Final answer:
Thermal energy is the average kinetic energy of particles due to their motion, while mechanical energy is the sum of kinetic and potential energies of particles in a system.
Step-by-step explanation:
Thermal energy is the average kinetic energy of the particles in a system due to their motion, while mechanical energy is the sum of the kinetic and potential energies of the particles in a system. Thermal energy is related to the temperature of the system and is transferred spontaneously due to a temperature difference, while mechanical energy is related to the motion and position of the particles in the system.