Chapter 13 Liquids

1. Chapter 13Liquids Physics 1 (Garcia) SJSU

2. 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 ) Physics 1 (Garcia) SJSU

3. Sample Problem “Gold” brick is roughly: 12 kilograms [Mass] 120 Newtons [Weight] (0.2 m)x(0.1 m) = (0.02 m2) [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. Physics 1 (Garcia) SJSU

4. B A B A Check Yourself In which case is the pressure greatest? Physics 1 (Garcia) SJSU

5. A B Check Yourself In which case is the pressure greatest? A B Physics 1 (Garcia) SJSU

6. 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. Physics 1 (Garcia) SJSU

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

8. 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. Physics 1 (Garcia) SJSU

9. Buoyancy Weight 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. Pressure Pressure Pressure If weight exceeds buoyancy force then object sinks, otherwise it floats. Physics 1 (Garcia) SJSU

10. Buoyancy & Depth For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth. 1 Buoyancy 2 3 Buoyancy 4 5 Buoyancy 6 Physics 1 (Garcia) SJSU

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

12. Demo: Take a Dip 1000 g A 1 kg mass (1000 g) submerged in water displaces 150 cm3. Water has density of 1 g/cm3 so displaced water has mass of 150 grams. Buoyant force equals weight of 150 grams. Beaker of Water 1000 g 850 g 150 cc displaced Physics 1 (Garcia) SJSU

13. 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 Physics 1 (Garcia) SJSU

14. 40 N Water 10 N Water Check Yourself Place block of wood in the water. Scale reading goes up, down, or stays the same? 10 N Block 50 N ? Physics 1 (Garcia) SJSU

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

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

17. 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/cm3) floating in a cup of mercury (13.6 g/cm3) Physics 1 (Garcia) SJSU

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

19. Ships Density of wood is about 0.5 to 0.8 g/cm3 so not surprising that wooden ships float. Density of iron 7.9 g/cm3. How is it that a battleship can float? Physics 1 (Garcia) SJSU

20. 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. Iron ball, 90% hollow Volume: 1000 cc Mass: 790 g Density: 0.79 g/cc Solid iron ball Volume: 100 cc Mass: 790 g Density: 7.9 g/cc Physics 1 (Garcia) SJSU

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

22. 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. Physics 1 (Garcia) SJSU

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

24. 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.” B A Physics 1 (Garcia) SJSU www.grapestompers.com