As per Newton's third law of Motion

When one object exerts a force on other object

The other object also exerts an equal and opposite force on the first object

 

In easy language

Every action has an equal and opposite reaction

 

Example 1

Walking

When we walk,our feet exert a force on ground in backward direction

The ground exert a force in forward direction

This makes us move forward

 

Example 2

Jumping to Land from Boat

When a sailor jumps back from boat,it exerts a pressure on boat backwards

The boat exerts equal force on sailor in forward direction

This makes person get out of board

 

Example 3

Slapping

When teacher slaps a student hard

This hurts students face as well as teacher's hand

Hand of teacher exert a force on students palm

Student's palm exert equal force on students hand

 

Example 4

Colliding of 2 footballers

If 2 footballers A and B collide, both of them are hurt

This is because force is exerted by both A and B on each other


Example 5

Recoil of Gun

When a gun is fired,

It exert forward force on bullet

Bullet also exert equal and opposite force on gun

This is called recoil of gun

 

Important Points of Third Law of Motion

There are 2 forces - one force is called action and the other force is called reaction

Both force act in opposite direction

Both forces occur on different bodies

Both forces are equal

However, Acceleration produced may be different because of different mass of 2 bodies

(Force = Mass x Acceleration)

 

Explanation

As per Newton's third law of Motion

Force of One body = Force of Other Body

Mass of First body x Acceleration of first body = Mass of Second Body x Acceleration of Second body

It can be seen that

If mass of first body is more than second body

then Acceleration of first body will be less than Acceleration of Second body

 

Example

A small boy of mass 40 kg collides with man with mass of 80 Kg

On Collision,boy will exert force on man

Man will exert equal and opposite force on boy

But since boy has less mass than man

Boy will feel more pain as acceleration produced in boy wil be more

 

Explanation

Suppose boy has mass of 40 kg and it collide with man(80 kg) with acceleration of 3 m/sec

Force Produced by Boy=40*3=120 kg m/s

As per Newton's Third Law of Motion ,Force Produced by Man =Force produced by boy

Force Produced by Man=120 kg m/s

Mass*Acceleration=120 kg m/s

80*Acceleration=120

Acceleration=120/80=1.5 m/sec

Hence Acceleration produced in man (1.5m/sec) will be much less than boy (3 m/sec)

  1. Class 9
  2. Chapter 9 Class 9 - Force and Laws Of Motion

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CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 9 years and a teacher from the past 16 years. He teaches Spoken English, Written English, Grammar and Vocabulary at Englishtan.