1

13 Liquids

• Demos:

• Surface tension (pepper, soap)

• Homework:

• RQ: 2, 4, 7, 10, 12, 14, 21.

• Problems: 2, 10.

2

Pressure

• pressure = force/area

• Example: 500N applied to 0.10m2. Pressure = 500/0.10 = 5,000N/m2.

• Example: atmospheric pressure is 100,000 N/m2.

3

Depth and Fluid Pressure• pressure = weight density x depth

• depth only (not area), pressure equal at bottom of all shapes below

4

Buoyancy and Archimedes’ Principle

5

sink or float?

• object more dense than fluid sinks

• object less dense than fluid floats

• object same density as fluid is neutral

6

Water Density & Volumes

• “L” = liter (about 33oz)

• 1mL = 0.001L = 1cc

• 1mL water = 1gram

• water density = 1gram/mL

Example Volumes

• An 800 gram object displaces 400mL (submerged). What is its density?

• Density = mass/volume

• = 800grams/400mL

• = 800grams/400cm3

• = 2g/cm3

7

Example 1 Arch.

• A 1kg object displaces 200mL of water (submerged). Does it sink or float?

• Density = 1000g/200mL

• = 5g/mL = 5g/cm3

• Density > 1 Sinks

8

Example 2 Arch.

• A 1kg object displaces 1200mL of water (submerged). Does it sink or float?

• Density = 1000g/1200mL

• = 0.83g/mL = 0.83g/cm3

• Density < 1g/cm3 Floats

9

10

Pascal’s Principle• pressure increases on an enclosed fluid are

transmitted undiminished to entire fluid.

• Hydraulic Application:

• output force = (area ratio)x(input force)

11

surface tension• cohesive nature of surface of a fluid, e.g.

waterbug, pin floating on water

• hot water has less surface tension, as does soapy water

Videos

• Metal boats

• Effect of Screen

• Soap in water, floating

• Soap in milk, food coloring

• Water, pepper

12

13

surface area

• sphere has smallest ratio of surface area/volume of any shape

• surface tension causes fluids to form ‘drops’ (water has more than 3 times the surface tension of alcohol)

14

Summary• pressure = force/area

• pressure ~ depth

• buoyant force equals weight of displaced fluid

• pressure changes are distributed throughout enclosed fluids

15

Summary 2

• surface tension phenomena

• surface area/volume ratio low for sphere, high for sheet

16

Ex: Sphere 12m3.

• area/volume = 4R2/(4/3)R3 = 3/R.

• Example: volume = 12 cubic meters.

• R = 1.42 meters

• area/volume = 3/1.42meter = 2.11/meter

17

12m3 rectangular plates

• 1) height = 3m, width = 2m, depth = 2m• 2) height = 0.75m, width = 4m, depth = 4m• 3) height = 0.12m, width = 10m, depth = 10m

• calculate area/volume ratio for these three objects.

• How do these ratios compare to the sphere?