Air Resistance, Free Fall
Motion and Falling Objects
Chapter 3.3
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)

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
(NO2)
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
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)
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
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)
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