Air Resistance, Free Fall Motion and Falling Objects

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Air Resistance, Free Fall Motion and Falling
Objects

• Chapter 3.3

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Lesson-Specific Learning Targets (10/20)

• I can identify the composition of the Earths
  atmosphere
• I can define air resistance and why it is a force
• I can define terminal velocity

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Air Resistance

• Starter Q (10/20) Provide answers to these three
  questions (see handout)

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Starter Q (10/24) Provide answers to these three
questions

• 1. Describe the Earths 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)

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Starter Q (10/20)

• 2. Describe air resistance (what causes it and
  why is it considered a force?)

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Starter Q (10/20)

• 3. Describe the term aerodynamic shape. (What
  does it mean? Give an example)

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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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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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Lesson-Specific Learning Targets(10/21)

• 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
  objects weight and gravitational force acting on
  the object.
• I can explain how the velocity and acceleration
  change for an objects as it falls

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Starter Q (10/21) Air resistance
Quick response how do these pictures relate to
the study of air resistance?
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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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Homework
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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.

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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?

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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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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
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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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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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Earth vs. Moon
Contrast These Characteristics of the Earth and
Moon
Earth Moon
Magnetic field Atmosphere Gravity Density Radius S
urface Plate Tectonics Water Cycle Rock Cycle
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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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Air Resistance and Falling Objects
F gravity or weight is the only force
How objects fall without air resistance?
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Air Resistance and Falling Objects
F gravity or weight is the only force
How objects fall without air resistance?
Objects accelerate equally.
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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.
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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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, its huge! But there are
actually larger moons in the Solar System. The
largest moon is Jupiters Ganymede (5,262 km),
followed by Saturns Titan, Jupiters Callisto,
Jupiters Io, and finally, the Earths Moon with
a mean diameter of 3475 km. 9.
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• 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 Earths gravity holds the
  Moon in orbit, but it pulls differently at
  various parts of the Moon. Over a long period,
  gravity slowed down the Moons 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.

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• 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 its also 400 times further away.
  This wasnt 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 its
  not the only thing pulling at the Earths 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 dont notice.

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• 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, youll 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.

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• 7. The official name for the Moon is the Moon
• I know its kind of confusing, but the only real
  name for the Earths Moon is the Moon. When the
  Moon was given its name, astronomers didnt 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 Earths moon is referred as
  the Moon, with a capital M. Other moons are
  given a lowercase m to show the difference.

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• 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,
  its huge! But there are actually larger moons in
  the Solar System. The largest moon is Jupiters
  Ganymede (5,262 km), followed by Saturns Titan,
  Jupiters Callisto, Jupiters Io, and finally,
  the Earths Moon with a mean diameter of 3475 km.

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• 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

• 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 objects 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.