A 64-kg base runner begins his slide into second base when he is moving at a speed of 3.4 m/s. The coefficient of friction between his clothes and Earth is 0.70. He slides so that his speed is zero just as he reaches the base. (a) How much mechanical energy is lost due to friction acting on the runner

First you need to find total mechanical energy which is kinetic energy+potential energy, or (1/2mv^2+mgh). Since there is no height, we can pretty much ignore the potential energy and just calculate the kinetic.

1/2mv^2=1/2 (64) (3.4) ^2 = 369.92J. Since it says the speed is 0 at the end of the run, we can assume that all the mechanical energy was lost. Therefore, 369.92 J of energy was lost