Final answer:
Including both surfaces in a system when friction is involved allows for a complete energy transfer analysis, accounting for the thermal energy generated by friction and adhering to the conservation of energy principle.
Step-by-step explanation:
When friction cannot be neglected, it is useful to include both surfaces in the system when analyzing processes using the energy approach because it allows for a more comprehensive understanding of the energy transfers involved.
Friction is an energy-dissipating mechanism that converts mechanical energy into thermal energy, and thus, it must be accounted for to maintain the accuracy of the energy conservation principle. Including both the object and the surface in the system means that the thermal energy generated by friction remains within the system's accounting boundary, allowing us to adhere to the conservation of energy.
Furthermore, understanding that macroscopic processes are never exactly reversible helps to clarify that when there is friction, the system and environment will not both return to their original states. Heat transfer due to friction will occur, which changes the internal energy of the system.
By considering both surfaces involved in the friction, one can calculate the total internal energy changes accurately, leading to a better understanding of the net work done and the final state of the system.