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

Physics 101: Lecture 17, Pg 2 Homework Help A block of mass m 1 = 3 kg rests on a table with which it has a coefficient of friction µ = 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. x y

Physics 101: Lecture 17, Pg 3Overview  F = m a  Fx = Change in Kinetic Energy  Ft = Change in momentum  = I   oday look at application of these ideas to molecules in fluids

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

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

Physics 101: Lecture 17, Pg 6 Atmospheric Pressure l Basically weight of atmosphere! l Air molecules are colliding with you right now! l Pressure = 1x10 5 N/m 2 = 14.7 lbs/in 2 ! l Example: Sphere w/ R = 0.1 m è Spheres demo

Physics 101: Lecture 17, Pg 7 Pascal’s Principle l 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

Physics 101: Lecture 17, Pg 8 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. A) P 1 > P 2 B) P 1 = P 2 C) P 1 < P 2 Under water P = P atmosphere +  g h 1 2

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

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

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

Physics 101: Lecture 17, Pg 12 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 13 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 14Summary l Pressure is force exerted by molecules “bouncing” off container P = F/A l Gravity/weight effects pressure  P = P 0 +  gd Buoyant force is “weight” of displaced fluid. F =  g V