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

7. Example Volumes
An 800 gram object displaces 400mL (submerged). What is its density?
Density = mass/volume
= 800grams/400mL
= 800grams/400cm3
= 2g/cm3

8. 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

9. 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

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

12. Videos Metal boats Effect of Screen Soap in water, floating
Soap in milk, food coloring
Water, pepper

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 = 4pR2/(4/3)pR3 = 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
calculate area/volume ratio for these three objects.
How do these ratios compare to the sphere?