Chapter 12 - Forces
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Transcript of Chapter 12 - Forces
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Chapter 12 - Forces
1 - Newton’s First and Second Laws
2 - Gravity
3 - Newton’s Third Law
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TN Standards
• CLE.3202.3.2: Investigate and apply Newton’s 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 – Newton’s 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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Newton’s 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 – Newton’s 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 Newton’s 1st Law – In your notebook ( lab grade )
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Newton’s Second Law
• The unbalanced force acting on an object equals the object’s 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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Newton’s 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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Newton’s 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 – Newton’s 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 Newton’s First Law (in your own words)• What is Newton’s 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 Earth’s 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 Earth’s 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 – Newton’s 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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Newton’s Third Law
• For every ACTION force there is an opposite and equal REACTION force
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Newton’s 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 Newton’s 3rd Law?
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Momentum
• A property of all moving objects
• Along a straight line, momentum is the product of an object’s mass and velocity– p = mv
• Increases as mass and velocity increases
• Force is related to change in momentum
• How does moving the catcher’s 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 – Newton’s 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 Newton’s 3rd Law ( in your own words )?