The internal energy of a system is the sum of mechanical energy of its particles, excluding internal motion energy of pointlike particles, remaining unaffected by position or motion.
Internal energy of a system is the total mechanical energy of all its constituent particles, comprising both kinetic and potential energies. It does not include the internal motion energy of pointlike particles as they have no internal motion energy. The internal energy is unaffected by the system's location or motion, emphasizing its focus on the energy within the particles.
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