1. Fluids: Floating & Flying
(Chapter 3b)

2. Student Leaning Objectives
Analyze fluid principles and apply to floating and flying
Distinguish between force and pressure
Recall factors that allow floating
Differentiate between cohesion and adhesion
Analyze Pascal’s principle and Bernoulli’s principle

3. How is pressure different from force?
Pressure is determined from how the force is distributed.
P = F A

4. Practice 1) Why do large trucks have so many tires?
2) Which would result in the greater pressure on your bare foot?
A 150 pound person steps on your foot with her bare foot.
A 150 pound person steps on your foot with a spike heal.
3) What is psi? Where do we typically measure pressure in psi?

6. What determines the amount of pressure in a fluid?
In a fluid, pressure is exerted in all directions.
Fluid pressure depends on density and depth.
P = Dgh
Same Depth
Same Pressure

7. Atmospheric Pressure
The normal atmospheric (air) pressure at sea level is one atmosphere.
The pressure in water (pure or sea) increases by about 1 atm for every 10 m of depth.
1 atm = 14.7 lbs/in2 = x 105 N/m2

8. Practice
1) How much pressure would a scuba diver experience 5 meters down in water? 2) Where would the air pressure be greater, Denver, CO (elevation 5000 ft) or at the beach in California (elevation 0 ft)? Why?

9. What factors allow floating?
Relative density allows objects or substances to float.
Density is the amount of mass contained in a standard volume. (Compactness)
D = m V

10. Practice Which of these would float on water?
Which of these is most compact?
Is it possible for steel to float on water?
Material
Density
Air
g/cm3
Water
1 g/cm3
Sea Water
1.025 g/cm3
Steel
7.85 g/cm3
Lead
g/cm3
Gold
19.3 g/cm3

11. Archimedes Principle FB = Wfluid
Displacement of water allows objects float.
The buoyancy force is equal to the weight of the fluid displaced.
FB = Wfluid

12. 500,000 Ton Ship =
500,000 Tons Water Displaced

13. Largest pressure vector
The buoyancy force is a result of the net upward pressure from the fluid.
Pressures balance when an object floats.
Pwater = Pboat
Largest pressure vector
from greatest depth

14. Practice
1) A helium balloon that is released into the sky will rise for a time, and then maintain the same altitude as it travels away from you. Explain this motion in terms of floating. 2) My body displaces the same volume of air as water; however I do not float in air. Why?

15. More Practice
 3) Two ships of equal mass are made of the same material, but have different shapes. Both ships are floating in water.
Is the buoyancy force greater on ship 1 or ship 2?
Which ship’s square cargo area sits higher on the water?
Which ship can carry more people? 1 2

16. The Math Practice
4) A toy raft weighs 20 lbs (89 N) and floats in fresh water. The boat has a flat bottom that measures 50 cm by 20 cm.
What is the buoyant force on the raft?
What does the water weigh that is displaced by the raft?
What is the height of the water line on the raft?
If the raft travels out into the cold sea, what will change?

17. Cohesion & Adhesion
Surface tension allows objects to “float” on top of a fluid.
Cohesion attraction between like molecules in a fluid (strongest at surface) Adhesion attraction between unlike molecules

18. How is Pascal’s Principle applied?
A change in pressure at any point in an enclosed fluid is exerted on all other parts of the fluid.
The same force is applied to every part of the fluid.

19. How is Bernoulli’s Principle applied?
As the velocity of the fluid increases, the pressure within the fluid decreases.
There is a change in pressure when fluid is flowing around an object. (excludes barriers)
v ↑ P ↓

20. Trucks & Trains

21. Airplanes
Less Pressure Down
Wing
Air
Flow
More Pressure Up