good evening! My question is the following: it is well stated that energy is conserved in any isolated system, and we can in most circumstances expand our system to a many-steps conversion of energy, but always conserved. The classic example could be the energy of the sun stored in form of chemical energy in the plants, which are eaten by a man that converts such an energy into motion when he pushes a ball, and so on. But when we consider fields of forces, who is the source of the energy they give to things (like an object who gains energy from the gravitational field as it gets near to the earth)? Thanks! Cheers!