Through the process of doing work, energy can move between the external world and the system as the result of ____.
The sentence is completed with the word FORCES. You have to rewrite the word to make the sentence true (if it is not already true.)
The process is the doing of mechanical work. This involves forces that cause motion. I can't think of a good single-word answer.
Energy (as work) can also be added or removed by electrical means.