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Chapter 8 Forces in Fluids

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Fluid Pressure

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Pressure Pressure- the result of a force distributed over an area Ex 1: A theater’s seat exerts less pressure than a bicycle seat because the large padded seat and back offer a larger area to support your weight. Ex 2: A sharp pencil point easily pokes a sheet of paper, but an eraser does not. The pencil point has a much smaller area than the eraser, so it exerts a much greater pressure than the eraser. The greater pressure exerted by the pencil point allows it to pierce the paper easily.

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Pressure Cont’d To calculate pressure, divide the force by the area over which the force acts. Pressure (N/m²) =Force (N) newtons Area (m²) square meters Pascal (Pa)- the SI unit of pressure, equal to 1 newton per square meter (N/m²). The pascal is named for French scientist Blaise Pascal. Pressures are often stated in units of kilopascals (kPa). 1 kPa=1000 Pa.

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Pressure Cont’d Equivalent values for pressure 1 atm = X 10 5 N/m 2 (Pa) = kPa 1 bar = 1 X 10 5 N/m 2 (used by meteorologists) 1 atm = 760 mm Hg (760 torr) kPa = 29.9 inHg = 14.7 psi

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Pressure Cont’d Ex: A box with a weight of 2700 newtons is resting on the ground. If the area of the box touching the ground is 1.5 square meters, what pressure does the box exert on the ground? Pressure=Force = 2700 N =1800 N/m² Area 1.5 m² 1800 Pa= 1.8 kPa 1000

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Pressure in Fluids Fluid- a substance or mixture that flows and has no shape of its own. It assumes the shape of its container. Liquids and gases: water, oil, gasoline, air, and helium are fluids. A glass is filled with water. Because the water is in contact with the walls and bottom of the glass, it exerts pressure on these surfaces. The amount of pressure exerted depends on several factors.

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Pressure in Fluids Cont’d Water pressure increases as depth increases. The pressure in a fluid at any given depth is constant, and it is exerted equally in all directions. For a fluid that is not moving, depth and the type of fluid are the two factors that determine the pressure the fluid exerts. Volume or weight does not affect pressure.

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Air Pressure and the Atmosphere Air pressure decreases as the altitude increases. When your ears pop, unbalanced pressures (pressure between the inside of your ears and the air outside) are equalizing as air passes through a small tube within your ear.

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Forces and Pressure in Fluids

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Pascal’s Principle According to Pascal’s 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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Hydraulic Systems Hydraulics is the science of applying Pascal’s principles. Hydraulic system- a device that uses a pressurized fluid acting on pistons of different sizes to increase force In a hydraulic lift system, an increased output force is produced because a constant fluid pressure is exerted on the larger area of the output piston. Because the pressure on each piston is the same, the difference in forces is directly related to the difference in areas.

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Bernoulli’s Principle According to Bernoulli’s principle, as the speed of fluid increases, the pressure within the fluid decreases. Ex: The air traveling over the top of an airplane wing moves faster than the air passing underneath. This creates a low-pressure area above the wing and a lift is created. Lift- an upward force due to a pressure difference between the top and bottom of a wing

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Buoyancy

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Buoyant Force Buoyancy- the ability of a fluid to exert an upward force on an object placed in it Buoyancy results in the apparent loss of weight of an object in a fluid. Every object in a fluid experiences buoyancy. Buoyant force- an upward force acting on an object in a fluid

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Archimedes’ Principle Archimedes’ Principle- the equivalence of the buoyant force on an object and the weight of the fluid displaced by the object Object is submerged: it pushes aside, or displaces, a volume of fluid equal to its own volume Object floats: it does not displace its entire volume. It displaces a volume equal to the volume of the part of the object that is submerged

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Density and Buoyancy 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. Different fluids can also float or sink in one another. Oil floats on water because oil is less dense than water. Two forces act on every object in a fluid- weight and the buoyant force. 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.

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