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Physics Lesson 10 Newton’s Third Law of Motion – Action & Reaction
Eleanor Roosevelt High School Chin-Sung Lin
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Review – Newton’s First Law
Law of inertia Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by an unbalanced force exerted upon it
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No Unbalanced Force Fnet = 0
No unbalanced force means the net force = 0 Fnet = 0 Stay at Rest or Constant Velocity
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Review - Newton’s Second Law
Law of Force & Acceleration The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object
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Unbalanced Force Fnet ≠ 0 a F m Fnet = m a
Unbalanced force means the net force ≠ 0 Fnet ≠ 0 a F m Fnet = m a
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Example
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Newton’s Third Law Law of Action & Reaction (or Interaction) Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object To every action there is always an equal opposing reaction
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Action force = Reaction force
Newton’s Third Law Action force = Reaction force Faction = Freaction
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Newton’s Third Law Action and reaction forces are acting on different objects It doesn’t matter which one is called action and which one is called reaction Force is not something an object has, like mass. Force is an interaction between two objects There cannot be action force without a reaction force
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System Concept A systems is a coherent entity which has certain function A system can be characterized and described by its input and output, and the relation between them A system can be viewed as a collection of interconnected subsystems
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Example: Identify the Interaction
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Example: Identify the Interaction
Earth attracts the vase Fg
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Example: Identify the Interaction
Earth attracts the vase Fg The vase attracts Earth
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Example: Identify the Interaction
The table supports the vase FN
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Example: Identify the Interaction
The table supports the vase FN The vase pushes the table
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Example: Identify the Interaction
Fg FN
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Example: Identify the Interaction
If the vase is our focus (or system) Fg FN
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Example: Identify the Interaction
If the vase is our focus (or system) Fg = FN Fg FN
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Exercise: Identify the Interaction
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Exercise: Identify the Interaction
Fg
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Exercise: Identify the Interaction
FT
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Exercise: Identify the Interaction
FT Fg
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Exercise: Identify the Interaction
FT Fg
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Exercise: Identify the Interaction
FT Fg = FT Fg
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Faction = Freaction Do they cancel each other out? Newton’s Third Law
Action force = Reaction force Faction = Freaction Do they cancel each other out?
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Why Does the Man-Box Move?
A Man-Box System
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Why Does the Man-Box Move?
A Man-Box System Horizontal forces only
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Why Does the Man-Box Move?
A Man System Horizontal forces only
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Why Does the Man-Box Move?
A Box System Horizontal forces only
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Why Does the Man-Box Move?
A Man-Box System Horizontal forces only
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Action force = Reaction force
Newton’s Third Law Action force = Reaction force Faction = Freaction They do not cancel each other out since they are acting on the different objects
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Why Does the Horse-Cart Move?
A Horse-Cart System
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Why Does the Horse-Cart Move?
A Horse-Cart System Horizontal forces only
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Horizontal forces only
Why Does the Horse Move? A Horse System Horizontal forces only
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Horizontal forces only
Why Does the Horse Move? A Horse System Horizontal forces only
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Horizontal forces only
Why Does the Cart Move? A Cart System Horizontal forces only
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Horizontal forces only
Why Does the Cart Move? A Cart System Horizontal forces only
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Why Does the Horse-Cart Move?
A Horse-Cart System Horizontal forces only
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Why Does the Horse-Cart Move?
A Horse-Cart System Horizontal forces only
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Exercise: Identify the Interaction
A Rear Wheel Drive Car
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Exercise: Identify the Interaction
A Front Wheel Drive Car
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Exercise: Identify the Interaction
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Exercise: Identify the Interaction
A 100 N pushing force at 30o angle is exerted on a 10 kg block against the wall. Identify and calculate all the forces acting on the block by drawing the free-body diagram 30o 100 N 10 kg
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Exercise: Identify the Interaction
10 kg 30o
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Action force = Reaction force Faction = Freaction m1 a1 = m2 a2
Newton’s Third Law Action force = Reaction force Faction = Freaction m1 a1 = m2 a2
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Interaction on Different Masses
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Interaction on Different Masses
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Interaction on Different Masses
40 kg 50 kg 2 m/s2 a = ?
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Interaction through a String
The reading of the scale is 50 N reading of the scale? The mass of the object is 5 kg reading of the scale?
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Interaction through a String
reading of the scale? reading of the scale?
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Interaction through a String
MA = MB = 5 kg FTA = ? FTC = ? MC = ?
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Interaction through a String
m 10 kg 20 kg
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Interaction through a String
M2 = 5 kg Θ = 30o T
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Interaction with acceleration
At rest or constant speed a = 0 m/s2
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Interaction with acceleration
Fg At rest or constant speed a = 0 m/s2
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Interaction with acceleration
Fg At rest or constant speed a = 0 m/s2 FN FN = Fg Scale Reading
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Interaction with acceleration
Accelerate upward a
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Interaction with acceleration
Fg Accelerate upward a
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Interaction with acceleration
Fg Accelerate upward a FN FN > Fg Scale Reading
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Interaction with acceleration
Accelerate downward a
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Interaction with acceleration
Fg Accelerate downward a FN FN < Fg Scale Reading
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Interaction with acceleration
Accelerate Downward with a = g a = g
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Interaction with acceleration
Fg Accelerate Downward with a = g a = g
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Interaction with acceleration
Fg Accelerate Downward with a = g a = g FN = 0 N Scale Reading = 0 N
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Interaction with acceleration
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Interaction with acceleration
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Interaction with acceleration
Reading?
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Interaction with acceleration
m1 = 10 kg a = 2 m/s2 Frictionless m2 = ?
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Newton’s Third Law Video
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Summary Newton’s third law - Faction = Freaction
Every action always pairs with an equal opposing reaction Action and reaction forces won’t cancel out each other since they are acting on different objects Identify the interactions Use system concept and free-body diagram to identify the forces exerting on a system
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Summary Use system concept and free-body diagram to explain motion
Interaction on different masses m1 a1 = m2 a2 Interaction through a string - Tension Interaction with acceleration
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