As U is evaluated only at the points $($i$)$ and $($f$)$ it is clear that for conservative forces the work depends only on the end points and not on the particular path traversed by the mass $($m$).$ This equation can be written as $=.$ Each side of this equation is called the total $($mechanical$)$ energy E of the mass. On the left, E has been evaluated at point $($f$)$ and on the right at point $($i$).$ The quantity E has been conserved and $=.$ This statement is called the conservation of energy. Energy is not conserved if friction or other non$-$conservative forces are present. Why?