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The Fluid States Section 13.1 Physics

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Objectives Describe how fluids create pressure and relate Pascal’s principle to some everyday occurrences. Apply Archimedes’ and Bernoulli’s principles. Explain how forces within liquids cause surface tension and capillary action, and relate the kinetic model to evaporation and condensation.

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Properties of Fluids Fluids: materials that flow and have no definite shape of their own. For the most part, Newton’s Laws and the Laws of Conservation can be applied to fluids. When we apply a force to a fluid, what do we call it?

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**Pressure We call a force applied to a fluid: pressure.**

Pressure is represented by the following equation: P = F/A Pressure equals force divided by area.

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Pressure The force is assumed to be perpendicular to the affected surface area. Pressure is measured in Pascals, Pa. The Pascal: 1N/m². Which has a greater pressure? The area under an elephant’s foot or the area under a woman’s high heel shoe? The area under a high heel shoe.

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Pressure from a Gas? It is easy to see how solids and fluids exert pressure on a surface. But how does a gas exert pressure on an area? The tiny gas particles are constantly hitting surfaces and applying a pressure value.

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**Pressure from a Gas Are we under pressure right now? Yep.**

In fact, on every square centimeter of Earth’s surface at sea level, the atmosphere exerts a force of approximately 10 N = 1 kg. Blue Barometer Demo.

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Practice Problems Pg 303 1-4

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Fluids at Rest Water also exerts pressure just as the atmosphere exerts pressure. Think of a diver, the pressure felt when under water does not depend on whether the diver is upright, sideways, or up-side-down. Pascal’s Principle accounts for this pressure.

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Pascal’s Principle Pascal’s Principle: any change in pressure applied at any point on a confined fluid is transmitted undiminished throughout the fluid. How does toothpaste demonstrate this principle?

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**Pascal’s Principle So why is this important?**

There are more important reasons than that of toothpaste. Hydraulic systems use this principle Force Exerted by lift = F = (F1A2)/A1

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Practice Problem Pg 304 5

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**Swimming Under Pressure**

When you dive under water, your body feels increasing pressure as you dive deeper. The downward pressure of water is illustrated by the following equation. P = ρhg Rho; density of fluid, h; depth, g; gravity.

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**Swimming Under Pressure**

While swimming, you may also notice the upward force from the water you are immersed in. This force is the buoyant force. Fbuoyant = ρVg Rho; density, V; volume, g; gravity. The net upward force is equal to the weight of the fluid displaced by the object.

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**Archimedes’ Principle**

The buoyant force and weight of displaced fluid relationship was discovered by the Greek scientist Archimedes. Archimedes’ Principle: states that an object immersed in a fluid has an upward force on it equal to the weight of the fluid displaced by the object; it does not depend on the weight of the object.

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So Will It Sink Or Float? When an object is placed in a fluid, it has the force of gravity pressing down and the buoyant force pressing up. The difference between the two determines if the object will sink or float. Fapparent = Fg – Fbuoyant A negative value for Fapparent indicates the object will float.

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Practice Problems Pg 309 6-7

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Fluids in Motion Ok, we have looked at static fluids, but fluids do not just stay still. Bernoulli’s Principle: describes the relationship between the velocity and pressure exerted by a moving fluid. Bernoulli’s Principle states that as the velocity of a fluid increases, the pressure exerted by that fluid decreases.

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Fluids in Motion Most airplanes get part of their lift by utilizing this principle. The curvature of the top wing is greater than that of the bottom. As the wing travels through the air, ;the air moving over the top surface travels faster than air moving past the bottom surface.

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Fluids in Motion The decreased air pressure created on the top surface results in a net upward pressure that produces an upward force on the wings, or lift, which helps hold the plane up. Race cars use this same principle…How?

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Forces Within Liquids All of the fluids examined thus far have been ideal fluids. In real liquids, particles exert electromagnetic forces of attraction on each other. These forces affect the behavior of liquids.

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**Forces Within Liquids These forces are cohesive forces.**

Cohesive forces are the forces of attraction between particles of a fluid. These forces account for water’s surface tension.

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Forces Within Liquids Another force that affects the way a fluid behaves is adhesion. Adhesive forces are the attraction between particles of a fluid and the particles of another surface. This is why capillary action occurs.

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Pressure Force per unit area Units: Pa (N/m 2 ), lb/in 2, atm, torr, mmHg P = pressure, N (psi) F=force, N (lb) A= area, m 2 (in 2 )

Pressure Force per unit area Units: Pa (N/m 2 ), lb/in 2, atm, torr, mmHg P = pressure, N (psi) F=force, N (lb) A= area, m 2 (in 2 )

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