Why 2nd Law of motion known …

Newton's second law is the real law of motion in the sense that the firest and third law of motion can be derived from the 2nd law of motion.

From Newton’s 2nd law, F = ma

If F = 0, then a = 0

This means that if no force is applied on the body, its acceleration will be zero. If the body is at rest, it will remain in the state of rest and if it is moving, it will remain moving in the same direction with the same speed. Thus, a body not acted upon by an external force, does not change its state of rest or motion. This is the statement of Newton’s first law of motion.

Also the 3 rd law of motion can be derived from the 2nd 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.