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  Physics (NTSE/Olympiad)  

2. Force & Laws Of Motion

Newton's third law of motion

(C) Newton's Third Law of Motion
Newton's first law of motion gives a qualitative idea of force, while the second law provides us an idea to measure the force.
Newton's third law of motion states that
"If a body A exerts a force on the body B, the body B will also exert an equal and opposite force on A."
The force exerted by A on B is called action while the force exerted by B on A is called the reaction.
Newton's Third Law is also stated as "to every action there is an equal and opposite reaction."
Forces always occur in pairs.
Action and reaction always act on different bodies.
Example. By hitting a table with palm we apply a force. The table also exerts a force on palm on hitting it.

Applications of Newtons III Law of Motion :
Recoil of a gun – when the bullet is fired from a gun, an equal and opposite force is applied on the gun, due to which the gun recoils in backward direction.
Application in walking : while moving in forward direction we push the ground backwards that is the action. An equal and opposite force is applied by the ground on the man, thus the reaction due to which man moves forward.
Rowing a boat in river : when we push the water backward with the help of oars (applying a force backward), an equal and opposite force acts on the boat. This is the reaction which moves the boat forward.
Launching Rocket : In rocket, gases are produced in large amount. Due to internal combustion they come out and move backwards with an equal and opposite force which in turn acts on the rocket and moves it forward.

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