1. Fluids: Floating & Flying
(Chapter 3)

2. Student Leaning Objectives
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) A 150 pound person steps on your foot with her bare foot.
b) A 150 pound person steps on your foot with a spike heal.
3) What is psi? Where do we typically measure pressure in psi?

5. 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.
Same Depth = Same Pressure
P = Dgh

6. Practice The normal atmospheric pressure at sea level is 1 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
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?
1.5 atm or 22 lbs/in2
Beach in California, pressure goes down with elevation

7. What factors allow floating?
Relative density allows objects or substances to float.
Density is the amount of mass contained in a standard volume.
Density measures how compact the material is.
Example: Golf Ball vs Ping Pong Ball
D = m V

8. Air
D = g/cm3
Water
D = 1 g/cm3
Sea Water
D = g/cm3
Steal
D = 7.85 g/cm3
Lead
D = g/cm3
Gold
D = 19.3 g/cm3
Practice A single gold bar from Fort Knox measures cm, 9.21 cm, and 4.45 cm. How many of these gold bars do you think you could carry?
17.78𝑐𝑚 9.21𝑐𝑚 4.45𝑐𝑚 =728.7 𝑐𝑚 3
(728.7 𝑐𝑚 3 )(19.3g/cm3)=14,063g=14kg≈7 bottle of soda

9. Weight of Floating Object = Weight of Fluid Displaced
Displacement of water allows objects float. (Archimedes Principle)
The buoyancy force is equal to the weight of the fluid displaced by the object.
The real story behind Archimedes’ Eureka!
FB = Wfluid
Weight of Floating Object = Weight of Fluid Displaced

10. The upward buoyancy force is a result of the net upward pressure from the fluid.
500,000 Ton Ship =
500,000 Tons Water Displaced
Largest Pressure vector from greatest depth
Pressures are balanced when a steel ship floats in water.
Pwater = Pboat

11. 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. Why does it have this motion?
2) My body displaces the same volume of air as water; however I do not float in air. Why?
3) Two ships of equal mass are made of the same material, but have different shapes. Both ships are floating in water.
a) Is the buoyancy force greater on ship 1 or ship 2?
b) Which ship’s square cargo area sits higher on the water?
c) Which ship can carry more people? 1 2

12. Practice
4) A toy boat 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 boat?
What does the water weigh that is displaced by the boat?
What is the height of the water line on the boat?
89 N
89 N
𝜌= 𝑚𝑎𝑠𝑠 𝑣𝑜𝑙𝑢𝑚𝑒 = 𝑚𝑎𝑠𝑠 𝑏∙𝑤∙ℎ = 1𝑘𝑔 1000 𝑐𝑚 3
ℎ= (𝑚𝑎𝑠𝑠)(1000 𝑐𝑚 3 ) (𝑏)(𝑤)(1𝑘𝑔) = (8.9𝑘𝑔)(1000 𝑐𝑚 3) (50𝑐𝑚)(20𝑐𝑚)(1𝑘𝑔) =8.9𝑐𝑚

13. Example: Overfull glass of water
Surface tension allows objects to “float” on top of a fluid.
Polar molecules
Electrical Attractions
Cohesion is the attraction between like molecules in a fluid. (water and water)
Adhesion is an attraction between unlike molecules. (water and glass)
Cohesive forces are strongest at the surface (surface tension).
How polarity makes water behave strangely
Example: Overfull glass of water

14. How are Pascal’s principle and Bernoulli’s principle used?
Pascal’s Principle: A change in pressure at any point in an enclosed fluid is exerted on all other parts of the fluid.
A force on an enclosed fluid is distributed throughout the fluid.
Examples:
Closed water bottle
Hydraulic lifts & Pistons

15. small force on small area = Large Force on Large Area
The pressure is equal on both sides of a piston; however, the Force and Area are different.
small force on small area = Large Force on Large Area
Application of Pascals Law

16. Examples: Trucks, Trains, & Airplanes
Bernoulli’s Principle: As the velocity of the fluid increases, the pressure within the fluid decreases.
There is a change in pressure when a fluid flows, or an object moves through the fluid. (excludes barriers)
Coanda Beach Ball
Examples: Trucks, Trains, & Airplanes

17. Unit I Test Chapters 1, 2, and 3 Study!!! Book Notes Online Text
Matching, Multiple Choice, Fill in the Blank, Short Answer
Notes
In-Class Practice
Online
Review Sheet
Quiz-5 points test extra credit