Free Fall motion

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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.
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Starter 11/15 In your spiral notebookProvide
answers to these three questions

• 1. Describe the Earths atmosphere.
• 2. Describe air resistance
• 3. Describe the term aerodynamic shape.

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Earths Atmosphere

• Major Constituents
• Nitrogen (N2) 78
• Oxygen (O2) 21
• Argon (Ar) lt 1
• Minor Constituents
• Water vapor (H2O)
• Carbon dioxide (CO2)
• Methane (CH4)
• Nitrous oxide (N2O)

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Without the Ozone Layer

• No terrestrial life (UV rays are dangerous to
  life DNA destroyed for example)

Temperature fluctuation
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(No Transcript)
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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)

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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
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AIR DENSITY
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AIR DENSITY
Less drag
More drag
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Suppose a bowling ball is falling

• How many forces are acting on it?

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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)
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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!
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Free Fall Free body diagram

• Only under the influence of gravitational force.

No air resistance force!
This object will continue to gain speed at a rate
of 9.8 m/s2.
Rock that weighs 100 N
Fgrav 100 N
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4.5-4.6 Free Fall

• The acceleration of an object in free fall is 9.8
  m/s2.

Neglecting air resistance!
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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
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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
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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
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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
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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?
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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)
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4.8 Air Resistance and Falling Objects
A feather and a coin accelerate equally when
there is no air around them.
Vacuum tube
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4.8 Air Resistance and Falling Objects
F gravity or weight is the only force
How objects fall without air resistance?
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4.8 Air Resistance and Falling Objects
F gravity or weight is the only force
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
Its just like on the __________________
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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
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6.6 Free Fall Explained
In Galileos 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.
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g weight/mass a force/mass
6.6 Free Fall Explained

• F stands for the force (or weight in newtons)
• m stands for the mass of the cannonball
• a is the rate of acceleration
• the weight-to-mass ratio is the same for these or
  any objects, which means that the acceleration
  rates are the same.

10x gravitational force or weight
1 kg rock
10 kg cannonball
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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?
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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.
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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.

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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
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Free Fall

• Physicists consider air resistance to be
  negligible for heavier objects that fall near the
  surface of the Earth.

Dont worry about air when making calculations!
Fg 71.2 N
Fg 11 N
Fg 100 N
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Air resistance does not depend upon the weight of
the object.
6.7 Falling and Air Resistance

• The amount of air resistance force an object
  experiences depends on the objects 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.
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Falling and Air Resistance
What two factors determine the air resistance
force on an object?
The speed and the exposed surface area
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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.

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Another variable

• How would the composition or density of an
  atmosphere influence air resistance?
• More density, more air resistance!

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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.

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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.
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4.6 Free Fall How Far
4.9 m
19.6 m
44.1 m
78.4 m
122.5 m
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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.