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Describe the Newtons laws of motion.
Newton has given three laws to describe the motion of bodies.These laws are known as Newton's Laws of Motion.The Newton's laws of motion give a precise definition of force and establish a relationship between the force applied on a body and the state of motion acquired by it.
According to Newton's first law of motion : A body at rest will remain at rest, and a body in motion will continue in motion in a straight line with a uniform speed,unless it is compelled by an external force to change its state of rest or of uniform motion.
Newton's first law recognizes that every body has some inertia.Inertia is that  property of a body due to which it resists a change in its state of rest or of uniform motion.Greater the inertia of a body,greater will be the force required to bring a change in its state of rest or of uniform motion. 
According to Newton's second law of motion : 
The rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction in which the force acts.The rate of change of momentum of a body can be obtained by dividing the 'Change in momentum' by 'time taken' for change.So,Newton's second law of motion can be expressed as :

                             Force   ?   Change in momentum / Time taken
Consider a boby of mass m having a n initial velocity u.The initial momentum of this body will be mu.Suppose a force F acts on this body for time t and causes the final velocity to become v.The final momentum of this body will be mv.Now ,the change in momentum of this body is mv - mu and the time taken for this change is t.So,according to Newton's second law of motion :
                               F  ?   mv - mu /t
or                            F  ?   m (v - u )/t

But v - u / t represents change in velocity with time which is known as acceleration'a'.So,by writing 'a' in the place of v-u/t in the above equation, we get :
F ?  m x a
Thus ,the force acting on a body is directly proportional to the product of 'mass' of the body and 'acceleration' produced in the body by the action of the force , and it acts in the direction of  acceleration.
The relation F  ?  m x a can be turned into an equation by putting in a constant k.
Thus, F = k x m x a   (where k is a constant)
The value of constant k in S.I. units is 1,so the above equation becomes :
            F = m x a 
or Force = mass x acceleration
Thus, Newton's second law of motion gives us a relationship between 'force' and 'acceleration'.

According to Newton's third law of motion : Whenever one body exerts a force on another body ,the second body exerts an equal and opposite force on the first body. 


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