Name: _______________________________ Class: ____ 12.1Forces

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Name _______________________________ Class
____12.1 Forces

• Chapter 12
• Forces and Motion

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What are we going to learn?
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• A soccer ball sits at rest. You come along and
  kick it, sending it flying across the field. It
  eventually slows to a stop. You applied a force
  to start it moving, and then it stopped all on
  its own, right?

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• No!
• Forces cause all changes in motion. Just as you
  applied a force to the ball to speed it up from
  rest, the ground applied a force to slow it down
  to a stop. If the ground didnt apply a force to
  the ball, it would keep rolling forever without
  slowing down or stopping.

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Forced to Change
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Whats the BIG Idea?
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She jumps with enough force to stay in the air
long enough to turn before she lands
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We will find out the answer as we go along...
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What is a Force?
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How are Forces Described?

• Force a push or pull
• When one object pushes or pulls another object,
  the first object exerts a force on the second
  object
• Ex You exert a force on a computer key when you
  push it
• Ex You exert a force on a chair when you pull it
  away from a table

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• Force is described by its strength and by the
  direction in which it acts
• Pushing to the left is a different force from
  pushing to the right
• The direction and strength can be represented by
  an arrow

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• The arrow points in the direction of the force
• The arrow length tells you the strength (ex the
  longer the arrow, the greater the force)

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• The strength of the force is measured in the SI
  unit called a newton (N), after the scientist Sir
  Isaac Newton
• 1 newton is the force that causes a 1-kilogram
  mass to accelerate at a rate of 1 meter per
  second each second (1 m/s2)

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• A force can cause a resting object to move, or it
  can accelerate a moving object by changing the
  objects speed or direction

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We will find out the answer as we go along...
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How do Forces Affect Motion?

• Often more than one force acts on an object at
  the same time
• The combination of all the forces on an object is
  called the net force
• The net force determines if and how an object
  will accelerate

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• When the forces on an object are balanced, the
  net force is zero and there is no change in the
  objects motion

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• When an unbalanced force acts on an object, the
  object accelerates

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• You can find the net force on an object by adding
  together the strengths of all the individual
  forces acting on the object
• Look at the figure below

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• The big dog pushes on the box with a force of 16
  N to the right
• The small dog pushes on the box with a force of
  10 N to the right
• The net force on the box is the sum of these
  forces

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• In the situation, there was a nonzero net force
• A nonzero net force causes a change in the
  objects motion

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What if the Forces on an Object are not acting in
the same Direction?

• The big dog pushes with a force of 20 N
• The small dog pushes with a force of 10 N
• The dogs are pushing against each other

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• When the forces on an object act in opposite
  directions, the strength of the net force is
  found by subtracting the strength of the smaller
  force (think negative) from the strength of the
  larger force (think positive)
• When forces act in opposite directions, the net
  force is in the same direction as the larger force

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• In the example, the box will accelerate to the
  right

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Lets try a few examples...
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• Use what you know about net force to describe the
  motion of the box in the figure. Assume the box
  starts at rest.

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• 1. You pull on your dogs leash to the right with
  a 12 N force. Your dog pulls to the left with a 6
  N force. Sketch this situation, including labeled
  force arrows.

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• 1.

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• 2. What is the net force on the leash? Calculate
  it. Draw and label it.

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• 2. 6 N to the right

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Tug of War Clip

• http//techtv.mit.edu/videos/850-mit-physics-demo-
  ---no-win-tug-of-war

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• What's the definition of balanced forces?

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• What's the definition of balanced forces?
• Equal but opposite forces in strength and
  direction resulting in a zero net force

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• What's the definition of unbalanced forces?

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• What's the definition of unbalanced forces?
• Direction and strength of forces acting on an
  object result in a non-zero net force which
  causes the object to accelerate

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• When forces are unbalanced and in opposite
  directions, what is the combined force equal to?
  What direction is it?

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• When forces are unbalanced and in opposite
  directions, what is the combined force equal to?
  What direction is it?
• The combined force is equal to the difference
  between the 2 forces
• It will be in the direction of the larger force

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• Use your answers above to explain what happens in
  a tug-of-war stand-off.

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• Use your answers above to explain what happens in
  a tug-of-war stand-off.
• The 2 sides each exert an opposite force. The
  side that exerts the most force will cause a
  movement of the other side toward them.

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• Use your answers above to explain what happens in
  a arm-wrestling stand-off.

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• Use your answers above to explain what happens in
  a arm-wrestling stand-off.
• Each arm wrestler is exerting a force opposite
  the other wrestler. The person with the most
  force will cause the others arm to move in the
  direction of the greater force.

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• How is a tug-of-war stand-off similar to an
  arm-wrestling stand-off?

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• How is a tug-of-war stand-off similar to an
  arm-wrestling stand-off?
• Same both involve opposite forces, if there is a
  net zero force there is no winner, if there is a
  net non-zero force the side/person with the
  greater force will cause the other to move in the
  direction of the greater force

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• How are the two stand-offs different?

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• How are the two stand-offs different?
• Different team compared to single person when
  there is a net non-zero force for the arm
  wrestler, the winners force will stop when their
  hands hit the table direction of force is
  straight line for tug of war but angled for the
  arm wrestler

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Catching a baseball
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In space
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30 N to the right
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The net force becomes 0 N so it wont accelerate
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5 N to the left
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Their strength and direction
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Non zero net forces
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Friction

• The force that 2 surfaces exert on each other
  when they rub against each other
• Ex If you slide a book across a table, the
  surface of the book rubs against the surface of
  the table

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Factors that Affect Friction

• 2 Factors
• types of surfaces involved
• how hard the surfaces are pushed together

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• Ex The football player is pushing on a blocking
  sled
• If his coach wanted to make it harder to move the
  sled, the coach could change the surface of the
  sled. Covering the bottom of the sled with rubber
  would increase friction and make the sled harder
  to move.

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• Smooth surfaces produce less friction than rough
  surfaces

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• Ex if you rubbed your hands together forcefully,
  there would be more friction than if you rubbed
  your hands together lightly
• Friction increases when surfaces push harder
  against each other

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• Friction acts in a direction opposite to the
  direction of the objects motion (opposes the
  motion of objects that touch as they move past
  each other)
• Without friction, a moving object will not stop
  until it strikes another object

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• How could you make it easier moving heavy objects?

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• How could you make it easier moving heavy
  objects?
• Put sliders, dolly, or slick surface under the
  object

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• How could you make it easier moving heavy
  objects?
• Put it on a dolly or put sliders under it

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• What would happen to the football player if he
  switched to a much heavier sled?

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• What would happen to the football player if he
  switched to a much heavier sled?
• He would find it harder to push because it pushes
  down harder against the ground

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Types of Friction

• There are 4 main types of friction
• Static friction
• Sliding friction
• Rolling friction
• Fluid friction

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Static Friction

• The friction force that acts on objects that are
  not moving

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Sliding Friction

• A force that opposes an objects direction of
  motion as it slides over a surface

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Rolling Friction

• The friction force that acts on rolling objects

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Fluid Friction

• This force opposes an objects motion through a
  fluid
• Ex Air resistance that acts on an object moving
  through the air

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• Why is it more difficult to walk on ice than on
  the floor?

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• Why is it more difficult to walk on ice than on
  the floor?
• Ex Because your feet slide on the ice (There is
  more friction between your shoes and the floor
  than there is between your shoes and ice)

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• Which type of friction keeps your shoes from
  slipping on the floor?

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• Which type of friction keeps your shoes from
  slipping on the floor?
• Static friction

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• Which type of friction keeps you skating on an
  icy pond?

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• Which type of friction keeps you skating on an
  icy pond?
• Sliding friction

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Question...

• How is friction between people like friction
  between objects?

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Question...

• How is friction between people like friction
  between objects?
• The strength of the friction depends on how
  intensely 2 people dislike each other, just as
  the strength of a frictional force depends on how
  hard the surfaces are pushed together
• The strength of the friction depends on the
  personalities of the 2 people, just as the
  strength of the frictional force depends on the
  types of surfaces involved

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• Name 4 types of friction and give an example of
  each.

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• Name 4 types of friction and give an example of
  each.
• Static pushing a car that is not moving
• Sliding a sled sliding down a hill
• Fluid a plane flying through the air
• Rolling skateboard wheels on cement
  

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• What types of friction occur between your bike
  tires and the ground when you ride over cement,
  ride through a puddle, and apply your brakes?
  

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• What types of friction occur between your bike
  tires and the ground when you ride over cement
  (rolling), ride through a puddle (fluid), and
  apply your brakes (sliding)?

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• What is friction affected by?

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• What is friction affected by?
• The surfaces involved and how hard they press
  against each other

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Gravity

• A force that acts between any two masses
• It is an attractive force
• Earths gravity acts downward toward the center
  of Earth

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• What forces are acting on the skydiver?

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• What forces are acting on the skydiver?
• Gravity and friction

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• Which force is greater?

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• Which force is greater?
• Gravity

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• How can you tell?

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• How can you tell?
• The net downward force causes them to fall toward
  Earth

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• How will the skydiver land safely?

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• How will the skydiver land safely?
• He will deploy his parachute which will slow his
  fall

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• Sir Isaac Newton concluded that a force pulled
  objects straight down toward the center of Earth
• We are so used to objects falling that we may not
  have thought about why they fall

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• Newton published his work on gravity in 1687.
• What observations might you make today that would
  lead you to the same conclusions about gravity?

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• What observations might you make today that would
  lead you to the same conclusions about gravity?
• Rain falling, apples falling from trees, thrown
  baseballs eventually hitting the ground

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Universal Gravitation

• Newton realized that gravity acts everywhere in
  the universe, not just on Earth
• Ex makes skydivers fall to the ground
• keeps the moon orbiting around Earth
• keeps all the planets in our solar
  system
• orbiting around the sun

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Law of Universal Gravitation

• The force of gravity acts between all objects in
  the universe that have mass
• This means that any 2 objects in the universe
  that have mass attract each other

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• You are attracted not only to Earth but also to
  the moon, the other planets in the solar system,
  and all other objects around you
• Earth and the objects around you are attracted to
  you as well
• You do not notice the attraction among small
  objects because these forces are extremely small
  compared to the force of Earths attraction

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• The gravitational force between some objects is
  stronger than the force between others
• You observe only the effects of the strongest
  gravitational forces

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• Ex You dont see your pencil fly toward the wall
  the way you see it fall toward Earth

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Factors Affecting Gravity

• 2 factors Mass
• Distance
• Mass is a measure of the amount of matter in an
  object (SI unit is the kilogram)

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• The more mass an object has, the greater the
  gravitational force between it and other objects
• Ex Earths gravitational force on nearby objects
  is strong because the mass of Earth is so large.
  The more massive planets interact with a greater
  gravitational force than the less massive planets

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• Gravitational force also depends on the distance
  between the objects centers
• Ex As distance increases, gravitational force
  decreases. Thats why Earth can exert a visible
  gravitational force on a pencil in your room and
  not on a pencil on the moon.

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• Ex Athletes in space would be able to perform
  feats that are impossible on Earth...
• Gravitational forces are weaker so golf balls and
  baseballs can float or fly farther in space
• Give an example of another sport which would be
  affected by reduced gravitational force

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• Give an example of another sport which would be
  affected by reduced gravitational force
• Gymnastics, jumping events in track

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Whats that Again?

• The more mass an object has, the greater its
  gravitational force
• The shorter the distance is between one object
  and another, the stronger the gravitational force
  between the objects

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• Between which 2 objects is the gravitational
  force stronger, a robin and a worm or a bear and
  a cub? Why?

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• Between which 2 objects is the gravitational
  force stronger, a robin and a worm or a bear and
  a cub? Why?
• Assuming the distances are the same,
  gravitational force is stronger between a bear
  and cub because they have greater mass

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• Between which 2 objects is the gravitational
  force stronger, 2 apples in a fruit bowl or 2
  apples on different branches of an apple tree?
  Why?

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• Between which 2 objects is the gravitational
  force stronger, 2 apples in a fruit bowl or 2
  apples on different branches of an apple tree?
  Why?
• Between 2 apples in a fruit bowl because the
  apples in the bowl are closer, and gravitational
  force decreases as distance increases

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• In the solar system, which body exerts the most
  gravitational force? Why?

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• In the solar system, which body exerts the most
  gravitational force? Why?
• The sun since it has the greatest mass

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• 1. Circle the object in the outermost orbit which
  experiences the greatest gravitational pull from
  the sun.

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• 2. Planet Bs force arrow from the gravitational
  pull of the sun should be (longer/shorter) than
  planet As arrow.

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• 3. Draw what a planet would look like it is was
  the same distance from the sun as Planet C but
  experienced a smaller gravitational pull from the
  sun.

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Mass and Weight

• Mass is sometimes confused with weight
• Mass is the measure of the amount of matter in an
  object
• Weight is a measure of the force of gravity on an
  object
• Ex Your bathroom scale displays the
  gravitational force Earth is exerting on you

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• What happens to the gravitational force between 2
  objects when their masses are increased?

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• What happens to the gravitational force between 2
  objects when their masses are increased?
• When mass increases, the force increases

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• What happens when the distance between the
  objects increases?

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• What happens when the distance between the
  objects increases?
• When distance increases, the force decreases

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• If the mass of Earth increased, what would happen
  to your weight?

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• If the mass of Earth increased, what would happen
  to your weight?
• My weight would also increase. The gravitational
  force would be stronger.

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• What about your mass?

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• What about your mass?
• My mass would stay the same.

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• At any time, your mass is the same on Earth as it
  would be on any other planet
• But your weight varies with the strength of the
  gravitational force

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• The dog has a different weight at different
  places in the solar system
• On the moon, he would weigh about 1/6 of what he
  does on Earth
• On Mars, he would weigh just over 1/3 of what he
  does on Earth

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The Ups and Downs of It...

• Gravity causes objects to accelerate downward
• Air resistance acts in the direction opposite to
  the downward motion of gravity and reduces
  acceleration

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Terminal Velocity

• The constant velocity of a falling object when
  the force of air resistance equals the force of
  gravity

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Projectile Motion

• The combination of an initial forward velocity
  and the downward vertical force of gravity causes
  the ball to follow a curved path
• Projectile motion is a falling objects motion
  after it is given a forward velocity

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Projectile Motion
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