Final answer:
The total sum of kinetic and potential energy within a system is called mechanical energy. Mechanical energy in a closed system remains constant due to the Law of Conservation of Mechanical Energy. Thermal energy and internal energy, while related, include different constituents, with internal energy combining both kinetic and potential energy of the system's particles.
Step-by-step explanation:
The total amount of kinetic energy and potential energy within a system is called mechanical energy. Gravitational potential energy, often encountered in physics problems, is the energy stored due to an object's position in a gravitational field, whereas kinetic energy is the energy of motion. In a closed system without external dissipative forces, like friction or air resistance, the mechanical energy remains constant, according to the Law of Conservation of Mechanical Energy. This law states that mechanical energy will not change; it cannot become more or less, but may only be converted from one form to another (e.g., from potential to kinetic energy).
Thermal energy is the energy associated with the random motion of constituent particles in a system, and tends to be referred to as the average kinetic energy of these particles. The internal energy, however, includes both this thermal component and the potential energies of the constituent particles. Hence, while thermal energy is a measure of the system's microscopic kinetic energy, internal energy combines both kinetic and potential energies at the microscopic level.