Physics 121: Lecture 20 Today's Agenda Announcements Homework 8: due Friday Nov. 11@6: 00 PM Chap8:#7,22,28,33,35,44,45,50,54,61,and65 Today's topics Fluid and solids Pressure measurement Archimedes principle Fluids in motion Physics 121: Lecture 20, Pg
Physics 121: Lecture 20, Pg 1 Physics 121: Lecture 20 Today’s Agenda Announcements Homework 8: due Friday Nov. 11 @ 6:00 PM. Chap. 8: # 7, 22, 28, 33, 35, 44, 45, 50, 54, 61, and 65. Today’s topics Fluid and solids Pressure measurement Archimedes’ principle Fluids in motion
Fluids: Chapter 9 At ordinary temperature, matter exists in one of three states Solid -has a shape and forms a surface Liquid -has no shape but forms a surface Gas-has no shape and forms no surface What do we mean by fluids"? F| uids are“ substances that flow”..."“ substances that take the shape of the container Atoms and molecules are free to move No long range correlation between positions Physics 121: Lecture 20, Pg 2
Physics 121: Lecture 20, Pg 2 Fluids : Chapter 9 At ordinary temperature, matter exists in one of three states Solid - has a shape and forms a surface Liquid - has no shape but forms a surface Gas - has no shape and forms no surface What do we mean by “fluids”? Fluids are “substances that flow”…. “substances that take the shape of the container” Atoms and molecules are free to move. No long range correlation between positions
Fluids What parameters do we use to describe fluids? Density units kg/m3=10-3 g/cm3 Pressure units 1 N/m2= 1 Pa(Pascal 1 bar =105 Pa 1 mbar= 102 Pa 1 torr 133.3Pa Any force exerted by a fluid is perpendicular to a surface of contact, and is proportional to the area of that surface Force(a vector)in a fluid can be expressed in terms of pressure (a scalar)as F=pAn A Physics 121: Lecture 20, Pg 3
Physics 121: Lecture 20, Pg 3 Fluids What parameters do we use to describe fluids? Density Pressure units : kg/m3 = 10-3 g/cm3 units : 1 N/m2 = 1 Pa (Pascal) 1 bar = 105 Pa 1 mbar = 102 Pa 1 torr = 133.3 Pa F = pAn ˆ A n Any force exerted by a fluid is perpendicular to a surface of contact, and is proportional to the area of that surface. Force (a vector) in a fluid can be expressed in terms of pressure (a scalar) as:
Fluids Bulk modulus △ B △∨/V) LIQUID: incompressible (density almost constant GAS: compressible(density depends a lot on pressure) Pb Gas(STP) HO Steel 10410 10 10 10 101l Bulk modulus(Pa=N/m2) Physics 121: Lecture 20, Pg
Physics 121: Lecture 20, Pg 4 Fluids Bulk Modulus ( V / V ) p B − = LIQUID: incompressible (density almost constant) GAS: compressible (density depends a lot on pressure) Bulk modulus (Pa=N/m2 ) Gas (STP) H2O Steel Pb
Pressure vs Depth Incompressible fluids (liquids) When the pressure is much less than the bulk modulus of the fluid we treat the density as constant y1 independent of pressure incompressible fluid A For an incompressible fluid the density is the same everywhere mg F2 but the pressure is not Consider an imaginary fluid volume(a cube, face area A) The sum of all the forces on this volume must be zero as it is in equilibrium: F2-F1-mg=0 mg=Py、01A F=p2A →|p2=p1+pg(y2-y y,)Ag Physics 121: Lecture 20, Pg 5
Physics 121: Lecture 20, Pg 5 When the pressure is much less than the bulk modulus of the fluid, we treat the density as constant independent of pressure: incompressible fluid For an incompressible fluid, the density is the same everywhere, but the pressure is NOT! Pressure vs. Depth Incompressible Fluids (liquids) Consider an imaginary fluid volume (a cube, face area A) The sum of all the forces on this volume must be ZERO as it is in equilibrium: F2 - F1 - mg = 0 y 1 y2 A p 1 p 2 F1 F 2 mg 0 p F2 − F1 = p2 A − p1 A mg = ( y2 − y1 )Ag p p g( y y ) 2 = 1 + 2 − 1