Liquids

1. Liquids

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 )

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.

4. B A B A Check Yourself In which case is the pressure greatest?

5. A B Check Yourself In which case is the pressure greatest? A B

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.

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

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.

9. Extensive and Intensive • Extensive properties – depends on how much of the substance is present. Ex. – mass, length, volume • Intensive properties – does not matter how much you have, the property is the same. Ex. – density, specific heat, freezing point, boiling point

10. 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.

11. 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

12. Archimedes’ Principle Weight of liquid displaced by floating or submerged object equals the buoyant force on the object.

13. 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

14. 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

15. 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 ?

16. 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 ?

17. 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?

18. 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)

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

20. Physics 1 (Garcia) SJSU 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?

21. 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. 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

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

23. 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.

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

25. Physics 1 (Garcia) SJSU What is Viscosity? • Resistance of a fluid to a change in shape, or movement of neighboring portions relative to one another. Viscosity denotes opposition to flow. It may also be thought of as internal friction between the molecules.

26. Physics 1 (Garcia) SJSU Viscosity • Viscosity is a major factor in determining the forces that must be overcome when fluids are used in lubrication or transported in pipelines. It also determines the liquid flow in spraying, injection molding, and surface coating.

27. Viscosity Notes • Fluids with low viscosity flow quickly • Fluids with high viscosity flow slowly • heating a fluid decreases viscosity; cooling a fluid increases viscosity • Ex: Warm honey flows more quickly than cold honey.