y is 2nd law of motion d real law of motion.how can v derive 1st and 3rd law from 2nd law of motion

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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.