13.2 Forces and Pressure in Fluids

Pascal’s PrincipleBernoulli’s Principle

Archimedes’ Principle

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Pressure in Fluids

The pressure in a fluid at any given depth is constant, and it is exerted equally in all directions.

So if I squeeze a water bottle, will the pressure be greater at the point where I squeeze?

http://dwellingintheword.files.wordpress.com/2009/04/17-water_bottle.jpg

Pascal’s PrincipleA change in pressure at any point in a

fluid is transmitted equally and unchanged in all directions throughout the fluid.

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http://www.schoolforchampions.com/science/pressure.htm

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Hydraulic SystemsIn a hydraulic system, an increased output force

is produced because a constant fluid pressure is exerted on the larger area of the output piston.

http://www.explainthatstuff.com/hydraulic2.png

Hydraulic Systems

• If you are working with a hydraulic system where the large piston has 8 times the area of the small piston, then the output force is how many times larger?

• 8 times.

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Bernoulli’s Principle

As the speed of a fluid increases, the pressure within the fluid decreases.

http://www.iboatnyharbor.com/Vee-wake.jpg

How does an airplane’s wings work?

• The wings of an airplane are shaped so that the air going over the top of the wing goes faster than the air on the bottom.

• More speed on top means less pressure on top, which means we get lift.

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Buoyancy

• Have you ever stood in a pool and tried lifting a friend who was submerged in water?

http://farm1.static.flickr.com/62/178690742_3eb7b3cc66.jpg

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Buoyancy• Buoyancy is the

ability of a fluid to exert an upward force (called the buoyant force) on an object in it.

• Buoyancy results in the apparent loss of weight of an object in a fluid.

http://avoca37.org/persinod/files/2009/10/buoyancy-archimedes-principle1.jpeg

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How is a buoyant force produced?

Because water pressure increases with depth, the pressure on the bottom of an object is greater than the pressure on top.

Archimedes’ Principle

• The buoyant force on an object is equal to the weight of the fluid displaced by the object.

• So if an object displaces 12 N of water, the buoyant force on the object is also 12 N.

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Will it float?

• If an object is less dense than the fluid it is in, it will float. If the object is more dense than the fluid it is in, it will sink.

http://upload.wikimedia.org/wikipedia/commons/thumb/7/74/Buoyancy.svg/301px-Buoyancy.svg.png

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Will it float?When the buoyant

force is equal to the weight, an object floats or is suspended. When the buoyant force is less than the weight, the object sinks.

http://rubaduck.com/graphx/rubber_duck_buoyancy_force_diagram.gif

How does a ship float?

The area of an output piston is 25 times greater than the area of the input piston. If the input force is 40 N, what is the output force?

• The high-speed winds of a hurricane sometimes cause houses to explode. Use a diagram to model the pressures acting on the house, and use Bernoulli’s principle to explain why this happens.