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h1. Conserved

{excerpt}A quantity that is constant in time (does not change) is said to be *conserved*. {excerpt}

h3. Conservation and System Specification

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 universally conserved, it is extremely rare to consider the entire universe as a single [system]. When considering a smaller [system], energy and/or momentum can be transferred into or out of the [system] of interest by exchange with the [environment]. 

{note}For this reason, it is sometimes possible to specifically choose a [system] of more than one object that will allow for conservation of momentum or energy even when individual objects do not conserve these quantities.  This trick is particularly powerful in analysis of collisions.{note}

h3. Conservation of Mechanical Energy

While energy is universally conserved, [*mechanical* energy|mechanical energy] is _not_.  Mechanical energy is a special type of energy that can be transformed into other types (thermal, radiative, chemical, etc.) without sacrificing conservation of energy _overall_.  For this reason, [*mechanical* energy|mechanical energy] is not always conserved, even in extremely isolated systems (such as the [totally inelastic collision|totally inelastic collision] of two satellites in space).

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