Chapter 12 - Forces
1 - Newton’s First and Second Laws
2 - Gravity
3 - Newton’s Third Law
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
Bellwork
• How many laws of motion did Newton create?
1 – Newton’s
st
1
and
nd
2
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?
Newton’s First Law
• An object at rest remains at rest and an object in
motion maintains its velocity unless it experiences
a non-zero ( or unbalanced ) net force
• Objects change their state of motion only when a
non-zero net force is applied
• Objects tend to maintain their state of motion
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
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 )
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
F/m = a
• Unbalanced force on an object determines how
much an object speeds up or slows down
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
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
1 – Newton’s
st
1
and
nd
2
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?
Bellwork
• What is Newton’s First Law (in your own words)
• What is Newton’s Second Law (in your own words)
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?
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
Law of Universal Gravitation
Law of Universal Gravitation
• All matter is affected by gravity
• Gravitational force increases with mass OR
decreases with distance
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 )
Projectile Motion
• Curved path of an object thrown, launched, kicked,
or otherwise projected near Earth’s surface
Projectile Motion
• Two components – horizontal and vertical
– Curved path when both are combined
Projectile Motion
• Orbiting is projectile motion
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?
Bellwork
• What is the difference between weight and mass?
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?
Action and Reaction Forces
• Foot on Ball – ACTION
• Ball on Foot – REACTION
Newton’s Third Law
• For every ACTION force there is an opposite and
equal REACTION force
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
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
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?
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?
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
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?
Bellwork
• What is Newton’s 3rd Law ( in your own words )?