Chapter 12 - Forces

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Chapter 12 - Forces

• - Newtons First and Second Laws
• - Gravity
• 3 - Newtons Third Law

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TN Standards

• CLE.3202.3.2 Investigate and apply Newtons
  three laws of motion
• CLE.3202.4.1 Explore the difference between mass
  and weight
• CLE.3202.4.2 Relate gravitational force to mass
• CLE.3202.3.3 Examine the Law of Conservation of
  Momentum in real-world situations
• CLE.3202.Math.1 Understand the mathematical
  principles behind the science of Physics

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Bellwork

• How many laws of motion did Newton create?

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1 Newtons 1st and 2nd Laws

• Key Questions
• What makes an object speed up, slow down, or
  change directions?
• What determines how much an object speeds up or
  slows down?

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Newtons First Law

• An object at rest remains at rest and an object
  in motion maintains its velocity unless it
  experiences a net force
• Objects change their state of motion only when a
  net force is applied
• Objects tend to maintain their state of motion

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Inertia

• Laziness of all matter based on mass
• Is the tendency of an object to stay at rest or
  to move at constant velocity ( if moving )
• Seat belts and car seats

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QuickLab Newtons 1st Law

• Place an Index Card over a glass Coin on card
• Flick the card sideways off the glass what
  happens to the coin?
• Does coin move with Index Card?
• Repeat but slowly pull card sideways what
  happens to the coin?
• Explain the results using Newtons 1st Law
• In your notebook ( lab grade )

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Newtons Second Law

• The unbalanced force acting on an object equals
  the objects mass times its acceleration
• Net force equals mass times acceleration
• F ma
• Unbalanced force on an object determines how much
  an object speeds up or slows down

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Newtons Second Law

• Force is measured in Newtons ( 1 N 1 kg x 1
  m/s2 )
• For equal forces, a larger mass accelerates less
• Acceleration depends on force and mass

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Newtons Second Law

• Force is measured in Newtons ( 1 N 1 kg x 1
  m/s2 )
• For equal mass, a larger force accelerates more
• Acceleration depends on force and mass

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1 Newtons 1st and 2nd Laws

• Key Questions
• What makes an object speed up, slow down, or
  change directions?
• What determines how much an object speeds up or
  slows down?

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Bellwork

• What is Newtons First Law (in your own words)
• What is Newtons Second Law (in your own words)

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2 Gravity

• Key Questions
• How are weight and mass related?
• Why do objects fall to the ground when dropped?
• What is the relationship between free-fall
  acceleration and mass?
• Why does a projectile follow a curved path?

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Weight and Mass

• Force on an object due to gravity is WEIGHT
• Weight is measured in Newtons
• Free-fall acceleration is a constant acceleration
  all objects on earth experience
• Weight is equal to mass times free-fall
  acceleration
• w mg

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Law of Universal Gravitation

•  

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Law of Universal Gravitation

• All matter is affected by gravity
• Gravitational force increases with mass OR
  decreases with distance

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Free Fall

• Without air resistance, all objects falling near
  Earths surface accelerate at the same rate
  regardless of their mass
• Due to the Law of Gravitation
• Air resistance can balance weight
• EXAMPLE Leaf falling at constant velocity
• TERMINAL VELOCITY ( max velocity obtained )

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Projectile Motion

• Curved path of an object thrown, launched,
  kicked, or otherwise projected near Earths
  surface

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Projectile Motion

• Two components horizontal and vertical
• Curved path when both are combined

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Projectile Motion

• Orbiting is projectile motion

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2 Gravity

• Key Questions
• How are weight and mass related?
• Why do objects fall to the ground when dropped?
• What is the relationship between free-fall
  acceleration and mass?
• Why does a projectile follow a curved path?

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Bellwork

• What is the difference between weight and mass?

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3 Newtons Third Law

• Key Questions
• What happens when an object exerts a force on
  another object?
• How do you calculate the momentum of an object?
• What is the total momentum after objects collide?

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Action and Reaction Forces

• Foot on Ball ACTION
• Ball on Foot REACTION

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Newtons Third Law

• For every ACTION force there is an opposite and
  equal REACTION force

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Newtons Third Law

• Forces always occur in pairs
• Forces in a force pair do not act on the same
  object
• Do not cancel each other
• Equal forces do not always have equal effects

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QuickLab Action/Reaction Forces

• Hang a 2 kg mass from a spring scale
• Observe and record the reading on the spring
  scale
• With the mass on the first scale, link a second
  scale to the first
• Observe/record readings from both scales

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QuickLab Action/Reaction Forces

• QUESTIONS
• What are the action and reaction forces in the
  spring scale-mass system that you have
  constructed?
• How did the readings on both scales compare in
  the last step? Explain how this demonstrates
  Newtons 3rd Law?

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Momentum

• A property of all moving objects
• Along a straight line, momentum is the product of
  an objects mass and velocity
• p mv
• Increases as mass and velocity increases
• Force is related to change in momentum
• How does moving the catchers glove back change
  the force needed to stop the ball?

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Conservation of Momentum

• The total amount of momentum in an isolated
  system is conserved
• Total momentum of two or more objects after a
  collision is the same as the momentum before
• Explained by 3rd Law

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3 Newtons Third Law

• Key Questions
• What happens when an object exerts a force on
  another object?
• How do you calculate the momentum of an object?
• What is the total momentum after objects collide?

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Bellwork

• What is Newtons 3rd Law ( in your own words )?