Im wondering how do we know when to substract Kinetic engery from potential engery or when do we add them? Is that making sense? question is asking: a women skis downhill at a constant speed of 8.0 m/s when she reaches an icy patch on which her skis move with negligible friction. If the icy patch is 10 m high, what is the skier's speed at its bottom? the book says to use this formula: KE2=KE1 + PE why are adding kinetic and potential together? I understand how to manipulate the equaiton to solve for speed but why are we using this formula?