Physics Lesson 10 Newton’s Third Law of Motion – Action & Reaction Eleanor Roosevelt High School Chin-Sung Lin

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

No Unbalanced Force Fnet = 0 No unbalanced force means the net force = 0 Fnet = 0 Stay at Rest or Constant Velocity

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

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

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

Newton’s Third Law Example

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

Action force = Reaction force Newton’s Third Law Action force = Reaction force Faction = Freaction

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

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

Example: Identify the Interaction

Example: Identify the Interaction Earth attracts the vase Fg

Example: Identify the Interaction Earth attracts the vase Fg The vase attracts Earth

Example: Identify the Interaction The table supports the vase FN

Example: Identify the Interaction The table supports the vase FN The vase pushes the table

Example: Identify the Interaction Fg FN

Example: Identify the Interaction If the vase is our focus (or system) Fg FN

Example: Identify the Interaction If the vase is our focus (or system) Fg = FN Fg FN

Exercise: Identify the Interaction

Exercise: Identify the Interaction Fg

Exercise: Identify the Interaction FT

Exercise: Identify the Interaction FT Fg

Exercise: Identify the Interaction FT Fg

Exercise: Identify the Interaction FT Fg = FT Fg

Faction = Freaction Do they cancel each other out? Newton’s Third Law Action force = Reaction force Faction = Freaction Do they cancel each other out?

Why Does the Man-Box Move? A Man-Box System

Why Does the Man-Box Move? A Man-Box System Horizontal forces only

Why Does the Man-Box Move? A Man System Horizontal forces only

Why Does the Man-Box Move? A Box System Horizontal forces only

Why Does the Man-Box Move? A Man-Box System Horizontal forces only

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

Why Does the Horse-Cart Move? A Horse-Cart System

Why Does the Horse-Cart Move? A Horse-Cart System Horizontal forces only

Horizontal forces only Why Does the Horse Move? A Horse System Horizontal forces only

Horizontal forces only Why Does the Horse Move? A Horse System Horizontal forces only

Horizontal forces only Why Does the Cart Move? A Cart System Horizontal forces only

Horizontal forces only Why Does the Cart Move? A Cart System Horizontal forces only

Why Does the Horse-Cart Move? A Horse-Cart System Horizontal forces only

Why Does the Horse-Cart Move? A Horse-Cart System Horizontal forces only

Exercise: Identify the Interaction A Rear Wheel Drive Car

Exercise: Identify the Interaction A Front Wheel Drive Car

Exercise: Identify the Interaction

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

Exercise: Identify the Interaction 10 kg 30o

Action force = Reaction force Faction = Freaction m1 a1 = m2 a2 Newton’s Third Law Action force = Reaction force Faction = Freaction m1 a1 = m2 a2

Interaction on Different Masses

Interaction on Different Masses

Interaction on Different Masses 40 kg 50 kg 2 m/s2 a = ?

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?

Interaction through a String reading of the scale? reading of the scale?

Interaction through a String MA = MB = 5 kg FTA = ? FTC = ? MC = ?

Interaction through a String m 10 kg 20 kg

Interaction through a String M2 = 5 kg Θ = 30o T

Interaction with acceleration At rest or constant speed a = 0 m/s2

Interaction with acceleration Fg At rest or constant speed a = 0 m/s2

Interaction with acceleration Fg At rest or constant speed a = 0 m/s2 FN FN = Fg Scale Reading

Interaction with acceleration Accelerate upward a

Interaction with acceleration Fg Accelerate upward a

Interaction with acceleration Fg Accelerate upward a FN FN > Fg Scale Reading

Interaction with acceleration Accelerate downward a

Interaction with acceleration Fg Accelerate downward a FN FN < Fg Scale Reading

Interaction with acceleration Accelerate Downward with a = g a = g

Interaction with acceleration Fg Accelerate Downward with a = g a = g

Interaction with acceleration Fg Accelerate Downward with a = g a = g FN = 0 N Scale Reading = 0 N

Interaction with acceleration

Interaction with acceleration

Interaction with acceleration Reading?

Interaction with acceleration m1 = 10 kg a = 2 m/s2 Frictionless m2 = ?

Newton’s Third Law Video

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

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