Physics 101 Lecture 23 Fluids l Today s lecture

Physics 101: Lecture 23 Fluids l Today’s lecture will cover Textbook Sections 11. 1 -11. 5 èDensity èPressure èPascal’s Principle Physics 101: Lecture 23, Pg 1

Physics 101: Density l l Density = Mass/Volume è = M/V èSI unit: [kg/m 3] Densities of some common things (kg/m 3) èWater 1000 èice 917 (floats on water) èblood 1060 (sinks in water) èlead 11, 300 èCopper 8890 èMercury 13, 600 èAluminum 2700 èWood 550 èair 1. 29 èHelium 0. 18 Physics 101: Lecture 23, Pg 2

Pressure that a fluid or gas exerts on a surface: P = F / A SI unit: [N/m 2= Pa] F: force acting perpendicular to surface A: Area of surface Atmospheric presure: 1. 01 105 Pa Physics 101: Lecture 23, Pg 3

Pressure and Depth Barometer: a way to measure atmospheric pressure p 2 = p 1 + gh p 1=0 patm = gh Measure h, determine patm p 2=patm h example--Mercury = 13, 600 kg/m 3 patm = 1. 05 x 105 Pa h = 0. 757 m = 757 mm = 29. 80” (for 1 atm) Physics 101: Lecture 23, Pg 4

Concept Question Suppose you have a barometer with mercury and a barometer with water. How does the height hwater compare with the height hmercury? 1. hwater is much larger than hmercury 2. hwater is a little larger than hmercury 3. hwater is a little smaller than hmercury 4. hwater is much smaller than hmercury CORRECT water is much less dense than mercury, so the same amount of pressure will move the water farther up the column. p 1=0 p 2=patm h Physics 101: Lecture 23, Pg 5

Concept Question Is it possible to stand on the roof of a five story (50 foot) tall house and drink, using a straw, from a glass on the ground? 1. No CORRECT 2. Yes Even if a person could completely remove all of the air from the straw, the height to which the outside air pressure moves the water up the straw would not be high enough for the person to drink the water. Physics 101: Lecture 23, Pg 6

Concept Question p=0 pa h Evacuate the straw by sucking How high will water rise? no more than h = Pa/ g = 10. 3 m = 33. 8 feet no matter how hard you suck! Physics 101: Lecture 23, Pg 7

Summary • Density • Pressure P 2 = P 1 + gh Pascal’s Principle Physics 101: Lecture 23, Pg 8