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MR. SURRETTE VAN NUYS HIGH SCHOOL CHAPTER 9: FLUID MECHANICS CLASSNOTES FLUIDS A fluid is a substance whose molecules move freely past one another and has the tendency to assume the shape of its container. In general, liquids and gases are fluids. DENSITY The density of a substance is mass per unit volume. It has units of kilograms per cubic meter (or grams per cubic centimeter) in the metric system: =m/V PRESSURE Pressure is force exerted per unit area: P=F/A Pressure [Pascals] = Force [Newtons] / Area [m2] BASIC LAW OF FLUID PRESSURE The pressure at the bottom of a fluid can be expressed as: P = gh ( = density of fluid, h = depth of fluid g = 9.8 m/s2) FLUID PRESSURE The pressure at any point in a fluid depends only on its density and its depth. It acts equally in all directions. ABSOLUTE PRESSURE The absolute pressure, P, at a depth, h, below the surface of a liquid which is open to the atmosphere is greater than atmospheric pressure, Pa, by an amount that depends on the depth below the surface. ABSOLUTE PRESSURE EQUATION P = Pa + gh ( = density of the liquid) Example 1. What is the absolute pressure at the bottom of a 5 m deep swimming pool? (Note the pressure contribution from the atmosphere is 1.01 x 105 N/m2, the density of water is 103 kg/m3, and g = 9.8 m/s2) 1A. (1) P = Pa + gh (2) P = (1.01 x 105 N/m2)+(1.0 x 103 kg/m3) (9.8 m/s2)(5 m) (3) P = 1.5 x 105 N/m2 1|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL PASCAL’S PRINCIPLE Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel: F1 / A1 = F2 / A2 PASCAL’S PRINCIPLE Pressure is equal (P1 = P2) Example 2. A hydraulic lift raises a 2000 kg automobile when a 500 N force is applied to the smaller piston. If the smaller piston has an area of 10 cm2, what is the cross-sectional area of the larger piston? 2A. (1) F1 / A1 = F2 / A2 (2) A2 = (F2A1)/F1 (3) F2 = w = mg (4) A2 = (mgA1) / F1 (5) A2 = (2000 kg)(9.8 m/s2)(1.0 x 10-3 m2) / 500 N (6) A2 = 3.92 x 10-2 m2 ARCHIMEDE’S PRINCIPLE Archimede’s Principle is the law of buoyancy. When an object is submerged in a fluid (completely or partially), there exists an upward force on the object that is equal to the weight of the fluid that is displaced by the object. 2|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL ARCHIMEDE’S PRINCIPLE BUOYANT FORCE One common expression for the upward buoyant force is: FB = Vg (FB = buoyant force, = density of liquid V = volume of submerged object, g = 9.8 m/s2) BUOYANT FORCE Another expression for buoyant force is: FB = w – w’ (w = actual weight of object w’ = apparent weight of submerged object) DENSITY OF OBJECTS The density of a submerged object can be determined: object = liquid / (w – w’) 3|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL Example 3. An object of mass 0.5 kg is suspended from a scale and submerged in a liquid. What is the buoyant force that the fluid exerts if the reading on the scale is 3.0 N? 3A. (1) w = mg (2) w = (0.5 kg)(9.8 m/s2) (3) w = 4.9 N (4) FB = w – w’ (5) FB = 4.9 N – 3.0 N (6) FB = 1.9 N upwards FLOW RATE The product Av is called the flow rate. The flow rate at any point along a pipe carrying an incompressible fluid is constant: A1v1 = A2v2 Example 4. If the flow rate of a liquid going through a 2.00 cm radius pipe is measured at 0.8 x 10-3 m3/s, what is the average fluid velocity in the pipe? 4A. (1) Flow rate = Av (2) v = Flow rate / A (3) v = Flow rate / r2 (4) v = 0.8 x 10-3 m3/s / (2.0 x 10-2 m)2 (5) v = 0.64 m/s TORRICELLI’S THEOREM The speed of efflux is the same as the speed a body would acquire in falling freely through a height h: v = (2gh)1/2 TORRICELLI’S THEOREM Torricelli’s Theorem can be analyzed another way: Point 1 is at the surface of water in a tank and Point 2 is the position of the hole. TORRICELLI’S THEOREM y1 is the vertical distance from the ground to Point 1 and y2 is the vertical distance from the ground to Point 2. TORRICELLI’S THEOREM EQUATION v = (2g(y1-y2))1/2 (Notice that (y1 – y2) = h = fluid depth) 4|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL TORRICELLI’S THEOREM Example 5. A liquid filled tank has a hole on its vertical surface just above the bottom edge. If the surface of the liquid is 0.4 m above the hole, at what speed will the stream of liquid emerge from the hole? Question 5A. 5|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL 5A. (1) v = (2gh)1/2 (2) v = [(2)(9.8 m/s2)(0.4 m)]1/2 (3) v = 2.8 m/s BERNOULLI’S EQUATION Bernoulli’s equation relates to the Work Energy Theorem and to the Conservation of Energy. It is derived from Newtonian mechanics. BERNOULLI’S EQUATION p1 + ½ v12 + gy1 = p2 + ½ v22 + gy2 [p1 = initial pressure, p2 = final pressure = density of fluid, g = 9.8 m/s2 v1 = initial velocity, v2 = final velocity y1 = initial height, y2 = final height] Example 6. Water (density = 1 x 103 kg/m3) is flowing through a pipe whose radius is 0.04 m with a speed of 15 m/s. This same pipe goes up to the second floor of the building, 3 m higher, and the pressure remains unchanged. What is the cross-sectional area of the pipe on the second floor? 6A. (1) First Equation: p1 + ½ v12 + gy1 = p2 + ½ v22 + gy2 (2) ½ v12 + gy1 = ½ v22 + gy2 (3) v12 = v22 + 2gy2 (4) v2 = [(v1)2 – 2gy2]1/2 (5) v2 = [(15 m/s)2 – 2(9.8 m/s2)(3 m)]1/2 (6) v2 = 12.9 m/s (7) Second Equation: A1v1 = A2v2 (8) A2 = A1v1 / v2 (9) A1 = r2 (10) A2 = (r2)v1 / v2 (11) A2 = (0.04 m)2(15 m/s) / (12.9 m/s) (12) A2 = 5.84 x 10-3 m2 BERNOULLI’S PRINCIPLE Bernoulli’s principle states that swiftly moving fluids exert less pressure than slowly moving fluids. AIR FOILS An air foil is a body (like an airplane wing or propeller blade) designed to provide a desired reaction force when in motion. AIR MOLECULE MOTION A popular expression in science is: “Nature abhors a vacuum.” This is very true for air. Like all other gases, air quickly fills in all available space. 6|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL AIR MOLECULE MOTION At the molecular level, air molecules randomly bounce off each other and prevent gaps. AIR MOLECULE DISTANCE Airplane wings are air foils designed to create greater distance for air molecules to travel above the wing than below the wing. AIR MOLECULE TIME Because air molecules quickly fill in available space, it takes the same amount of time for a parcel of air (think of a small invisible cloud) to split, move around both sides of an airplane wing, and then reassemble on the other side of the wing. VELOCITY EQUATION As we saw in Chapter 1 (Kinematics), velocity is equal to distance divided by time: v=d/t WINGS AND AIR VELOCITY Air molecules above an airplane wing travel a greater distance in the same amount of time as air molecules below an airplane wing. WINGS AND VELOCITY EQUATION Since v = d/t: (1) d ABOVE WING > d BELOW WING (2) V ABOVE WING = d ABOVE WING / t CONSTANT (3) V BELOW WING = d BELOW WING / t CONSTANT WINGS AND VELOCITY EQUATION Therefore: V ABOVE WING > V BELOW WING (v2 > v1 in diagram below) BERNOULLI’S PRINCIPAL AND FLIGHT According to Bernoulli’s principle, faster air exerts less pressure than slower air. BERNOULLI’S PRINCIPAL AND FLIGHT Since V ABOVE WING > V BELOW WING, air pressure below the wing must be greater than air pressure above the wing: P ABOVE WING < P BELOW WING (P2 < P1 in diagram below) 7|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL APPLICATION OF BERNOULLI’S PRINCIPLE CROSS-SECTION OF WING (AIR FOIL) APPLICATION OF BERNOULLI’S PRINCIPLE P = P1 – P2. When the force exerted by P exceeds the weight of the airplane, the airplane is provided lift and flies. BERNOULLI’S TUBE The velocity of a fluid at the constricted end of a pipe is greater than the velocity at the wider ends if steady flow is maintained. BERNOULLI’S TUBE BERNOULLI’S TUBE At Point 1, the velocity is less, but the pressure is greater. At Point 2, the velocity is greater, but the pressure is less. 8|Page PHYSICS