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

*moves with a velocity*

**m***then its momentum is*

**v***.*

**mv**According to second law ;

*(an external force) varies as*

**F**

**(d/dt) (mv)**therefore

*where*

**F=K.(d/dt)(mv)***is a suitable constant*

**K***, if*

**=K.m.dv/dt***is a constant .*

**m***where*

**=K.m.f ,***is the acceleration .*

**f**We now define a unite of force as that force which acting on unit mass produces unit acceleration .

therefore

*so that*

**1=k.1.1**

**K=1.**therefore

**F=mf**=

*.*

**mass . acceleration**