Explain how Newton’s first law of motion follows from the second law.

We have,

From Second Law of Motion,

F = ma

If no external force is applied on the body then F= 0,

ma = 0

Since, m ≠ 0

a = 0

i.e. the body is either at rest or in uniform motion. This proves, if no external force is applied on the body then the body continues in its state of rest or uniform motion.