Newton's Third Law Of Motion

Newton’s Third Law Of Motion

Defnition: To every action there is always an equal and opposite reaction 
Action and reaction do not occur on the same body they act on different bodies at same instant of time.
Action and reaction, known as pair of forces, equal in magnitude opposite in directions acting on diffrerent bodies in interaction. So they never cancel each other. 

Limitation of Newton’s Third law
a)  Newtons third law is not strictly applicable for the interaction between two bodies seperated by larger distances, of the order of astronomical units.
b) It does not apply strictly when the objects move with velocity nearer to that of light of light.
c) It does not apply where the gravitational fields are very strong.

Characterstics of action and reaction :
1) Action and reaction are equal in magnitude and opposite in direction
2) Action and reaction do not act on the same body, therefore they do not cancel each other
3) Action and reaction are mutual and simultaneous. This means they always exist in     pairs and one exists only as long as the other exists.
4) The force of action and reaction may appear due to actual physical contact of the two bodies or even from a distance. But they are always equal and opposite.
5) When taken together action and reaction be come internal forces. 

Some of the examples of Newton’s third law of motion are given below:
1. Book kept on a table. A book lying on a table exerts a force on the table, which is equal to the weight of the book.This may be called the force of action. The table supports the book,by exerting an equal force on the book.This is the force of reaction, 
           
Since the book is observed to be at rest, the net external force on it must be zero.This implies that the normal reaction must be equal and opposite to the weight of the book.

2. Walking : while walking, a person presses the ground in the backward direction (action)by his feet.The ground pushes the person in forward direction with anequal force (reaction).The component of reaction in the horizontal direction makes the person move forward.

3. Swimming. A swimmer pushes the water backwards (action).The water pushes the swimmer forward(reaction) with the same force.Hence the swimmer swims

4. Firing from a gun. When a gun is fired, the bullet moves forward (action).The gun recoils backwards (reaction)

5. Flight of jet planes and rockets.The burnt fuel which appears in the form of hot and highly compressed gases escapes through the nozzle (action) in the backward direction. The escaping gases push the jet plane or rocket forward (reaction)with the same force.Hence the jet or rocket moves.

6. Rebounding of a rubber ball. When a rubber ball is struck against a wall or floor, it exerts a force on wall (action).The ball rebonds with an equal force (reaction) exerted by the wall or floor on the ball.

7. It is difficult to walk on sand or ice. This is because on pushing sand gets displaced and reaction from sandy ground is very little.In case of ice, force of reaction is again small,because friction between our feet and ice is very little.