Forces in Fluids

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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.
• Pressure Force/Area
• What label?
• N/m2
• 1 N/m2 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 area should be in square meters (m2).
• The resulting unit would be N/m2.
• One N/m2 is a pascal (Pa).
• 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.

• P F / A

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Pressure in Liquids

• Deeper the more pressure.
• 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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Pressure Increases with Depth

• 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.
• Air pressure decreases as the altitude increases.

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Air pressure
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.
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"Standard Pressure"

• 1 atmosphere (at sea-level)
• 1 atm 101.3 kPa 14.7 psi
• kPa is kiloPascals.

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Air Pressure the Atmosphere

• 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.
• Pascals 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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Pascals Principle
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Pascals Principle

• A 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 Pascals principle and moving pistons with
  an enclosed pressurized fluid
• Examples
• Car brakes, jacks, and loaders

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Hydraulic Systems

• Hydraulics is the science of applying Pascals
  principle.
• Hydraulic systems use pressurized fluid acting on
  a piston to change the force.

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Hydraulic Systems
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Bernoullis Principle

• 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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Bernoullis 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 Bernoullis principle
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Applications of Bernoullis principle

• OOOh B-58 Husler

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Applications of Bernoullis principle

• OOOh B-58 Husler

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Applications of Bernoullis principle

• Go IRL

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Applications of Bernoullis principle

• Go John

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Applications of Bernoullis principle

• HAIRDRYER AND PING PONG BALL

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Applications of Bernoullis principle
Air
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Applications of Bernoullis 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?
Interest Grabber

• Do heavy objects always sink when placed in
  water?

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What Makes Objects Float and Sink?
Interest Grabber

• 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?

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What Makes Objects Float and Sink?
Interest Grabber

• 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?

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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 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.
• This produces the buoyant force.

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13.3 Buoyancy in a Liquid

• Buoyancy
• The apparent loss of weight of submerged objects.
• 4.9 N object in air.
• 4.3 N object in water.
• Buoyant force 0.6 N

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Buoyant force

• Buoyant 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.
• 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

                             
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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.

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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
Ascend
Hover
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 gas
• An object surrounded by air is buoyed up by a
  force equal to the weight of the air displaced.
• Rise
• 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
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Hot Air Balloons