In a closed system, which physical quantity is always conserved during an interaction?

In a closed system, energy is the physical quantity that is always conserved during an interaction. This principle is rooted in the law of conservation of energy, which states that energy cannot be created or destroyed; it can only be transformed from one form to another. In any interaction, whether it involves kinetic energy, potential energy, thermal energy, or other forms, the total energy of the system before and after the interaction remains constant, provided there are no external forces or energy losses.

While mass is also conserved in closed systems according to the conservation of mass principle, it is important to note that in modern physics, especially in contexts involving relativistic speeds or nuclear reactions, mass can be converted to energy and vice versa, as described by Einstein's famous equation (E=mc^2). Therefore, energy conservation encompasses mass conservation when considering rest mass energy.

Work, on the other hand, refers to the transfer of energy and is not a conserved quantity by itself; it is the process that can alter the energy of a system. Force is related to the interactions and changes in velocity or acceleration but is not a quantity that gets conserved in the same sense as energy during interactions. Thus, energy is the fundamental quantity that remains conserved in a closed system during interactions