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Forces in Fluids Chapter 13

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What is pressure? The result of a force acting over a given area.The result of a force acting over a given area. Pressure = Force/AreaPressure = Force/Area What label?What label? N/m 2N/m 2 1 N/m 2 is known as a pascal (Pa)1 N/m 2 is known as a pascal (Pa)

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Blaise Pascal 1623-1662 French physicist and mathematician Performed some of the first experiments dealing with pressure

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13.1 Pressure The force should be in newtons (N).The force should be in newtons (N). The area should be in square meters (m 2 ).The area should be in square meters (m 2 ). The resulting unit would be N/m 2.The resulting unit would be N/m 2. One N/m 2 is a pascal (Pa).One N/m 2 is a pascal (Pa). The SI unit of pressure is the pascal.The SI unit of pressure is the pascal.

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13.1 Fluid Pressure Pressure: is the force per unit area. A seat that reduces pressure will be more comfortable than a chair with higher pressure. How can you reduce pressure on a chair?

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13.1 Pressure To calculate pressure, divide the force by the area over which the force acts.To calculate pressure, divide the force by the area over which the force acts. P = F / A

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Pressure in Liquids Deeper the more pressure.Deeper the more pressure. Weight of water (and air) above pushing against you.Weight of water (and air) above pushing against you. –Twice weight, twice pressure. –Pressure of air above transmitted down through water and adds pressure.

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Density and Depth Liquid pressure = weight density x depth. Pressure does not depend on amount of liquid, just the depth.

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Density and Depth Pressure exerted by a liquid is the same at any given depth below surface no matter what its shape. Exerted equally in all directions.

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Pressure on Dams Liquid Pressure = Weight density x depth

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Pressure Increases with Depth

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Why are the metal bands round the water tower closer together at the bottom?Why are the metal bands round the water tower closer together at the bottom?

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Air Pressure Air pressure at sea level is approx. 101 kPa. Air pressure will decrease with increases in altitude. Examples: flying in a plane, driving in the mountains

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Air Pressure & the Atmosphere Just as water pressure increases with depth the weight of the atmosphere results in air pressure.Just as water pressure increases with depth the weight of the atmosphere results in air pressure. Air pressure decreases as the altitude increases.Air pressure decreases as the altitude increases.

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Air pressure is equal to the weight (per unit area) of the column of air extending above that location to the top of the atmosphere. Air pressure

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"Standard Pressure" 1 atmosphere (at sea-level)1 atmosphere (at sea-level) * 1 atm = 101.3 kPa = 14.7 psi * 1 atm = 101.3 kPa = 14.7 psi kPa is kiloPascals.kPa is kiloPascals.

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Air Pressure & the Atmosphere Your ears pop when you go up a hill because the pressure changes.Your ears pop when you go up a hill because the pressure changes. –Pressure inside our bodies equal surrounding air –Pressure inside a balloon is equal to the pressure of the surrounding air

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13.2 Forces and Pressure in Fluids Pressure is exerted equally in all directions.Pressure is exerted equally in all directions. Pascal’s Principle states that: a change in pressure at any point in a fluid is transmitted equally and unchanged in all directions.Pascal’s Principle states that: a change in pressure at any point in a fluid is transmitted equally and unchanged in all directions.

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Pascal’s Principle

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A change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluidA change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid

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hydraulics Uses Pascal’s principle and moving pistons with an enclosed pressurized fluid:Uses Pascal’s principle and moving pistons with an enclosed pressurized fluid: Examples:Examples: –Car brakes, jacks, and loaders

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Hydraulic Systems Hydraulics is the science of applying Pascal’s principle.Hydraulics is the science of applying Pascal’s principle. Hydraulic systems use pressurized fluid acting on a piston to change the force.Hydraulic systems use pressurized fluid acting on a piston to change the force.

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Hydraulic Systems

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Bernoulli’s Principle When the speed of a fluid increases, pressure in the fluid decreases.When the speed of a fluid increases, pressure in the fluid decreases.

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Why is wing shaped the way it is? Fast air! Low pressure. Slow air! High pressure.

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Bernoulli’s principle Bernoulli's Principle is an example of an inverse relationship. An inverse relationship means that when one value goes down, the other one goes up.

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Other applications of Bernoulli’s principle:

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Applications of Bernoulli’s principle “OOOh” B-58 Husler

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Applications of Bernoulli’s principle “OOOh” B-58 Husler

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Applications of Bernoulli’s principle “Go IRL”

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Applications of Bernoulli’s principle “Go John”

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Applications of Bernoulli’s principle HAIRDRYER AND PING PONG BALL

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Applications of Bernoulli’s principle Air

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Applications of Bernoulli’s principle

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A Hose-End Sprayer

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Straw Pressure Pressure in straw –You reduce air pressure in straw –Atmospheric pressure pushes liquid into reduced pressure region

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What Makes Objects Float and Sink? Do heavy objects always sink when placed in water? Interest Grabber

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What Makes Objects Float and Sink? Consider a dime and a large cruise ship. When placed in water the dime quickly sinks, while the cruise ship floats. 1. Which object is heavier, the cruise ship or the dime? Interest Grabber

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What Makes Objects Float and Sink? Consider a dime and a large cruise ship. When placed in water the dime quickly sinks, while the cruise ship floats. 2. Knowing that weight acts downward, what can you infer about other forces acting on a floating object? Interest Grabber

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13.3 Buoyancy Buoyancy is the ability of a fluid to exert an upward force on an object placed in it.Buoyancy is the ability of a fluid to exert an upward force on an object placed in it. Buoyancy causes the apparent loss of weight when an object is placed in a fluid.Buoyancy causes the apparent loss of weight when an object is placed in a fluid.

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Buoyant Force The pressure on the bottom of the ball is greater than the pressure on the top.The pressure on the bottom of the ball is greater than the pressure on the top. This produces the buoyant force.This produces the buoyant force.

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13.3 Buoyancy in a Liquid BuoyancyBuoyancy –The apparent loss of weight of submerged objects. 4.9 N object in air.4.9 N object in air. 4.3 N object in water.4.3 N object in water. –Buoyant force = 0.6 N 4.9 N4.3 N 0.6 N

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Buoyant force Buoyant force- a consequence of pressure increasing with depthBuoyant force- a consequence of pressure increasing with depth –Pressure is greatest at bottom –Upward force against the bottom are greater than the downward forces against top

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13.3 Archimedes

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Archimedes’ Principle An immersed body is buoyed up by a force equal to the weight of the fluid it displaces.An immersed body is buoyed up by a force equal to the weight of the fluid it displaces. –True to all fluids, liquids and gases –Ex. 7lb object displaced 3lb of water, the buoyant force is 3lbs and the apparent weight is 4lbs.

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Archimedes’ Principle

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Buoyant force Weight is greater than the buoyant force the object sinks Weight is still greater than the buoyant force Buoyant force is = to the weight and the object floats

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Weight and the Buoyant Force

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Buoyant force A sunken object displaces its own volume of liquid. A floating object displaces its own mass of liquid. Buoyant force is equal to the weight of the displaced liquid, whether the object is submerged or floating.

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Partial Submersion Density of the fluid makes things float AlcoholWaterGlycerin

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Why do steel ships float? The weight of the water displaced is less than the weight of the cube, the cube will sink. The weight of the water displaced equal to the weight of the ship, the ship will float.

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Principle of flotation Floating object displaces a weight of fluid equal to its own weight. A ship must be built to displace enough fluid to equal its weight.

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Density and Buoyancy The weight of a floating object equals the weight of the water displaced by the submerged part.The weight of a floating object equals the weight of the water displaced by the submerged part.

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Principle of flotation If a ship weighs 100 tons, it must displace 100 tons of water. Ship floats higher in salt than fresh water. Salt is denser.

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Principle of flotation Weight of water displaced equals the weight of the cargo.

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SCUBA Divers Buoyancy Largest = Ascend Forces Balanced = Hover Weight Largest = Sink Hover Ascend Sink

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Make sinkers floaters By either removing ballast (weight) or increasing the size. Removing ballast by while keeping the same size makes the object a better floater.

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Make floaters sinkers By either adding ballast or making their size smaller. Adding ballast makes them heavier than the buoyant force and the object sinks.

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Good Floaters Objects which are big and weigh little are good floaters.

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Buoyancy in Gas Archimedes principle in gasArchimedes principle in gas –An object surrounded by air is buoyed up by a force equal to the weight of the air displaced. RiseRise – Object has mass less than mass of equal volume of air rises –Only rise so long as it displace volume of air that weighs more than it does

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Hot Air Balloons Buoyancy Largest = Ascend Forces Balanced = Hover Weight Largest = Sink Hover Sink Ascend

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Hot Air Balloons

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