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Forces in Fluids p

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Pressure Since fluids take different shapes, the concept of a force on a fluid is better described as PRESSURE. You can’t really “push” a fluid but you can exert pressure on it. Pressure is the amount of force applied to a fluid per unit surface area Bike tire has 100 psi (pounds per square inch) Car tire has 35 psi (pounds per square inch)

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**Formula to calculate pressure**

Pressure (Pa) = Force (N) area (m2) OR P = F/a P is pressure measured in Pascals (Pa) F is force exerted measured in Newtons (N) a is surface area measured in m2

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Pressure in a Liquid In a liquid, pressure depends on the density of the liquid and the depth Higher the density the higher the pressure Increased depth means increased pressure SCUBA divers experience increased pressure the deeper they dive….you can feel it in your ears!!!

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Pressure in a Gas In a gas, pressure depends on the temperature, volume and number of gas particles Higher temperature means higher pressure because the molecules are moving faster Increasing the volume results in lowering the pressure because there is more room for the molecules to move Increasing the number of gas molecules will increase the pressure

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**As volume increases, pressure decreases**

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Pascal’s Principle An increase in the pressure on a fluid is transmitted uniformly in all directions Used in water pistols, hydraulic breaks, syringes. See Fig on p. 94 (hydraulic lift)

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**Hydraulic breaks and jacks use Pascal’s principal**

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**Archemedes’ Principle or “Why an 80 000 ton battleship can float!”**

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Which will float?

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**Archimedes’ Principle**

Since the pressure in a liquid increases with depth, an upward force called “buoyancy” is exerted on objects placed in liquids Archimedes’ Principle: the magnitude of the buoyant force will equal the weight of the fluid displaced by the object

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**Archimedes’ Principle explains why some objects will float in water and some will sink.**

It also explains why objects feel “lighter” when underwater. Why does a steel ship float in water? It all has to do with the shape of the hull and the weight of water displaced by the ship’s hull.

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**Archemedes’ Principle**

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**Archimedes’ Principle**

If the buoyant force is less than the force of gravity (Fb<Fg) the object will sink If the buoyant force is greater than the force of gravity (Fb>Fg) the object will float If the buoyant force is equal to the force of gravity (Fb=Fg) the object will maintain the same depth

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How a submarine works See also Fig. 3.37, p. 97

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How a submarine works

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**Bernoulli’s Principle or how a 875 000 pound airplane can fly**

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**Bernoulli’s Principle**

When a fluid or gas is in motion, like the wind in the air, its pressure varies with its speed Bernoulli’s Principle: the higher the speed of the fluid or gas, the lower its pressure This explains how a plane can fly (Fig. 3.38, p. 98)

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**Bernoulli’s Principle**

When the force from the lift exceeds that of gravity, the plane rises into the air.

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Pressure Pressure is a force exerted over an area on the surface

Pressure Pressure is a force exerted over an area on the surface

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