#### Transcript Monday, May 2, 2011

PHYS 1443 – Section 001 Lecture #22 Monday, May 2, 2011 Dr. Jaehoon Yu • • • • Absolute and Gauge Pressure Buoyant Force and Archimedes’ Principle Equation of Continuity Bernoulli’s Principle Announcements • Planetarium extra credit sheets due today!! • Quiz #5 this Wednesday, May 4 – Covers from CH. 11 – CH13.14 • Final comprehensive exam – 11am, Monday, May 9, in SH103 – Covers: Chapter 1.1 – CH13. 14 + appendices – Review: Wednesday, May 4th, in the class after the quiz • Attendance will be taken • Colloquium this Wednesday at 4pm Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 2 Absolute and Relative Pressure How can one measure pressure? P0 P h One can measure the pressure using an open-tube manometer, where one end is connected to the system with unknown pressure P and the other open to air with pressure P0. The measured pressure of the system is P P0 gh This is called the absolute pressure, because it is the actual value of the system’s pressure. In many cases we measure the pressure difference with respect to the atmospheric pressure to avoid the effect of the changes in P0 that G depends on the environment. This is called gauge or relative pressure. P P P0 gh The common barometer which consists of a mercury column with one end closed at vacuum and the other open to the atmosphere was invented by Evangelista Torricelli. Since the closed end is at vacuum, it does not exert any force. 1 atm of air pressure pushes mercury up 76cm. So 1 atm is P0 gh (13.595103 kg / m3 )(9.80665m / s 2 )(0.7600m) 1.013105 Pa 1atm Monday, May 2,the 2011tire pressure with a PHYS Spring 3 If one measures gauge1443-001, at 220kPa the2011 actual pressure is 101kPa+220kPa=303kPa. Dr. Jaehoon Yu Finger Holds Water in Straw You insert a straw of length L into a tall glass of your favorite beverage. You place your finger over the top of the straw so that no air can get in or out, and then lift the straw from the liquid. You find that the straw strains the liquid such that the distance from the bottom of your finger to the top of the liquid is h. Does the air in the space between your finger and the top of the liquid in the straw have a pressure P that is (a) greater than, (b) equal to, or (c) less than, the atmospheric pressure PA outside the straw? Less pinA What are the forces in this problem? Gravitational force on the mass of the liquid Fg mg A L h g Force exerted on the top surface of the liquid by inside air pressure Fin pin A mg Force exerted on the bottom surface of the liquid by the outside air Fout pA A Since it is at equilibrium F out p AA Cancel A and solve for pin Monday, May 2, 2011 Fg Fin 0 pin pA g L h PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu pA A g L h A pin A 0 So pin is less than PA by ρg(L-h). 4 Buoyant Forces and Archimedes’ Principle Why is it so hard to put an inflated beach ball under water while a small piece of steel sinks in the water easily? The water exerts force on an object immersed in the water. This force is called the buoyant force. How large is the The magnitude of the buoyant force always equals the weight of the buoyant force? fluid in the volume displaced by the submerged object. This is called the Archimedes’ principle. What does this mean? Let‘s consider a cube whose height is h and is filled with fluid and in its equilibrium so that its weight Mg is balanced by the buoyant force B. pressure at the bottom of the cube B Fg Mg The is larger than the top by gh. h Mg B Therefore, P B / A gh B PA ghA Vg B Vg Mg Fg Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu Where Mg is the weight of the fluid in the cube. 5 More Archimedes’ Principle Let’s consider the buoyant force in two special cases. Case 1: Totally submerged object Let’s consider an object of mass M, with density 0, is fully immersed in the fluid with density f . The magnitude of the buoyant force is B f Vg The weight of the object is Fg Mg 0Vg h Mg B Therefore total force in the system is What does this tell you? Monday, May 2, 2011 F B Fg f 0 Vg The total force applies to different directions depending on the difference of the density between the object and the fluid. 1. If the density of the object is smaller than the density of the fluid, the buoyant force will push the object up to the surface. 2. If the density of the object is larger than the fluid’s, the object will sink to the bottom of the fluid. PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 6 More Archimedes’ Principle Case 2: Floating object h Mg B Let’s consider an object of mass M, with density 0, is in static equilibrium floating on the surface of the fluid with density f , and the volume submerged in the fluid is Vf. The magnitude of the buoyant force is The weight of the object is Therefore total force of the system is Since the system is in static equilibrium What does this tell you? Monday, May 2, 2011 B f Vf g Fg Mg 0V0 g F B Fg f Vf g 0V0 g 0 f Vf g 0V0 g Vf 0 f V0 Since the object is floating, its density is smaller than that of the fluid. The ratio of the densities between the fluid and the object determines the submerged volume under the surface. PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 7 Ex.13 – 10 for Archimedes’ Principle Archimedes was asked to determine the purity of the gold used in the crown. The legend says that he solved this problem by weighing the crown in air and in water. Suppose the scale read 7.84N in air and 6.86N in water. What should he have to tell the king about the purity of the gold in the crown? In the air the tension exerted by the scale on Tair mg 7.84 N the object is the weight of the crown In the water the tension exerted mg B T 6.86 N water by the scale on the object is Therefore the buoyant force B is B Tair Twater 0.98 N Since the buoyant force B is B wVw g wVc g 0.98 N The volume of the displaced water by the crown is Vc Vw Therefore the density of the crown is c 0.98N 0.98 1.0 10 4 m 3 w g 1000 9.8 7.84 m c mc g 7.84 8.0 10 3 kg / m 3 4 1.0 10 9.8 Vc Vc g Vc g 3kg/m 3, this Monday, May 2,the 2011 1443-001, Spring 2011 Since density of pure gold PHYS is 19.3x10 crown is not made of pure gold. Dr. Jaehoon Yu 8 Example for Buoyant Force What fraction of an iceberg is submerged in the sea water? Let’s assume that the total volume of the iceberg is Vi. Then the weight of the iceberg Fgi is Fgi iVi g Let’s then assume that the volume of the iceberg submerged in the sea water is Vw. The buoyant force B caused by the displaced water becomes B wVw g Since the whole system is at its static equilibrium, we obtain Therefore the fraction of the volume of the iceberg submerged under the surface of the sea water is iVi g wVw g 3 917kg / m Vw i 0.890 3 Vi w 1030kg / m About 90% of the entire iceberg is submerged in the water!!! Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 9 Flow Rate and the Equation of Continuity Study of fluid in motion: Fluid Dynamics If the fluid is water: Water dynamics?? Hydro-dynamics •Streamline or Laminar flow: Each particle of the fluid Two primary follows a smooth path, a streamline types of flows •Turbulent flow: Erratic, small, whirlpool-like circles called eddy current or eddies which absorbs a lot of energy Flow rate: the mass of fluid that passes the given point per unit time m / t m1 1V1 1 A1l1 1 A1v1 t t t since the total flow must be conserved m1 m2 1 A1v1 2 A2v2 t t Equation of Continuity Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 10 Ex. 13 – 14 for Equation of Continuity How large must a heating duct be if air moving at 3.0m/s through it can replenish the air in a room of 300m3 volume every 15 minutes? Assume the air’s density remains constant. Using equation of continuity 1 A1v1 2 A2v2 Since the air density is constant A1v1 A2v2 Now let’s imagine the room as the large section of the duct A2l2 / t V2 A2 v2 300 2 0.11 m A1 3.0 900 v1 v1 t v1 Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 11 Bernoulli’s Principle Bernoulli’s Principle: Where the velocity of fluid is high, the pressure is low, and where the velocity is low, the pressure is high. Amount of the work done by the force, F1, that exerts pressure, P1, at point 1 W1 F1l1 P1 A1l1 Amount of the work done by the force in the other section of the fluid is W2 P2 A2 l2 Work done by the gravitational force to move the fluid mass, m, from y1 to y2 is W3 mg y2 y1 Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 12 Bernoulli’s Equation cont’d The total amount of the work done on the fluid is W W1 W2 W3 P1 A1l1 P2 A2 l2 mgy2 mgy1 From the work-energy principle 1 1 2 mv2 mv12 P1 A1l1 P2 A2 l2 mgy2 mgy1 2 2 Since the mass m is contained in the volume that flowed in the motion and m A1l1 A2 l2 A1l1 A2l2 Thus, 1 1 2 2 A2 l2 v2 A1l1v1 2 2 P1 A1l1 P2 A2 l2 A2 l2 gy2 A1l1 gy1 Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 13 Bernoulli’s Equation cont’d Since 1 1 A2 l2 v22 A1l1v12 P1 A1l1 P2 A2 l2 A2 l2 gy2 A1l1 gy1 2 2 We obtain 1 2 1 2 v2 v1 P1 P2 gy2 gy1 2 2 Reorganize P1 1 2 1 2 Bernoulli’s v1 gy1 P2 v2 gy2 Equation 2 2 Thus, for any two points in the flow 1 2 P1 v1 gy1 const. 2 For static fluid P2 P1 g y1 y2 P1 gh 1 For the same heights P2 P1 v12 v22 2 Result of Energy conservation! Pascal’s Law The pressure at the faster section of the fluid is smaller than slower section. Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 14 Ex. 13 – 15 for Bernoulli’s Equation Water circulates throughout a house in a hot-water heating system. If the water is pumped at the speed of 0.5m/s through a 4.0cm diameter pipe in the basement under a pressure of 3.0atm, what will be the flow speed and pressure in a 2.6cm diameter pipe on the second 5.0m above? Assume the pipes do not divide into branches. Using the equation of continuity, flow speed on the second floor is 2 A1v1 r12 v1 0.020 v2 0.5 1.2m / s 2 A2 r2 0.013 Using Bernoulli’s equation, the pressure in the pipe on the second floor is 1 P2 P1 v12 v22 g y1 y2 2 1 5 3.0 10 1103 0.52 1.22 1103 9.8 5 2 2.5 105 N / m2 Monday, May 2, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 15 Congratulations!!!! You all are impressive and have done very well!!! I certainly had a lot of fun with ya’ll and am truly proud of you! Good luck with your exam!!! Have safe holidays!!