Answer:
Newton's second law only applies in an inertial frame of reference.
Explanation:
An inertial frame of reference is not accelerating. An observer on the ground is in an inertial frame of reference. He can use Newtons laws to describe a rocket launch. The person inside the rocket is not in an inertial frame of reference when the rocket is launched. Newtons laws can not be used by the person inside the rocket.