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Free Fall Motion
Questions
• Does every object fall the same?
• Should every object fall the same?
• What is a fluid?
• Do we live in a fluid?
• What is the composition of our
atmosphere?
• Make a table showing the differences
between the Earth and Moon.
AIR DENSITY
4.5-4.6 Free Fall
Falling Objects
Imagine there is no air
resistance…
An object moving under
the influence of the
gravitational force only is
said to be in free fall.
4.5-4.6 Free Fall
The acceleration of an object in
free fall is 9.8 m/s2.
Neglecting air resistance!
4.8 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
4.8 Air Resistance and Falling Objects
A feather and a coin accelerate equally
when there is no air around them.
Vacuum tube
4.8 Air Resistance and Falling Objects
How objects fall without air resistance?
F gravity or
weight
is the only
force
4.8 Air Resistance and Falling Objects
How objects fall without air resistance?
Objects accelerate equally.
Why?
F gravity or
weight
is the only
force
4.8 Air Resistance and Falling Objects
How objects fall without air resistance?
Objects accelerate equally.
Why?
No atmosphere means no air resistance (no
drag force), so surface area and weight make
no difference. All objects fall at the same rate.
Moon
It’s just like on the __________________
F gravity or
weight
is the only
force
4.5 Free Fall: How Fast
During each second of fall the instantaneous speed of the
object increases by an additional 9.8 meters per second.
This gain in speed per second is the acceleration.
4.5 Free Fall: How Fast
v = gt
v represents both speed and velocity.
g represents acceleration due to gravity (9.8 m/s2)
t represents time the object is free-falling
4.5 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
• An object is said to be in free fall if it is
only under the influence of gravitational
force.
This object will fall at
a rate of acceleration
equal to 9.8 m/s2.
No
support or
resistance
force!
Fg = 100 N
Free Fall
• Physicists consider air resistance to be
negligible for heavier objects that fall near
the surface of the Earth.
Don’t worry
about air
when making
calculations!
Fg = 71.2 N
Fg = 100 N
Fg = 11 N
4.5 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 is accelerating because its
velocity is changing.
How much does its speed decrease
each second?
4.8 Air Resistance and Falling Objects
How objects fall without air resistance?
F gravity or
weight
is the only
force
4.8 Air Resistance and Falling Objects
How objects fall without air resistance?
Objects accelerate equally.
Why?
F gravity or
weight
is the only
force
4.8 Air Resistance and Falling Objects
How objects fall without air resistance?
Objects accelerate equally.
Why?
No atmosphere means no air resistance (no
drag force), so surface area and weight make
no difference. All objects fall at the same rate.
Moon
It’s just like on the __________________
F gravity or
weight
is the only
force
6.6 Free Fall Explained
In Galileo’s famous
demonstration, a 10-kg
cannonball and a 1-kg
stone strike the ground at
practically the same time.
This experiment
demolished the
Aristotelian idea that an
object that weighs ten
times as much as another
should fall ten times faster
than the lighter object.
6.6 Free Fall Explained
g = weight/mass
F stands for the force (or weight in newtons)
m stands for the mass of the cannonball
a is the rate of acceleration
a = force/mass
• the weight-to-mass ratio is the same for these or any
objects, which means that the acceleration rates are
10x gravitational force
the same.
or weight
1 kg rock
10 kg
cannonball
6.6 Free Fall Explained
Since the ratio of weight
(F) to mass (m) is the
same for the 10-kg
cannonball and the 1-kg
stone, they both fall at the
same rate of acceleration.
Why?
6.6 Free Fall Explained
Since the ratio of weight
(F) to mass (m) is the
same for the 10-kg
cannonball and the 1-kg
stone, they both fall at the
same rate of acceleration.
Why?
Weight-mass ratios for
each are identical!
It takes a larger force to keep the
larger mass accelerating because a
larger mass has more inertia.
6.6 Free Fall Explained
All freely falling objects fall with the
same acceleration because the ratio of
weight to mass is the same for all
objects.
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.
6.7 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.
Another variable…
• How would the composition or density of
an atmosphere influence air resistance?
Composition of Air
Helium
Earth’s atmosphere
Same concentration, different masses
AIR DENSITY
Less
drag
More
drag
6.7 Falling and Air Resistance
What two factors determine the air
resistance force on an object?
The speed and the exposed surface area
6.7 Falling and Air Resistance
The air resistance force an object
experiences depends on the object’s
_______ and ___________ _______.
4.5 Free Fall: How Fast
think!
How fast would a free-falling rock be moving 4.5 seconds
after it drops from rest?
How about 8 seconds after it is dropped?
4.5 Free Fall: How Fast
What is the acceleration of an object in
free fall?
9.8 m/s2
4.6 Free Fall: How Far
For each second of free fall, an object
falls a greater distance than it did in
the previous second.
4.6 Free Fall: How Far
These distances form a mathematical pattern: at the
end of time t, the object starting from rest falls a
distance d.
4.6 Free Fall: How Far
4.9 m
19.6 m
44.1 m
78.4 m
122.5 m
4.6 Free Fall: How Far
think!
An apple drops from a tree and hits the ground in one
second. What is its speed upon striking the ground? What
is its average speed during the one second? How high
above ground was the apple when it first dropped?
4.6 Free Fall: How Far
think!
An apple drops from a tree and hits the ground in one
second. What is its speed upon striking the ground? What
is its average speed during the one second? How high
above ground was the apple when it first dropped?
Answer:
4.6 Free Fall: How Far
For a falling object, how does the
distance per second change?
For each second of free fall,
an object falls a greater
distance than it did in the
previous second.