To obtain Newton’s first law of motion from the second law of motion —
From Newton’s second law, F=ma
If F = 0, then a = 0
This means that if no force is applied to the body, its acceleration will be zero. If the body is at rest, it will remain at 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 any external force, does not change its state of rest or of motion.
This statement is Newton’s first law of motion.
