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A conserved quantity is constant in time, it does not change. |
Before deciding that a quantity is conserved in a given situation it is important to carefully consider the choice of system. Although quantities like energy, linear momentum, and angular momentum are globally conserved, it is extremely rare to consider the entire universe as a system. When considering a subsystemsmaller system, energy and/or momentum can be transferred to other parts of the universe and so lost from the subsystem of interestinto or out of the system of interest. Mechanical energy can also be lost to or gained from thermal energy within in the system.