# Chapter 7 Section 1 Fluids and Pressure

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Chapter 7 Section 1 Fluids and Pressure
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 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.

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.