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13.1 Fluid Pressure


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13.1 Fluid Pressure. After completing this section, you will be able to: Describe and calculate pressure Identify appropriate SI units for measuring pressure Describe the relationship between water depth and the pressure it exerts

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13.1 Fluid Pressure

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13 1 fluid pressure l.jpg

13.1 Fluid Pressure

  • After completing this section, you will be able to:

    • Describe and calculate pressure

    • Identify appropriate SI units for measuring pressure

    • Describe the relationship between water depth and the pressure it exerts

    • Describe how forces from pressure are distributed at a given level in a fluid

    • Explain how altitude affects air pressure


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Pressure

  • Result of the force distributed over an area

  • P = F÷A

    • P = pressure (Pa)

    • F = force (N)

    • A = area (m2)


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Pressure in a fluid

  • Fluid is a substance that assumes the shape of its container

  • As the depth of a fluid increases, the pressure increases

  • At a particular depth, the pressure in a fluid is constant and exerted equally in all directions


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  • Shape of the container and the area of the container’s bottom do not affect the fluid pressure

  • Different fluids exert different pressures

  • Air pressure is a type of fluid pressure

    • As altitude increases, air pressure decreases


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13.2 Forces and Pressure in Fluids

  • After completing this section, you will be able to:

    • Describe how pressure is transmitted in a fluid according to Pascal’s principle

    • Explain how the speed and pressure of a fluid are related according to Bernoulli’s principle


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  • Fluids exert pressure equally in all directions at a given depth

    • Amount of pressure depends on the type of fluid and the fluid depth


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  • Any change in pressure at any point in a fluid is transmitted equally and unchanged in all directions

    • Called Pascal’s principle

    • Example: when you squeeze a bottle filled with water, the pressure change is equally spread throughout the whole bottle

    • Application of Pascal’s principle is a hydraulic system

      • Device that used pressurized fluid acting on pistons of different sizes to change a force


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  • The speed of a fluid can change the pressure of a fluid

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

      • Called Bernoulli’s principle

      • The faster the fluid, the lower the pressure


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  • Application of Bernoulli’s principle is the wings of planes and birds

    • Pressure difference created by the fluid moving at different speeds causes an upward force called lift

  • Spoilers on cars are upside down wings

  • Spray bottles


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13-3 Buoyancy

  • After completing this section, you will be able to:

    • Explain the effect of buoyancy on the apparent weight of an object

    • Explain the relationship between the volume of fluid displaced by an object and buoyant force acting on the object according to Archimedes’ principle


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  • Describe the relationship among object density, fluid density, and whether an object sinks or floats in a fluid

  • Describe the relationship among object weight, buoyant force, and whether an object sinks or floats in a fluid


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Buoyancy

  • Ability of a fluid to exert an upward force on any object placed in it

  • Results in the apparent loss of weight of an object in a fluid

    • Called the apparent weight

  • Upward force that acts opposite of gravity is buoyant force


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  • Since water pressure increases with depth, the forces pushing up on the bottom of an object are greater than the buoyant forces pushing down on the top of an object

  • Buoyant force on an object is equal to the weight of the fluid displaced by the object

    • Called Archimedes’ principle


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  • Buoyancy is closely related to density

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

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


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  • When the buoyant force is greater or equal to the weight, an object will float

  • When the buoyant force is exactly equal to the weight, an object is suspended

    • Floats at any level in the fluid

  • When the buoyant force is less than the weight, an object will sink


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  • Why does a clay block sink while a clay boat floats?

    • The shape of the boat allows it to displace a larger volume of water relative to its weight

    • The heavier the boat, the more water it must displace in order to float

    • The larger the boat, the more volume it has and the less its density