Fluids: Floating & Flying

(Chapter 3)

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.

How is pressure different from force?

Pressure is determined from how the force is distributed.

P = F A

Practice1) 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?

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

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 = 1.013 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?

What factors allow floating?

Relative density allows objects or substances to float.

Density is the amount of mass contained in a standard volume.

D = m V

Density measures how compact the material is.

Golf Ball vs Ping Pong Ball

Material Density

Air 0.00129 g/cm3

Water 1 g/cm3

Sea Water 1.025 g/cm3

Steal 7.85 g/cm3

Lead 11.389 g/cm3

Gold 19.3 g/cm3

Practice

A single gold bar from Fort Knox is about

7 x 3.5 x 2 inches

The actual dimensions are 17.78 cm, 9.21 cm, and 4.45 cm. How many of these gold bars do you think you could carry?

Archimedes Principle

Displacement of water allows objects float.

The buoyancy force is equal to the weight of the fluid displaced by the object.

FB = Wfluid

Two objects with the same mass/weight

WarmFreshWater

ColdFreshWater

WarmSea

Water

ColdSea

Water

Weight of Floating Object = Weight of Fluid Displaced

500,000 Ton Ship=

500,000 Tons Water Displaced

The upward buoyancy force is a result of the net upward pressure from the fluid.

Pressures are balanced when a boat floats in water.

Pwater = Pboat

Largest Pressure vector from greatest depth

Practice1) 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?

More Practice 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

The Math 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.

 a) What is the buoyant force on the boat?

b) What does the water weigh that is displaced by the boat?

c) What is the height of the water line on the boat?

Surface tension allows objects to “float” on top of a fluid.

Polar moleculesElectrical Attractions

Surface Tension

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 are Pascal’s principle and Bernoulli’s principle used?

Pascal’s Principle allows force to be multiplied.

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.

Cartesian DiverHydraulic Lifts

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

Bernoulli’s Principle allows jet planes to fly.

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 ↓

Trucks & Trains

Airplanes

Less Pressure Down

More Pressure Up

AirFlow

Wing