Question

Derive Newton’s 3rd Law and 1st Law using 2nd Law

Answer 1

 Consider an isolated system of 2 bodies A & B.An isolated system means where there is no external force acting.Now let FAB be the force acting on B by A & FBA  be the force acting on A by B.Now rate of change of momentum of A =dpA/dt and rate of change of momentum of B =dpB/dt

thus FAB=dpB/dt      (i)......

        FBA=dpA/dt       (ii)........

Adding 1 and 2 we get FAB+FBA=dpB/dt+dpA/dt =d(pB+pA)/dt

But if no force is applied momentum will also be 0 becoz no velocity will be there.so rate of change of momentum will also be 0

thus d(pA+pB)/dt=0

therefore FAB+FBA=0 or,FAB=-FBA(3RD LAW OF MOTION)

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