Question

why Newtons second law is called real law?

Answer 1

Other laws of Newton can be derived from Newton's second law. Hence it is called real law of Newton

For eg…

2nd law is…F=ma

And 1st law is …initial velocity=final velocity

Assume that force acting on body is zero

F=ma

so…0= ma

0/m = a

0=a

0= (v-u)/t

0/t = v-u

0= v-u

U=v

Thus ….we proved that 1st law can be derived from 2nd law of Newton
Consider an isolated system of 2 bodies A & B. An isolated system means where there is no external force acting.Now let F be the force acting on B by A & F 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 because 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)

Answer 2

We shall derive the 1 st and the 3 rd law of motion fron the 2 nd law.

Derivation of 1 st law:

According to Newton�s 2 nd law of motion,

F = ma

Where, F is the force applied, m is the mass of the body and a is the acceleration of the body.

So,

In absence of a force,

F = 0

=> ma = 0

=> a = 0

Since, m cannot be zero for a body.

Zero acceleration means, body at rest will remain at rest and a body in uniform motion will continue its uniform motion. Which is Newton�s 1 st law of motion.

Derivation of 3 rd law:

Consider an isolated system of two bodies A and B. An isolated system is such that no force acts on the system.

This is the Newton�s third law of motion for a body exerting some force on another.

Thus, Newton�s 2 nd law of motion is the most basic law of motion.

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