jets need SMALL wings
 Gliders need large wings
 The forward force produced by a plane’s engine is called
THRUST.
 THRUST INCREASE LIFT!
 Jets have a lot of thrust, but gliders have none
Drag and Motion in Fluids
 DRAG is the force that opposes or
restricts motion in a fluid
 TURBULENCE is an irregular or
unpredictable flow of fluids
 Airplanes reduce drag by using wing
flaps.
 Birds reduce drag by adjusting their
wing feathers.
Pascal’s Principle
 According to Pascal, changes in water
pressure will be transmitted equally through
an enclosed fluid.
 Pascal’s Principle is used by hydraulic
devices to move or lift objects
 Hydraulic devices can multiply forces
 When breaks are used to stop a car,
Pascal’s Principle is in effect.