If a system is isolated from its environment, then there can be no energy transfers to or from it. For that case, the law of conservation of energy states.
We discussed a special situation for isolated systems-namely, the situation in which non conservative forces (such as a kinetic frictional force) do not act within them. In that special situation, , are both zero, and so. In other words, the mechanical energy of an slated system is conserved when non conservative forces do not act within it.