CBSE Class 9 Answered

We cannot entertain multiple question of this type, please post each question individually and let us know where you are getting stuck so that we would be able to explain things better.

Newton's second law is the real law of motion, as it contains both , the 1st as well as 3'rd law.

1st law  from 2nd law :

A/c to 2nd law ,  F = ma

  If there is no external force, i.e. F= 0

                                               then, ma= 0

which implies ,                                 a = 0  [ as mass can't be zero ] 

Acceleration is zero only when :

(i) body is at rest ;  OR

(ii) body is moving with a uniform speed .

Which is actually the 1st law of motion.