Newton’s third law of motion:

To every action, there is always an equal but opposite reaction.

Explanation:

Newton's third law of motion deals with the reaction of a body when a force acts on it. Let body A exert a force on another body B, the body B reacts against this force and exerts a force on body A. The force exerted by body A on B is the action force whereas the force exerted by body B on A is called the reaction force.

Note that action and reaction forces act on different bodies.

 

Examples:

A book lying on a table:

           Consider a book lying on a table. The weight of the book is acting on the table in a downward direction. This is the action. The reaction of the table acts on the book in the upward direction.

 

An air-filled balloon:

Take an air-filled balloon. When the balloon is set free, the air inside it rushes out and the balloon moves forward. In this example, the action is by the balloon that pushes the air out of it when set free. The reaction of the air which escapes out from the balloon acts on the balloon. It is due to this reaction of the escaping air that moves the balloon forward.

 

Taking off a rocket:

A rocket moves on the same principle. When its fuel burns, hot gases escape out from its tail at a very high speed. The reaction of these gases on the rocket causes it to move opposite to the gases rushing out of its tail.

 

Quick Quiz

 

Stretch out your palm and hold a book in it.

 

1. How much force do you need to prevent the book from falling?

Ans:  The force which is needed to prevent the book from falling is equal to the weight of the book, i.e. R = W

 

2. Which is the action?

Ans:  The weight of the book acting in a downward direction is called action.

 

3. Is there any reaction? If yes, then what is its direction?

Ans:  Yes, the force applied to prevent the book from falling is called the reactional force. The reactional force is acting in the upward direction. Opposite to the weight of the book i.e.  R = W