why Newton’s second law (F=m*a) is called the real law of mo…

Deriving the First Law: If the external force is zero, then . Since mass (m) can’t be zero, the acceleration (a) must be zero. This means an object at rest stays at rest, and an object in motion stays in motionwhich is exactly what the First Law states.
Deriving the Third Law: By considering an isolated system of two interacting particles and applying the Second Law (in terms of momentum), you can show that the force exerted by the first body on the second is equal and opposite to the force exerted by the second on the first (F12 = F21).

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