Physics 207: Lecture 21, Pg 1Physics 207,Physics 207, Lecture 21, NovLecture 21, Nov 16 16 ‘‘0909TODAY:TODAY:Static FluidsStatic Fluids Density Buoyancy Bulk modulusASSIGNMENTS:ASSIGNMENTS:Read Read Chaper Chaper 15 15 ‘‘FluidsFluids’’Read Chapter 20.1 - 20.6 Read Chapter 20.1 - 20.6 Travelling Travelling WavesWavesRead Read Chaper Chaper 21.1 - 21.421.1 - 21.4 SuperpositionSuperpositionLab thisLab this week -week - Rotational Acceleration and Moment of InertiaRotational Acceleration and Moment of InertiaDonDon’’t forget the t forget the Prelab Prelab questions!questions!Physics 207: Lecture 21, Pg 4Atmospheric PressureAtmospheric PressureThe global average sea-level pressure is 101,300 Pa.Consequently we define the standard atmosphere as1 Pa = 1 N/m2Physics 207: Lecture 21, Pg 5Pressure in LiquidsPressure in LiquidsThe pressure at depth d in a liquid iswhere ρ isyou proved this for HW last week…. theliquid’s density, and p0 is the pressure at the surface of theliquid. Because the fluid is at rest, the pressure is calledthe hydrostatic pressure. The fact that g appears in theequation reminds us that there is a gravitationalcontribution to the pressure.Physics 207: Lecture 21, Pg 6Gauge PressureGauge PressureMany pressure gauges, such as tire gauges and the gaugeson air tanks, measure not the actual or absolute pressure pbut what is called gauge pressure pg.where 1 atm = 101.3 kPa.Physics 207: Lecture 21, Pg 7the net force due to the atmospheric pressure on the flat plate.Physics 207: Lecture 21, Pg 9Physics 207: Lecture 21, Pg 10Atmospheric Pressure at the Earth's Surface:Atmospheric Pressure at the Earth's Surface:Lines indicate "Isobars": -points of equal pressure (normalized to sea level)Pressures indicated in millibars 1 bar =100 kPaAverage atmospheric pressure at sea level = 101.3kPa ~ 1 barPhysics 207: Lecture 21, Pg 17ElasticityElasticityandandYoungYoung’’s Moduluss Modulus(15.6)(15.6)! "LL=FYAY - Young’s Modulus! F =YAL" # $ % & ' (LSpring constant, kPhysics 207: Lecture 21, Pg 18Volume Stress and the Bulk ModulusVolume Stress and the Bulk ModulusPhysics 207: Lecture 21, Pg 19Volume Stress and the Bulk ModulusVolume Stress and the Bulk Modulus• A volume stress applied to an object compresses its volume slightly.• The volume strain is defined as ΔV/V, and is negative when the volume decreases.• Volume stress is the same as the pressure. where B is called the bulk modulus. The negative sign in the equation ensures that the pressure is a positive number.Physics 207: Lecture 21, Pg 20Physics 207: Lecture 21, Pg 22WhatWhat’’s the pressure at thes the pressure at thebottom of the ocean at a depthbottom of the ocean at a depthof 10 km?of 10 km?A. 10 atmB. 100 atmC. 1000 atmD. 10,000 atmPhysics 207: Lecture 21, Pg 23By what percentageBy what percentage is the density of wateris the density of waterhigherhigher at a depth of 10 km than at atmosphericat a depth of 10 km than at atmosphericpressure?pressure?A. Same densityB. 1%C. 5%D. 10%E. 100%Physics 207: Lecture 21, Pg 25PascalPascal’’s Principle:s Principle:Pressure in a closed tubePressure in a closed tubeQUESTION:“Pressure applied to an enclosed liquid is transmitted undiminished to every point in the fluid and to the walls of the container.”Physics 207: Lecture 21, Pg 26Physics 207: Lecture 21, Pg 27HydraulicsHydraulicsConsider a hydraulic lift, such as the one that lifts your carat the repair shop. The system is in static equilibrium if! F1A1=F2A2+"ghPhysics 207: Lecture 21, Pg 32BuoyancyBuoyancyWhen an object (or portion of an object) is immersed in afluid, it displaces fluid. The displaced fluid’s volumeequals the volume of the portion of the object that isimmersed in the fluid.Suppose the fluid has density ρf and the object displacesvolume Vf of fluid. Archimedes’ principle in equation
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