Fluids

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 15

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 Demo A = 4  r 2 =.125 m 2 F = 12,000 Newtons (over 2,500 lbs)! 23

Pascal’s Principle l A change in pressure at any point in a confined fluid is transmitted everywhere in the fluid. l Hydraulic Lift l 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 27

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

Dam Question 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 A B F = P A, and pressure is  gh. Same pressure, same Area same force even though more water in B! 35

h p 2 =p atm p 1 =0 37 Example: Barometer: a way to measure atmospheric pressure

h p 2 =p atm p 1 =0 37 Example:Water Barometer:

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 2. Yes 40

Archimedes’ Principle l Determine force of fluid on immersed cube è Draw FBD 43 l Buoyant force is weight of displaced fluid!

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? 48 mg FbFb Mg h

Summary 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