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Physics 2 Chapter 9 Section 2

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Pressure Ratio of the magnitude of the force acting perpendicular to the surface to the area over which the force acts P = F / A[P] = N/m 2 = Pascal, Pa since Pa is very small pressure is usually measured in kPa

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Pressure cont’d. Other units for pressure [P] = lbs / in 2 = psi [P] = torr = 133.3 Pa or 1/760 atm [P] = atm = 1.01 x 10 5 Pa = 14.7 lb/in 2 which is pressure at sea level

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Question At sea level the atmosphere presses down on us with a pressure equivalent to 14.7 pounds per square inch. Why aren’t we crushed by the air pushing in on us at every surface?

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Sample Problem A man weighs 200 lb. At one particular moment when he walks, his right heel is the only part of his body that touches the ground. If the heel of his shoe measures 3.5 in. by 3.25 in., what pressure does the man exert on the ground?

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Sample Problem A woman weighs 100 lb and is wearing high- heeled shoes. The cross section of her high-heel shoe measures ½ in. by 5/8 in. At a particular moment when she is walking, only one heel of her shoe makes contact with the ground. What is the pressure exerted on the ground by the woman?

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Pressure cont’d. Pressure in water is created by the weight of the water Pressure in air is created by the air being pulled down by gravity toward the center of the earth Air pressure (like water pressure) increases with depth

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Sample Problem An airplane window has an area of 825 cm 2. Cabin pressure is 1 atm and the outside pressure is 0.3 atm. Find the net force produced by air pressure on the window.

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Pressure cont’d. Pressure increases with depth Pressure at top surface equals F 1 /A Pressure at bottom surface equals F 2 /A ΣF = P 2 A – P 1 A – mg Since fluid is static can show P 2 = P 1 + ρgh Only good if ρ is same at any depth

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Pascal’s Principle Any change in the pressure applied to a completely enclosed fluid is transmitted undiminished to all parts of the fluid and the enclosing walls Since P 1 = P 2 the F 2 = F 1 (A 2 /A 1 ) Principle behind hydraulic lifts and hypodermic needles

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Barometer 1643, Torricelli invented the barometer to measure atmospheric pressure Filled a glass tube with Hg and inverted it in a container of Hg When Hg drop in tube it creates a vacuum Using Pascal’s principle can show P outside = ρgh Pressure is indicated by the height of Hg

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Gauge Pressure Amount by which the container pressure exceeds atmospheric pressure Ex – tire gauge if car tire has a gauge pressure of 2 atm its actual pressure is 3 atm

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