Why does work equals kinetic energy of friction?? 1. The problem statement, all variables and given/known data After hitting a long ﬂy ball that goes over the right ﬁelder’s head and lands in the outﬁeld, the batter decides to keep going past second base and try for third base. The 62.0-kg player begins sliding 3.40 m from the base with a speed of 4.35 m/s. If the player comes to rest at third base, (a) how much work was done on the player by friction? 2. Relevant equations w=fd ke=.5mv^2 3. The attempt at a solution I already know how to do this but KE= .5X6.2x4.35^2 = -587J But Why do we use Kinetic energy equation to find the work of friction???