Are there elementary mechanics problems that illustrate the phenomena of time varying force fields and energies associated with such fields? Problems in elementary mechanics are often solved by using the principle of conservation of energy even though "the system" (of masses, pulleys, inclined planes etc.) does not name the earth itself, which supplying the force of gravity. I think the fact that the conservation of energy works in such cases is because the force field of gravity is assumed to be conservative. So, by including the gravitational field in calculating potential energy, the effect of the earth is included in the system. Can we add a time varying force field to an elementary mechanics scenario and teach anything useful? Would the scenario have to account for the mechanism causing the time-varying force in detail?