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Air Resistance, Free Fall
Motion and Falling Objects
Chapter 3.3
Lesson-Specific Learning Targets
(10/20)
I can identify the composition of the
Earth’s atmosphere
 I can define air resistance and why it is a
force
 I can define terminal velocity

Starter Q (10/20) Provide
answers to these three
questions: (see handout)
Starter Q (10/24) Provide answers to these three
questions:
1. Describe the Earth’s atmosphere.
(What is it and what is it made of?)
 2. Describe air resistance (what causes it
and why is it considered a force?)
 3. Describe the term aerodynamic shape.
(What does it mean? Give an example)

Starter Q (10/20)
2.
Describe air resistance (what causes it
and why is it considered a force?)
Starter Q (10/20)

3. Describe the term aerodynamic shape.
(What does it mean? Give an example)
Earth’s Atmosphere
Major Constituents
•Nitrogen
(N2)
78%
•Oxygen
(O2)
21%
•Argon
(Ar)
< 1%
Minor Constituents
Water vapor
(H2O)
Carbon dioxide
(CO2)
Methane
(CH4)
Nitrous oxide
(N2O)
AIR RESISTANCE
A resistance force caused by air molecules
opposing the motion of an object as it
moves through the air.
 A form of friction sometimes called drag.

Aerodynamic
shape
Lesson-Specific Learning Targets(10/21)
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I can explain two factors that determine the air
resistance acting on any falling object
I can determine what changes applied to a falling
object would increase the air resistance force and
I can give examples.
I can explain the relationship between an object’s
weight and gravitational force acting on the
object.
I can explain how the velocity and acceleration
change for an objects as it falls
Starter Q (10/21) Air resistance
Quick response: how do these
pictures relate to the study of
air resistance?
Suppose a bowling ball is falling…

How many forces are acting on it?
2
Air Resistance
Force
Gravitational
force
The weight of the bowling ball is the
same as the gravitational force acting
on the ball (reported in Newtons)
Homework
Interpreting Motion Graphs
The velocity-time graph is useful for
determining whether a falling object is
accelerating or at a constant velocity
(terminal velocity)
 Interpret the graph given to you.

Questions
 Does
every object fall the same? Why or
why not?
 What happens to the air resistance when
an object ‘s exposed surface area
increases? Decreases?
 Does air resistance increase, decrease or
stay the same when an object travels
faster through the air?
Free Fall
An object moving only
under the influence of the
gravitational force is in
free fall.
The acceleration of
an object in free fall
on Earth is 9.8 m/s2.
For free fall, neglect air
resistance!
Free Fall Free body diagram

Only under the influence of gravitational
force.
No air
resistance
force!
Rock
that
weighs
100 N
This object will
continue to gain
speed at a rate of
9.8 m/s2.
Fgrav = 100 N
Free Fall: How Fast
During each second of fall the speed of
by the object increases by an additional
9.8 meters per second.
This gain in speed per second is the
acceleration.
After 1 second = 9.8 m/s
After 2 seconds = 9.8 m/s x 2
After 3 seconds = 9.8 m/s x 3… and so on
Free Fall: How Fast
9.8 m/s
19.6 m/s
29.4 m/s
39.2 m/s
49 m/s
9.8 m/s x t
Free Fall: How Fast
Rising Objects
Rising objects decelerate at the same
rate that falling objects accelerate.
During the upward part of this motion,
the object slows from its initial upward
velocity to zero velocity.
The object decreases in speed at the
same rate that it increases in speed
as it rises and falls
Air Resistance and Falling Objects
Drop a feather and a hammer on earth and the hammer reaches
the floor far ahead of the feather.
What about on the Moon?
http://history.nasa.gov/40thann/videos.htm
Earth vs. Moon
Contrast These
Characteristics of the
Earth and Moon
Magnetic field
Atmosphere
Gravity
Density
Radius
Surface
Plate Tectonics
Water Cycle
Rock Cycle
Earth
Moon
Air Resistance and Falling Objects
A feather and a coin accelerate equally when
there is no air around them.
Vacuum tube
Air Resistance and Falling Objects
How objects fall without air resistance?
F gravity or
weight
is the only
force
Air Resistance and Falling Objects
How objects fall without air resistance?
Objects accelerate equally.
F gravity or
weight
is the only
force
Free Fall

Physicists consider air resistance to be
negligible for heavier objects that fall near
the surface of the Earth.
Fg = 71.2 N
Fg = 100 N
Don’t worry
about air
when making
calculations!
Fg = 11 N
Falling and Air Resistance
Air resistance
does not depend
upon the weight
of the object.
The amount of air resistance force an
object experiences depends on the
object’s speed and exposed surface
area.
1. Speed
The greater the speed, the greater
the air resistance.
2. Surface Area (exposed or frontal)
The greater the surface area, the greater
the air resistance.
Falling and Air Resistance
What two factors determine the air
resistance force on an object?
The speed and the exposed surface area
1. The Moon formed out of the Earth

Scientists now think that the Moon was formed
when a Mars-sized object crashed into our planet
about 4.5 billion years ago. The collision was so
large that a huge spray of material was ejected
into space. The orbiting ring of debris gathered
itself into a sphere, and formed the Moon. How
do we know that this is how the Moon probably
formed? The Moon seems to be much less dense
than the Earth and lacks a lot of iron in its core.
Scientists think that the Moon is made up of the
upper crust material, which has mostly lower
density, than the composition of the Earth.

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2. The Moon only shows one face to
the Earth
Although the Moon used to rotate in the sky compared to
our point of view, it has been slowing down billions of
years. And at some point in the distant past it just stopped
turning from our perspective. The Earth’s gravity holds the
Moon in orbit, but it pulls differently at various parts of the
Moon. Over a long period, gravity slowed down the Moon’s
rotation so that it finally stopped, and always displayed one
face to the Earth. A similar situation has happened with
most of the large moons in the Solar System. In fact, in the
case of Pluto and Charon, but objects are tidally locked to
each other, so they present only one face to the other.


3. The Moon is slowly drifting away
Although the orbit of the Moon seems nice and stable, our
only natural satellite is actually drifting away from us at a
rate of 4 centimeters a year. This is happening because of
the conservation of momentum in the orbit of the Earth. In
about 50 billion years from now, the Moon will stop moving
away from us. It will settle into a stable orbit, taking about
47 days to go around the Earth (it takes 27.3 days today).
At that point, the Earth and the Moon will be tidally locked
to each other. It will look like the Moon is always in the
same spot in the sky. Of course, the Sun is expected to
consume the Earth in about 5 billion years, so this event
may not happen.

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4. The Moon looks the same size as
the Sun
This is an amazing coincidence. From our perspective here
on Earth, but the Moon and the Sun look approximately the
same size in the sky. Of course, the Sun is much much
bigger than the Moon. The Sun happens to be 400 times
larger than the Moon, but it’s also 400 times further away.
This wasn’t always the case. Billions of years ago, the Moon
was much closer than the Sun, and would have looked
larger in the sky. And the Moon is moving away from us, so
in the distant future, the Moon will look much smaller than
the Sun.

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5. The Moon causes most of the
tides… but not all
You might know that the tides on Earth are caused by the
gravitational pull of the Moon. But it’s not the only thing
pulling at the Earth’s water, the Sun is helping out too. This
is why we get very high and low tides from time to time.
When the gravity of the Moon and the Sun line up, we get
the biggest and smallest tides. Did you know that the Moon
is also pulling at the crust of the Earth causing it to bulge
up? You actually move a few meters every time the Moon is
overhead, but you just don’t notice.


6. Gravity on the Moon is only 17% of
the Earth
Want an easy way to lose some weight? Take a
trip to the Moon and stand on its surface. Since
the pull of gravity on the Moon is only 17% the
pull of gravity on the Earth, you’ll feel much
lighter. Just imagine, if you weighed 100 kg on
the Earth, you would feel like you only weighed
17 kg on Earth. You would be able to jump 6
times further and carry objects 6 times as heavy.
In fact, you had wings attached to your arms,
you could even fly around inside a dome on the
Moon under just your own muscle power.


7. The official name for the Moon is…
the Moon
I know it’s kind of confusing, but the only real
name for the Earth’s Moon is “the Moon”. When
the Moon was given its name, astronomers didn’t
know that there were moons orbiting other
planets. And so they just called it the Moon. Now
that we know there are other moons, it all comes
down to the capitalization. The Earth’s moon is
referred as “the Moon”, with a capital “M”. Other
moons are given a lowercase “m” to show the
difference.
8. The Moon is the 5th largest natural
satellite in the Solar System
 You might think that the Moon is the
largest satellite in the Solar System. I
mean look at it, it’s huge! But there are
actually larger moons in the Solar System.
The largest moon is Jupiter’s Ganymede
(5,262 km), followed by Saturn’s Titan,
Jupiter’s Callisto, Jupiter’s Io, and finally,
the Earth’s Moon with a mean diameter of
3475 km.

Only 12 people have ever stepped
onto the surface of the Moon
 Only a tiny group of astronauts have ever
set foot on the surface of the Moon. These
were the astronauts on board the Apollo
missions going from 1969 to 1972. The
first person to ever walk on the Moon was
Neil Armstrong. And the last person on the
Moon was Gene Cernan, who followed his
partner Jack Schmitt into the lunar lander
on December 14, 1972.
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Lesson-Specific Learning Targets
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I can explain how the velocity and acceleration change for an objects as it falls from
a tall building
I can explain two factors that determine the air resistance acting on any falling object
I can determine what changes applied to a falling object would increase the air
resistance force and I can give examples.
I can draw free-body diagrams showing how the weight (gravitational force) of an
object is influenced by air resistance.
I can determine how objects of different masses and shapes would fall without air
resistance on Earth.
I can identify that an object’s weight equals the gravitational forces acting on the
object.
I can identify the rate of acceleration due to gravity on Earth
I can identify that in the absence of air resistance, all objects regardless of size,
shape or mass will fall at the same rate.
I can explain key differences between the Moon and the Earth that influence how
objects fall.
I can explain why a feather and a hammer fall differently on the Moon than on the
Earth
I can explain why falling objects reach terminal velocity.
I can determine why objects could never reach terminal velocity.
I can analyze motion graphs to identify when objects are accelerating and when they
reach terminal velocity.