9.1 Fluids Under Pressure Fluids always move from high pressure to low pressure w Fluids under pressure and compressed gases are used for a variety of everyday tasks Air molecules pulled by gravity = atmospheric pressure w Air pressure increases as altitude decreases The more air there is above, the more it compresses the air molecules below Air pressure is lower at high altitudes When humans change altitude, our bodies try to equalize the pressure differences by having our ears pop See pages 314-316
Pressure Differences Fluids will always attempt to move from high pressure to low pressure w When we drink with a straw, we first remove air from the closed straw, which lowers the pressure inside. The atmosphere, having a higher pressure, then tries to get into the straw, and pushes the fluid up and out of the way to try to get into the straw! w This same idea is used for many purposes, including hydraulics, water rockets and dental tools. See page 316
Liquid Pressure The pressure of fluids increases with depth w When you dive deep, you can feel more pressure w In Earth s atmosphere and oceans, pressure also increases with depth (air behaves like as fluid as well!) Sea level is about the deepest the atmosphere gets Sea level = 1 atmosphere = 101.3 kpa (kilopascals) Top of Mount Everest = 1/3 atm = 330 kpa From sea level, every 10 m in water depth = +1 atm A submarine at a depth of 500 m has the equivalent of a 500 000 kg object resting on every square metre! See page 317
density The density of something is indication of how much mass per unit volume it has. A typical unit of density is kg/m 3. The density of fresh water is 1000 kg/m 3 = 1g/cm 3. Solids would more dense than most fluids, and fluids would be more dense than gases. (why?) Is the density of the atmosphere increase or decrease as you go up in altitude? Density can be calculated by the following formula: See page 261
density Example 1): the mass of an object is 250 g and it has a volume of 0.15 m 3. What is its density in kg/m 3? Solution: Density= mass/volume= 0.25 kg/0.15m 3 = 1.67 kg/m 3 Example 2): What volume does 1 500 kg of water occupy, if its density is 1000 kg/m 3? Solution: Volume=mass/density = 1500 kg/ 1000 kg/m 3 = 1.5 m 3 See page 261
Buoyancy The buoyant force is an upward force exerted by a fluid. This force is equal to the weight of fluid displaced by the object. This is known as Archimedes principle. w Fb= (m fluid )(g), where g= 9.80 m/s 2 at the Earth s surface. When an object is in a liquid (floating or submerged) there are two forces acting on this object: the buoyant force, and the force of gravity [Fg= (m object )(g) The diagram on the right shows the FBD of an object submerged in a liquid. Depending on its density it may stay where it is, start to rise, or sink. Fb Fg
Buoyancy For submerged objects when the buoyant force is equal to the weight of the object, it is neutrally buoyant. This also means the density of the fluid is equal to the density of the object. A submerged object rises if the buoyant force is greater than the object s weight. The object is less dense than the surrounding fluid. A submerged object will sink if the weight is greater than the buoyant force. The object is more dense than the surrounding liquid For a floating object on the surface of the liquid, Fg=Fb, similar neutral buoyancy Fb Fg Sinking object Fb Fg Neutrally buoyant Fb Fg Rising object
Rising and Sinking Many vehicles, including submarines, airplanes and space shuttles all must consider pressure changes w Submarine designers must ensure the sub is safe design a way to change depths w When water is pumped in, density increases = sink w When water is pumped out, density decreases = rise Compressed air, kept onboard, pushes out water Convection refers to the movement of low density over top of high density fluids (e.g. thermals) See page 318