Final answer:
Using Newton's Second Law, the force exerted by the spring on the 10 kg mass is calculated to be 120 N. Consequently, the net force on the 20 kg mass is 80 N, considering the external force and the spring's reaction. The energy stored in the spring is not calculated as per the student's instruction.
Step-by-step explanation:
When addressing the schoolwork question involving masses connected by a spring and their accelerations, we are dealing with a concept from Physics known as Newton's Second Law and the conservation of energy within a spring-mass system. Specifically, the student is asked to focus on the instant when the 10 kg mass has an acceleration of 12 m/s². To find the force exerted by the spring on the 10 kg mass we use Newton's Second Law (F=ma).
The force exerted by the spring is therefore Fspring = 10 kg × 12 m/s² = 120 N. Given that an external force of 200 N is acting on the system, and following Newton's Third Law about action and reaction being equal and opposite, the spring must be exerting an equal and opposite force on the 20 kg mass. Since the 20 kg mass is being pulled by the spring with a force of 120 N and an external force of 200 N is also applied, the net force acting on the 20 kg mass is 80 N (200 N - 120 N).
As the energy calculation part of the question has been disregarded by instruction, we stop our analysis here. The energy stored in the spring would typically require additional information such as spring constant or displacement for calculation, which is not provided or necessary in this scenario based on the given instructions.