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Laws of Mechanics


The foundation of Mechanics is based on the following four laws.

1.3.1 Newton’s First and Second Laws of Motion

Newton’s first law states that, Every particle continues in a state of rest or uniform motion in a straight line unless it is compelled to change that state by forces imposed on it. Newton’s second law states that, The rate of change of momentum is proportional to impressed force and the change takes place in a direction of the straight line in which the force is impressed.

1.3.2 Newton’s Third Law

Newton’s third law states that, To every action there is always equal and opposite reaction”.

Let us consider Fig. 1.3 (a) where blocks A and Bare placed one above the other on a horizontal surface.

We can show the forces as per Fig. 1.3 (b).

Here,     F1 = Force exerted by horizontal surface on body B (action)

– F1 = Opposite and equal amount of force exerted by body Bon horizontal surface (Reaction).  

F2 =Force exerted by body Bon body A (action)

– F2 = Opposite and equal amount of force exerted by body A on body B (Reaction).


1.3.3 The Law of ‘Gravitational’ Attraction 

This law states that, Two particles will be attracted toward each other along their connecting li11e with a force whose magnitude is directly proportional to the product of the masses and inversely proportional to the distance squared between the particles. Refer Fig. 1.4.

F= G . m1 m2/r2 

Fig. 1.4

Where              G == Universal gravitational constant

r = Distance between the centres of bodies.

F = Force of attraction between the bodies.

m1 = Mass of the first body

m2 =Mass of the second body.