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Section 1 of Forces in Fluids

Christopher Landry

on 29 April 2014

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Transcript of Pressure

Fluids in Forces
- Pressure - Objectives What is Pressure The word pressure is related to the word press and refers to a force pushing on a surface. Pressure and Area Force and pressure are related, but they are not the same. The larger the area over which the force is distributed, the less pressure exerted. In order to stand on snow without sinking, you can't make yourself weigh the same as a bird. However, you can change the area over which you exert the force of your weight. If you wear snowshoes, you'll exert pressure over a much greater area – about 2,200 cm2, as opposed to sneakers – about 500 cm2. Because the force of your weight is distributed over a greater area, the overall pressure exerted on the snow is much less. What does pressure depend on? Calculating Pressure Pressure depends on the area over which a force is distributed. Pressure is equal to the force exerted on a surface divided by the total area over which the force is exerted. Force is measured in newtons (N), and area is measured in square meters (m2). Since force is divided by area, pressure is measured as one newton of force on one square meter. The SI unit of pressure is called the pascal (Pa). One pascal of pressure is 1N/m2 = 1 Pa. Fluid Pressure A fluid is a substance that can easily flow. Fluids can change shape. Liquids and gases are both examples of fluids. What causes Fluid Pressure? Fluids exert pressure against the surfaces they touch. Fluids are made up of tiny particles called molecules. In fluids, molecules are constantly moving in all directions. As each molecule collides with a surface, it exerts a force on the surface. All of the forces exerted by the individual particles in a fluid combine to make up the pressure exerted by a fluid. Because the number of molecules is large, you can consider the fluid as a whole. Fluid pressure is the total force exerted by the fluid divided by the area over which the force is exerted. How do fluids exert pressure? Air Pressure The air in Earth's atmosphere is also a fluid. These gases press down on everything on Earth's surface, all the time. Air has mass and weight. Each cubic meter of air around you has a mass of about 1 kilogram. Because the force of gravity pulls down on this mass of air, the air has weight. The pressure exerted by this gas is called air pressure or atmospheric pressure. Air pressure is great because the atmosphere is over 100km high. Balanced Pressure In a fluid that is not moving, pressure at any point is exerted equally in all directions. Equal and opposite pressures balance each other out. Balanced pressure explains why the tremendous air pressure pushing on you from all sides does not crush you. Your body contains fluids that exert outward pressure. For example, your lungs contain air. Your cells and blood vessels contain liquids. So pressure from fluids inside your body balances the air pressure outside your body. You are not crushed by the weight of the atmosphere because it pushes against you equally in all directions, inside and out. What happens when air is not balanced? Imagine a can full of air. When the can is full, the air pressure inside the can balances the atmospheric pressure outside the can. When air is removed from the can, the unbalanced force of the outside air pressure crushes the can. Variations in Fluid Pressure Pressure of a fluid can change when you increase or decrease in elevation within a fluid. Atmospheric Pressure and Elevation Atmospheric pressure decreases as elevation increases. At higher elevations, there is less air above and therefore less weight of air to support. The air pressure at higher elevations is less than the air pressure at lower elevations. The fact that air pressure decreases as you move up in elevation explains why your ears pop. When the air pressure outside your body changes, the air pressure inside adjusts, but more slowly. So, for a moment, the air pressure behind your eardrums is greater than it is in the air outside. Your body releases this pressure with a “pop”, balancing the pressures. Water Pressure and Depth Water pressure increases as depth increases. At greater depths, there is more water above and therefore more weight of water to support. The water pressure at greater depths is more than the water pressure at lesser depths. In addition, air in the atmosphere pushes down on the water. The total pressure underwater is a sum of the water pressure plus the air pressure above the water. How does fluid pressure change with elevation? Measuring Pressure Air pressure can be measured with a device called a barometer. There are two types of barometers: a mercury barometer and an aneroid barometer. The aneroid barometer is the barometer you usually see hanging on a wall. Weather forecasters use the pressure reading from a barometer to help forecast the weather. Rapidly decreasing atmospheric pressure usually means a storm is on the way. Increasing pressure is often a sign of fair weather. You may hear barometric pressure readings expressed in millimeters, inches or another unit called a millibar. For example, the standard barometric pressure at sea level may be reported as 760 millimeters, 29.92 inches, or 1,013.2 millibars.
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