Laws of Motion discovered by Newton , They are as follows ;
Law 1 :- Every body perseveres in its state of rest or of uniform motion in a straight line unless it will compelled to change that state by impressed force .
Law 2 :- The rate of change of momentum is proportional to the impressed force and take place in the direction of the straight line in which the force acts .
Law 3 :- To every section there is always an equal and opposite reaction or the mutual action of any two bodies are always equal and oppositely directed .
The first law implies :
1)----- That the internal force in a body do not play a part in the state of rest or of uniform motion .
2)----- The definition of an external force as that which tends to change the inertia of a body on which it acts .
The second law enables us to measure force . If a mass m moves with a velocity v then its momentum is mv .
According to second law ;
F (an external force) varies as (d/dt) (mv)
therefore F=K.(d/dt)(mv) where K is a suitable constant
=K.m.dv/dt , if m is a constant .
=K.m.f , where f is the acceleration .
We now define a unite of force as that force which acting on unit mass produces unit acceleration .
therefore 1=k.1.1 so that K=1.
= mass . acceleration .