Transcript Free Fall motion - Fort Thomas Independent Schools

LT 1 I can calculate how fast, how far and how long
an object will fall, neglecting air resistance.
LT 2 I can explain why all objects, neglecting air
resistance, fall at the same rate.
LT 3 I can explain freefall and identify when
terminal velocity occurs for falling objects.
LT 4 I can explain the two factors that influence the
amount of air resistance a falling object experiences,
and analyze the forces interacting on a falling object
to determine its motion.
Starter 11/15
In your spiral notebook…Provide
answers to these three questions:
• 1. Describe the Earth’s atmosphere.
• 2. Describe air resistance
• 3. Describe the term aerodynamic
shape.
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)
Without the Ozone Layer
• No terrestrial life (UV rays are dangerous
to life: DNA destroyed for example)
Temperature fluctuation
Atmospheric Layers
• Four Layers
– Thermosphere (outermost layer, receives intense
solar radiation)
– Mesosphere (coldest layer)
– Stratosphere (contains the ozone layer, warmer due
to absorption of ultra-violet light)
– Troposphere (the weather layer, the densest layer)
• Also, the Ionosphere (forms due to the
interaction of cosmic radiation from the Sun with
the faint nitrogen and oxygen concentration in
the thermosphere)
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
AIR DENSITY
AIR DENSITY
Less
drag
More
drag
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)
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
4.5-4.6 Free Fall
The acceleration of an object in
free fall is 9.8 m/s2.
Neglecting air resistance!
Free Fall: How Fast
During each second of fall the speed of
by the object increases by an additional
9.8 meters per second. V = gt
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
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: 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
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?
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?
(see video clips in class)
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?
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
4.8 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
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.
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.
Fg = 71.2 N
Fg = 100 N
Don’t worry
about air
when making
calculations!
Fg = 11 N
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.
Falling and Air Resistance
What two factors determine the air
resistance force on an object?
The speed and the exposed surface area
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.
Another variable…
• How would the composition or density of
an atmosphere influence air resistance?
– More density, more air resistance!
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
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.