Physics 101 Lecture 17 Fluids l Today s lecture

Physics 101: Lecture 17 Fluids l Today’s lecture will cover Textbook Chapter 9. 1 -9. 6 Physics 101: Lecture 17, Pg 1

Homework Help A block of mass m 1 = 3 kg rests on a table with which it has a coefficient of friction µ = 0. 73. A string attached to the block passes over a pulley to a block of mass m 3 = 5 kg. The pulley is a uniform disk of mass m 2 = 0. 7 kg and radius 15 cm. As the mass m 3 falls, the string does not slip on the pulley. y x Physics 101: Lecture 17, Pg 2 10

Overview l SF =ma l SFx = Change in Kinetic Energy l SFt = Change in momentum l St =Ia l Today look at application of these ideas to molecules in fluids Physics 101: Lecture 17, Pg 3 10

States of Matter l Solid è Hold Volume è Hold Shape l Liquid Fluids è Hold Volume è Adapt Shape l Gas è Adapt Volume è Adapt Shape Physics 101: Lecture 17, Pg 4 11

Qualitative Demonstration of Pressure l Force due to molecules of fluid colliding with container. è Impulse = Dp l Average Pressure = F / A 1 N/m 2 = 1 Pa y ball drop Physics 101: Lecture 17, Pg 5 16

Atmospheric Pressure l Basically weight of atmosphere! l Air molecules are colliding with you right now! l Pressure = 1 x 105 N/m 2 = 14. 7 lbs/in 2! l Example: Sphere w/ R = 0. 1 m è Spheres demo Physics 101: Lecture 17, Pg 6 21

Pascal’s Principle A change in pressure at any point in a confined (i. e. not moving) fluid is transmitted everywhere in the fluid. l Hydraulic Lift l The truck is raised a distance d 2. Compare the work done by F 1 with the work done by F 2 A) W 1 > W 2 B) W 1 = W 2 C) W 1 < W 2 l Physics 101: Lecture 17, Pg 7 26

Gravity and Pressure l Two identical “light” containers are filled with water. The first is completely full of water, the second container is filled only ½ way. Compare the pressure each container exerts on the table. 1 A) P 1 > P 2 l Under B) P 1 = P 2 2 C) P 1 < P 2 water P = Patmosphere + r g h Physics 101: Lecture 17, Pg 8 30

Dam ACT B A A Two dams of equal height prevent water from entering the basin. Compare the net force due to the water on the two dams. A) FA > FB B) FA=FB C) FA< FB Physics 101: Lecture 17, Pg 9 33

Pressure and Depth Barometer: A way to measure atomospheric pressure For non-moving fluids, pressure depends only on depth. p 1=0 p 2=patm h Physics 101: Lecture 17, Pg 10 35

Preflight 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? p=0 1. No pa h 2. Yes Physics 101: Lecture 17, Pg 11 40

Archimedes’ Principle l Determine force of fluid on immersed cube è Draw FBD l Buoyant force is weight of displaced fluid! Physics 101: Lecture 17, Pg 12 43

Archimedes Example A cube of plastic 4. 0 cm on a side with density = 0. 8 g/cm 3 is floating in the water. When a 9 gram coin is placed on the block, how much sinks below water surface? h Physics 101: Lecture 17, Pg 13 49

Summary l Pressure is force exerted by molecules “bouncing” off container P = F/A l Gravity/weight effects pressure è = P 0 + rgd P l Buoyant force is “weight” of displaced fluid. F = r g V Physics 101: Lecture 17, Pg 14