19-Sep-15 Physics 1 (Garcia) SJSU Chapter 13 Liquids

19-Sep-15 Physics 1 (Garcia) SJSU Pressure Pressure is defined as Pressure = Metric unit of pressure is Pascal. 1 Pascal = 1 Newtons per square meter Atmospheric pressure is about 100,000 Pascals ( Force ) ( Area )

19-Sep-15 Physics 1 (Garcia) SJSU Sample Problem “Gold” brick is roughly: 12 kilograms [Mass] 120 Newtons [Weight] (0.2 m)x(0.1 m) = (0.02 m 2 ) [Area] Pressure on surface is (120)/(0.02) = 6000 Pascals Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the “gold” brick.

19-Sep-15 Physics 1 (Garcia) SJSU Check Yourself In which case is the pressure greatest? A B A B

19-Sep-15 Physics 1 (Garcia) SJSU Check Yourself In which case is the pressure greatest? A B A B

19-Sep-15 Physics 1 (Garcia) SJSU Demo: Bed of Nails One may safely lay or sit on a bed of nails, as long as there are enough nails so that the pressure, measured as force per nail, is small. Weight of 150 pounds is distributed over 300 nails. Force per nail is ½ lb. Need 5 lb per nail to pierce skin.

19-Sep-15 Physics 1 (Garcia) SJSU Pressure in Liquids Pressure in a liquid depends on depth. As with bricks, weight of what’s above determines pressure. High Medium Low High Medium Low

19-Sep-15 Physics 1 (Garcia) SJSU Demo: Pressure & Weight A can full of water has holes in the sides through which water comes out. What happens when you drop the can? The can is now in freefall and weightless. Water stops flowing as the can falls since the pressure was due to the water’s weight.

19-Sep-15 Physics 1 (Garcia) SJSU Buoyancy Since pressure depends on depth, a submerged object has more force due to pressure below it than above it. Net effect is to have a net upward force, which we call buoyancy. Buoyancy Weight If weight exceeds buoyancy force then object sinks, otherwise it floats. Pressure

19-Sep-15 Physics 1 (Garcia) SJSU Buoyancy & Depth For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth Buoyancy

19-Sep-15 Physics 1 (Garcia) SJSU Archimedes’ Principle Weight of liquid displaced by floating or submerged object equals the buoyant force on the object.

19-Sep-15 Physics 1 (Garcia) SJSU Demo: Take a Dip A 1 kg mass (1000 g) submerged in water displaces 150 cm 3. Water has density of 1 g/cm 3 so displaced water has mass of 150 grams. Buoyant force equals weight of 150 grams g 850 g 150 cc displaced Beaker of Water

19-Sep-15 Physics 1 (Garcia) SJSU Demo: Floating in the Tub Buoyant force depends only on the volume of fluid displaced, not on the volume of fluid in which the object is immersed. Rubber duck floats the same in a little water or in a full beaker of water

19-Sep-15 Physics 1 (Garcia) SJSU Check Yourself ? Place block of wood in the water. Scale reading goes up, down, or stays the same? 50 N 10 N Block 10 N Water 40 N Water

19-Sep-15 Physics 1 (Garcia) SJSU Check Yourself Use similar block of petrified wood. Scale reading goes up, down, or stays the same? ? 50 N 30 N Block 10 N Water 40 N Water

19-Sep-15 Physics 1 (Garcia) SJSU Check Yourself Did the designers of this “water bridge” have to account for the weight of ships or just the water?

19-Sep-15 Physics 1 (Garcia) SJSU Density & Floating By Archimede’s principle, a solid object will float if the density of the object is less than the density of the liquid. Billiard ball (4.0 g/cm 3 ) floating in a cup of mercury (13.6 g/cm 3 )

19-Sep-15 Physics 1 (Garcia) SJSU Demo: Diet or Regular? What can you say about the density of diet cola as compared with regular cola?

19-Sep-15 Physics 1 (Garcia) SJSU Ships Density of wood is about 0.5 to 0.8 g/cm 3 so not surprising that wooden ships float. Density of iron 7.9 g/cm 3. How is it that a battleship can float?

19-Sep-15 Physics 1 (Garcia) SJSU Density & Floating, Revisited When an object is not solid then it floats if the average density, (total mass)/(total volume), is less than the density of the liquid. Solid iron ball Volume: 100 cc Mass: 790 g Density: 7.9 g/cc Iron ball, 90% hollow Volume: 1000 cc Mass: 790 g Density: 0.79 g/cc

19-Sep-15 Physics 1 (Garcia) SJSU Iron Ships, Floating & Sinking Iron ship floats since it is hollow inside. If water floods the inside then it sinks.

19-Sep-15 Physics 1 (Garcia) SJSU Floating & Liquid Density The greater the density of a liquid, the greater the buoyant force on objects floating or immersed in the liquid. Floating in Great Salt Lake, Utah is easy because the lake water is dense due to high concentration of salt.

19-Sep-15 Physics 1 (Garcia) SJSU Demo: Hydrometer Hydrometer is a calibrated float; denser the liquid, the higher the hydrometer floats.

19-Sep-15 Physics 1 (Garcia) SJSU Density & Winemaking “Take a look at Diagram A, on the left, which represents a test jar full of must before the yeast is pitched. Do you see how the hydrometer is floating rather high? This is because the liquid is "heavy" with all the sugar... the hydrometer is pushed up because of this. As the yeast turns the sugar into alcohol and carbon dioxide, the wine becomes lighter (alcohol weighs less per unit volume) and the hydrometer doesn't float as high as it once did. Diagram B (seen on the right) represents a wine that has fermented to dryness and is lighter than water.” A B