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Section 1: Pressure 1. What is Pressure? a. It refers to a force exerted over an area on the surface of an object. i. Due to gravity, your feet exert a force on the surface of Earth—your feet exert pressure on the ground b. Pressure and Area i. Pressure decreases as the area over which a force is distributed increases 1. The larger the area over which the force is distributed, the less pressure is exerted c. Calculating Pressure i. Pressure = Force / Area 1. Pressure equals force divided by area 2. Force is measured in newtons (N) 3. Area is measured in square meters (m2) 4. The SI unit of pressure is the newton per square meter (N/m2) a. This unit of pressure is also called the pascal (Pa) i. 1 N/m2 = 1 Pa 2. Fluid Pressure a. What causes fluid pressure? i. Particles in a fluid constantly move in all directions 1. The particles collide with each other and with any surface that they meet ii. All of the forces exerted by the individual particles in a fluid combine to make up the pressure exerted by the fluid iii. Fluid pressure is the total force exerted by the fluid divided by the area over which the force is exerted b. Air Pressure i. 100 kilometers of fluid, air, surrounds Earth ii. Air is a mixture of gases 1. These gases press down on everything on Earth’s surface, all the time iii.Air exerts pressure because it has mass 1. Because gravity pulls down on this mass, air has weight 2. The weight of the air produces air pressure, or atmospheric pressure c. Balanced Pressure i. Hold out your hand, palm up. You are holding up air with atmospheric pressure about equal to 10.13 N/cm2 1. The weight supported by the surface area of your hand is about 1,000 newtons, or about the same weight as that of a large washing machine! ii. In a stationary fluid, the air is pressing in all directions so they balance each other. iii. Balanced Pressure explains why the air pressure around us does not crush us 1. We have fluids in our body that exert outward pressure iv. When air pressure becomes unbalanced 1. A can full of air has balanced air pressure inside and outside the can 2. When the air is removed, the can crushes 3. Variations in Fluid Pressure a. Atmospheric Pressure and Elevation i. As your elevation increases, atmospheric pressure decreases ii. The reason our ears “pop” 1. Has to do with changing air pressure 2. At higher elevations, there is less air above you and therefore less air pressure 3. When the air pressure outside your body changes, the air pressure inside adjusts, but more slowly a. The air pressure behind your eardrums is greater than it is in the air outside. b. Your body releases this pressure with a “pop” as pressures balance b. Water Pressure and Depth i. Fluid pressure depends on depth ii. The deeper you go into a body of water, the greater the pressure iii.Water pressure increases with depth iv.Just like air, you can think of water pressure as being due to the weight of the water above a particular point v. The total pressure at a given point beneath the water results from the weight of the water plus the weight of the air above it c. Measuring Pressure i. Barometer: instrument used to measure atmospheric pressure 1. Two types: A mercury barometer and an aneroid barometer a. Aneroid barometer i. The kind you usually see hanging on a wall ii. Used by weather forecasters iii.Rapidly decreasing atmospheric pressure usually means a storm is on its way iv.Increasing atmospheric pressure is a sign of fair weather v. Readings may be read in millimeters, inches, or millibars vi.Ex: The barometric pressure at sea level may be reported as 760 millimeters, 29.92 inches, or 1,013.2 millibars What is the SI unit for pressure? What two factors does pressure depend on? How do fluids exert pressure? Why must an astronaut wear a pressurized suit in space?