Fluids: Floating & Flying (Chapter 3)

Student Leaning Objectives Distinguish between force and pressure Recall factors that allow floating Differentiate between cohesion and adhesion Analyze Pascal’s principle and Bernoulli’s principle.

How is pressure different from force? Pressure is determined from how the force is distributed. P = F A

Practice 1) Why do large trucks have so many tires?   2) Which would result in the greater pressure on your bare foot? a) A 150 pound person steps on your foot with her bare foot. b) A 150 pound person steps on your foot with a spike heal. 3) What is psi? Where do we typically measure pressure in psi?

What determines the amount of pressure in a fluid? In a fluid, pressure is exerted in all directions. Fluid pressure depends on density and depth. Same Depth = Same Pressure P = Dgh

Practice The normal atmospheric pressure at sea level is 1 atmosphere. The pressure in water (pure or sea) increases by about 1 atm for every 10 m of depth. 1 atm = 14.7 lbs/in2 = 1.013 x 105 N/m2 Practice 1) How much pressure would a scuba diver experience 5 meters down in water?   2) Where would the air pressure be greater, Denver, CO (elevation 5000 ft) or at the beach in California (elevation 0 ft)? Why? 1.5 atm or 22 lbs/in2 Beach in California, pressure goes down with elevation

What factors allow floating? Relative density allows objects or substances to float. Density is the amount of mass contained in a standard volume. Density measures how compact the material is. Example: Golf Ball vs Ping Pong Ball D = m V

Air D = 0.00129 g/cm3 Water D = 1 g/cm3 Sea Water D = 1.025 g/cm3 Steal D = 7.85 g/cm3 Lead D = 11.389 g/cm3 Gold D = 19.3 g/cm3 Practice A single gold bar from Fort Knox measures 17.78 cm, 9.21 cm, and 4.45 cm. How many of these gold bars do you think you could carry? 17.78𝑐𝑚 9.21𝑐𝑚 4.45𝑐𝑚 =728.7 𝑐𝑚 3 (728.7 𝑐𝑚 3 )(19.3g/cm3)=14,063g=14kg≈7 bottle of soda

Weight of Floating Object = Weight of Fluid Displaced Displacement of water allows objects float. (Archimedes Principle) The buoyancy force is equal to the weight of the fluid displaced by the object. The real story behind Archimedes’ Eureka! FB = Wfluid Weight of Floating Object = Weight of Fluid Displaced http://physics.weber.edu/carroll/archimedes/principle.htm http://resources.yesican-science.ca/lpdd/g08/lp/unit1_archimedes.html

The upward buoyancy force is a result of the net upward pressure from the fluid. 500,000 Ton Ship = 500,000 Tons Water Displaced Largest Pressure vector from greatest depth Pressures are balanced when a steel ship floats in water. Pwater = Pboat

Practice 1) A helium balloon that is released into the sky will rise for a time, and then maintain the same altitude as it travels away from you. Why does it have this motion?   2) My body displaces the same volume of air as water; however I do not float in air. Why? 3) Two ships of equal mass are made of the same material, but have different shapes. Both ships are floating in water. a) Is the buoyancy force greater on ship 1 or ship 2? b) Which ship’s square cargo area sits higher on the water? c) Which ship can carry more people? 1 2

Practice 4) A toy boat weighs 20 lbs (89 N) and floats in fresh water. The boat has a flat bottom that measures 50 cm by 20 cm.   What is the buoyant force on the boat? What does the water weigh that is displaced by the boat? What is the height of the water line on the boat? 89 N 89 N 𝜌= 𝑚𝑎𝑠𝑠 𝑣𝑜𝑙𝑢𝑚𝑒 = 𝑚𝑎𝑠𝑠 𝑏∙𝑤∙ℎ = 1𝑘𝑔 1000 𝑐𝑚 3 ℎ= (𝑚𝑎𝑠𝑠)(1000 𝑐𝑚 3 ) (𝑏)(𝑤)(1𝑘𝑔) = (8.9𝑘𝑔)(1000 𝑐𝑚 3) (50𝑐𝑚)(20𝑐𝑚)(1𝑘𝑔) =8.9𝑐𝑚

Example: Overfull glass of water Surface tension allows objects to “float” on top of a fluid. Polar molecules Electrical Attractions Cohesion is the attraction between like molecules in a fluid. (water and water) Adhesion is an attraction between unlike molecules. (water and glass) Cohesive forces are strongest at the surface (surface tension). http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html How polarity makes water behave strangely Example: Overfull glass of water

How are Pascal’s principle and Bernoulli’s principle used? Pascal’s Principle: A change in pressure at any point in an enclosed fluid is exerted on all other parts of the fluid. A force on an enclosed fluid is distributed throughout the fluid. Examples: Closed water bottle Hydraulic lifts & Pistons

small force on small area = Large Force on Large Area The pressure is equal on both sides of a piston; however, the Force and Area are different. small force on small area = Large Force on Large Area Application of Pascals Law

Examples: Trucks, Trains, & Airplanes Bernoulli’s Principle: As the velocity of the fluid increases, the pressure within the fluid decreases. There is a change in pressure when a fluid flows, or an object moves through the fluid. (excludes barriers) Coanda Beach Ball Examples: Trucks, Trains, & Airplanes

Unit I Test Chapters 1, 2, and 3 Study!!! Book Notes Online Text Matching, Multiple Choice, Fill in the Blank, Short Answer Notes In-Class Practice Online Review Sheet Quiz-5 points test extra credit