Work Is Done by . . . on . . .
Not only must you identify the system,
you must also identify what
agent in the environment is doing
work on the system. When discussing
work, always use the phrase, “the
work done by . . . on . . . .” After “by,”
insert the part of the environment
that is interacting directly with the
system. After “on,” insert the system.
For example, “the work done by the
hammer on the nail” identifies the
nail as the system, and the force
from the hammer represents the
influence from the environment.