Physics Lesson 10

Newton’s Third Law of Motion – Action & Reaction

Eleanor Roosevelt High SchoolChin-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

No unbalanced force means the net force = 0

Fnet = 0

Stay at Rest orConstant 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

Unbalanced force means the net force ≠ 0

F

Fnet ≠ 0

Fnet = m a

am

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

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

The vase attracts Earth

Fg

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

Fg

FN

If the vase is our focus (or system)

Example: Identify the Interaction

Fg = FN Fg

FN

If the vase is our focus (or system)

Exercise: Identify the Interaction

Exercise: Identify the Interaction

Fg

Exercise: Identify the Interaction

FT

Exercise: Identify the Interaction

Fg

FT

Exercise: Identify the Interaction

Fg

FT

Exercise: Identify the Interaction

Fg = FT

Fg

FT

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 SystemHorizontal forces

only

Why Does the Man-Box Move?

A Man SystemHorizontal forces

only

Why Does the Man-Box Move?

A Box SystemHorizontal forces

only

Why Does the Man-Box Move?

A Man-Box SystemHorizontal forces

only

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 SystemHorizontal forces

only

Why Does the Horse Move?

A Horse SystemHorizontal forces

only

Why Does the Horse Move?

A Horse SystemHorizontal forces

only

Why Does the Cart Move?

A Cart SystemHorizontal forces

only

Why Does the Cart Move?

A Cart SystemHorizontal forces

only

Why Does the Horse-Cart Move?

A Horse-Cart SystemHorizontal forces

only

Why Does the Horse-Cart Move?

A Horse-Cart SystemHorizontal 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

30o

100 N

10 kg

Newton’s Third Law

Action force = Reaction force

Faction = Freaction

m1 a1 = m2 a2

m1 a1 = m2 a2

Interaction on Different Masses

Interaction on Different Masses

Interaction on Different Masses

50 kg

40 kg2

m/s2a = ?

Interaction through a String

The mass of the object is 5 kg

The reading of the scale is 50 N

reading of the scale?

reading of the scale?

Interaction through a String

reading of the scale?

reading of the scale?

Interaction through a String

MA = MB = 5 kg

MC = ?

FTC = ?

FTA = ?

Interaction through a String

m

10 kg 20

kg

Interaction through a String

M2 = 5 kgΘ = 30o

T

Interaction with acceleration

At rest orconstant speed

a = 0 m/s2

Interaction with acceleration

FgAt rest orconstant speed

a = 0 m/s2

Interaction with acceleration

Fg

FN

At rest orconstant speed

FN = Fg

Scale Reading

a = 0 m/s2

Interaction with acceleration

Accelerate upward

a

Interaction with acceleration

FgAccelerate upward

a

Interaction with acceleration

Fg

FN

Accelerate upward

a

FN > Fg

Scale Reading

Interaction with acceleration

Accelerate downward

a

Interaction with acceleration

FgAccelerate downward

a

FN FN < Fg

Scale Reading

Interaction with acceleration

Accelerate Downward with a = g

a = g

Interaction with acceleration

FgAccelerate Downward with a = g

a = g

Interaction with acceleration

FgAccelerate 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

Frictionless

T = ?

a = 2 m/s2 m1 = 10 kg

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