Final answer:
The magnitude of the force that the door exerts on the person is 40 Newtons, following Newton's Third Law of Motion which states that action and reaction are equal and opposite.
Step-by-step explanation:
In accordance with Newton's Third Law of Motion, often summarized as 'action and reaction are equal and opposite,' the magnitude of the force the door exerts on the person is equal to the force the person exerts on the door. Therefore, when the person applies a 40 N force to the door, the door applies a 40 N force back on the person. This reciprocal force is the reaction to the action force applied by the person, and it is of the same magnitude but in the opposite direction.
The acceleration of the door is not directly relevant to the magnitude of the force the door exerts on the person, because Newton's third law pertains to forces and states that for every action, there is an equal and opposite reaction. Hence, regardless of the acceleration that the door experiences, the reciprocal force on the person remains consistent with the initial force exerted by the person, thus being 40 N.