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
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?
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.
1 atm = 14.7 lbs/in2 = 1.013 x 105 N/m2
 The pressure in water (pure or sea) increases by
about 1 atm for every 10 m of depth.
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
Air
Water
Sea Water
Steal
Lead
Gold
Density
0.00129 g/cm3
1 g/cm3
1.025 g/cm3
7.85 g/cm3
11.389 g/cm3
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
Warm
Fresh
Water
Cold
Fresh
Water
Warm
Sea
Water
Cold
Sea
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
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?
More Practice
3) Two ships of equal mass are made of the same
material, but have different shapes. Both ships are
floating in water.
1
2
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?
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 molecules
 Electrical 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 Diver
Hydraulic 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.
v ↑ P↓
There is a change in pressure when fluid is flowing
around an object. (excludes barriers)
Trucks & Trains
Airplanes
Less Pressure Down
Air
Flow
Wing
More Pressure Up