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CHAPTER 4 Forces in Fluids

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**4.1 Fluid Pressure Objectives**

Define pressure and explain how it is measured. Identify examples of fluid pressure. Predict the effects of fluid pressure in land and water environments.

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**Fluid Name as many fluids as you can think of in 60 seconds…**

Air is a fluid. A fluid is any substance that can flow. People often think of fluids as liquids, such as water or oil. But a gas, like air, is also a fluid.

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**Pressure If you squeeze a bicycle tire, you feel an opposing force.**

The force comes from billions of air particles striking the inside of the tire producing pressure. What happens if you put too much air in the tire? Too much air pressure in the tire will cause it to burst.

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Pressure Any force (including any fluid force) exerted on a surface is called pressure. To calculate pressure, you divide the force by the area over which it is applied.

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**Pressure Remember force is measured in**

Write the equation: pressure = force area The official SI unit of pressure, the pascal (Pa), is equal to Newtons per square meter or 1 N/m2 NEWTONS ?????

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Pressure What is the pressure exerted by a 10 kg backpack put onto a rectangular table? The table is 0.5 m2. F=ma m=10kg a=9.8m/s2 F=100N P= force P=(100) area (0.5) P= 50 N/m2 or 50 Pa Pressure is a measure of force per unit of area, so you first need to find the force that is acting on the table. The force acting on the table is the weight of the backpack, (F=ma) which is equal to its mass times the acceleration of gravity.

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**Fluid Pressure in the Environment**

Air and water are the most common fluids on the earth's surface. As you move up or down in air or water, the pressure around you changes. The change may be too small for you to notice. However, your eardrums sometimes detect small pressure changes. Do you ever feel your ears "pop" when you ride in a plane or elevator? Or feel a sharp ear pain when you swim underwater? These experiences result from fluid pressure in the environment.

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**Pressure and the Atmosphere**

Gas (air) particles are pulled toward the earth by the force of gravity. The weight of air causes atmospheric pressure. Over the earth's entire surface, the average pressure produced by the atmosphere is about 100,000 Pa. Air exerts a downward force on the earth's surface. We live at the bottom of an ocean of air – the atmosphere, but you can’t feel it because the fluids in your body push back with the same force – the forces on your body are balanced.

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**Atmospheric Pressure at Different Places**

AIR PRESSURE CHANGE Altitude is the most important factor in surface air pressure. Mountains As you climb a mountain, air pressure steadily decreases. Moving air currents and storms also affect surface air pressure. Changes in surface pressure help people to predict weather. Sea Level The average surface pressure measured at sea level is 101, 300 Pa.

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**Atmospheric Pressure at Different Places**

Edge of Space At an altitude of 150 km, air contains so few gas particles that the pressure is almost ZERO. OZONE LAYER The air pressure in the ozone layer is less than 100 Pa. The absence of all particles is called a vacuum. Space is near a vacuum. AIRCRAFT Jets travel at an altitude of about 11km. The air pressure there is only about 25,000 Pa.

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Pressure and the Ocean The particles that make up water are packed more closely together than the particles in air. Water is denser than air. So, water exerts more force per unit area than air does.

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**Water Pressure Beneath the Ocean**

Intertidal Zone Continental Shelf Near shore, the water level changes with the tides. In one day, the pressure can vary as much as 200, 000 Pa. The shelf is up to 200 m deep. Organisms living on the deepest part must withstand water pressure of 2 million Pa.

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**Water Pressure Beneath the Ocean**

Mid-Ocean At depths of up to 1 km, organisms can withstand a pressure of 10 million Pa. Trench The trenches are the deepest parts of the sea. Pressures can exceed 100 million Pa. If you continue to descend in the ocean, the water pressure builds rapidly. The total pressure is equal to the air and water pressure combined.

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Pressure and the Ocean .

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**DO NOW Page 90 – Read Answer:**

Follow the path of pressure 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.

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**Check & Explain pg. 90 1. What is fluid pressure?**

2. Give two examples of fluid pressure. 4. Predict What changes in both air and water pressure occur on your body as you leap from a diving board?

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4.2 Buoyancy

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**4.2 Buoyancy Objectives Explain buoyant force.**

Compare and contrast density and buoyancy. Predict whether an object will float in water.

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Unbalanced Forces REMEMBER Charles Law? As temperature rises, volume rises, too (less dense). The particles strike the container with greater force, if the container is flexible, the volume increases. The less dense the air, the less pressure. The air below the balloon in exerted a greater pressure on the balloon than the air above, causing the forces to be unbalanced – the balloon rises! Why doesn't the balloon keep rising? The forces above and below the balloon eventually become equal.

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**Two forces act on an iceberg.**

Buoyant Force Two forces act on an iceberg. The downward force is gravity. The upward force acting on the iceberg is the buoyant force. The buoyant force opposes gravity. What force makes an object float in air or in water?

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Buoyant Force If the weight of a submerged object is greater than the buoyant force, the object will sink. If the weight is less than the buoyant force, the object will rise to the surface and float. If the weight is equal to the buoyant force, the submerged object will remain at any level. The forces are balanced!! Is the weight of the iceberg greater, the same, or less than its buoyant force?

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Buoyant Force To understand buoyant force, think about what happens to the water level in a glass when you add an ice cube… The ice displaces, some of the water. The weight of the water that is displaced is equal to the buoyant force. The greater the volume displaced, the greater the buoyant force !

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**DO NOW The volume of each of these three cubes is the same.**

2 1 3 The volume of each of these three cubes is the same. Why is each cube at a different level in the water? 1 - the buoyant force acting upon the block is greater than the downward force of gravity. These unbalanced forces result in an upward force that causes the block to float. 2 - the block's weight is greater than the buoyant force. A downward force results, and the block sinks. 3 - the downward force of gravity equals the upward buoyant force, no movement occurs. The forces balance each other.

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**ARCHIMEDE‘S PRINCIPLE**

More than 2,000 years ago, the Greek mathematician and inventor Archimedes observed a relationship between the buoyant force and the fluid displaced by an object. According to legend, when he stepped into his bathtub, he noticed that the water level rose. He later reasoned that the weight of the fluid displaced would be equal to the buoyant force. On the other hand, an object with a density less than 1 g/cm3 will float in water.

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**ARCHIMEDE‘S PRINCIPLE**

For example, if an object immersed in water displaces a volume of water that weighs 4.9 N, then a buoyant force of 4.9 N acts on the object. Archimedes' principle applies to all fluids, including air.

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**The chest sinks because its density is greater than that of water.**

Density and Buoyancy To predict whether an object will sink or float, you need to consider its density. DENSITY = MASS / VOLUME The density of water is 1 g/cm3. Any object with a density greater than 1 g/cm3 will sink in water. Higher density means the weight of water displaced is less than the weight of the object so buoyancy force will not be able to make the object float and it will sink , so less buoyancy for heavier objects. lower density means the weight of water displaced is more than the weight of object so buoyancy force will be able to make the object float , so more buoyancy for lighter objects. The chest sinks because its density is greater than that of water.

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Density and Buoyancy

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Check & Explain pg. 95 Describe the forces acting on a floating object. Buoyant force (upward force) and gravity (downward force). 2. What is Archimedes' principle? The weight of the fluid displaced by an object is equal to the buoyant force acting on it 3. Compare and Contrast: How are density and buoyancy similar? How are they different. Density is a property of matter. Buoyancy is a force that acts on matter in a fluid to overcome gravity. Buoyancy depends on the relative densities of both the object and the fluid. 1. Buoyant force (upward force) and gravity (downward force). 2. The weight of the fluid displaced by an object is equal to the buoyant force acting on it. Density is a property of matter. Buoyancy is a force that acts on matter in a fluid to overcome gravity. Buoyancy depends on the relative densities of both the object and the fluid.

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**4.3 Forces in Moving Fluids**

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**4.3 Forces in Moving Fluids**

Objectives Describe Bernoulli 's principle. Explain how airplanes and birds fly. Generalize about things that fly. Infer how an object's shape affects its movement through a fluid.

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**Pressure Differences in Moving Fluids**

Air is a fluid. When a fluid moves, its pressure drops. As the student blows across the paper, he creates a low-pressure region above it. The air beneath the paper is not affected by the blowing, therefore, its pressure remains the same. The pressure is greater on the bottom of the paper. The unbalanced pressures produce an upward force on the paper. When the boy stops blowing, the air above the paper returns to its original pressure so it sinks back down.

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Bernoulli's Principle In the 1700s, Swiss scientist Daniel Bernoulli studied the relationship between moving fluids and pressure. He observed that as the speed of a fluid increases, the pressure in the fluid decreases. This concept is known as Bernoulli's Principle

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**Using Pressure Differences for Flight**

The special shape of an airplane wing is called an airfoil. An airfoil has a curved upper surface and a relatively flat-bottom surface. As air moves past an airfoil, the airfoil's shape creates a pressure difference. (say after 2nd bullet) An airfoil's design uses Bernoulli's principle.

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Airplane Lift The difference between the pressure on the airfoil's upper and lower surfaces results in an upward force, called lift. ( say last ) When the lift and the plane's weight are equal, the plane levels off. When the lift is greater than the plane's weight, the plane rises. When the lift is less than the plane's weight, the plane sinks..

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Airplane Thrust The force that pushes the plane forward is called thrust. The shape and movement of the propeller blades produce thrust. As the blades spin, they create a low-pressure region in front of the propeller and a high pressure region behind it, pushing the plane forward.

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**By designing streamlined aircraft, drag is reduced.**

The forward motion of an aircraft is slowed by an opposing force called drag. Like friction, drag opposes the movement of objects. Disturbances in the flow of air or other fluids increase drag. By designing streamlined aircraft, drag is reduced.

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**Bird bones have pockets of air to make them light**

Bird Flight Like the wing of an airplane, a birds wing is an airfoil. Air passing over the birds wing creates lift. A bird flaps its wing producing thrust. Bird Skeleton Bird bones have pockets of air to make them light

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**Co-operative Learning Activity 101**

GROUP 3 GROUP 1 GROUP 2

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Check Your Vocabulary 1. Planes, birds, and boats are streamlined to reduce ________ . 2. The _________ on an object is equal to the weight of the fluid the object displaces. 3. Force per unit area is _________ . 4. The shape of an airplane wing is a(n) ________ . 5. Any substance that can flow is a(n)_________ . 6. The force that moves a plane forward is _________ . 7. As air moves over an airfoil. ___ is generated. DRAG Buoyant force pressure airfoil fluid thrust Lift

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**Check Your Vocabulary Pair each numbered word with a vocabulary term.**

8. Friction Wing DRAG AIRFOIL 10. Float Pascal buoyant force pressure 12. Flow Up fluid LIFT 14. Forward Area thrust pressure 16. Pressure Sink lift buoyant force

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