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MR. SURRETTE VAN NUYS HIGH SCHOOL CHAPTER 9: FLUID MECHANICS CLASS NOTES 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) 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: p = pa + gh ( = density of the liquid) Example 1. What is the total 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 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 m 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. ARCHIMEDE’S PRINCIPLE 2|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL 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’ 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 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 m/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 m/s / (2.0 x 10-2 m)2 (5) v = 0.64 m/s 3|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL 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. y1 is the vertical distance from the ground to Point 1 and y2 is the vertical distance from the ground to Point 2: v = (2g(y1-y2))1/2 (Notice that (y1 – y2) = h = fluid depth) 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 5. 5A. (1) v = (2gh)1/2 (2) v = [(2)(9.8 m/s2)(0.4 m)]1/2 (3) v = 2.8 m/s 4|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL BERNOULLI’S EQUATION Bernoulli’s equation relates to the Work Energy Theorem and to the Conservation of Energy. It is derived from Newtonian mechanics: 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) Third Equation: 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. 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. 5|Page PHYSICS MR. SURRETTE VAN NUYS HIGH SCHOOL 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 6|Page PHYSICS