The stair stepper is a novel exercise machine that attempts to reproduce the work done against gravity by walking up stairs. with each step, brad (of mass 60 kg) simulates stepping up a vertical distance of 0.2 m with this machine. if brad exercises for 13 min per day with a stair stepper at a frequency of 50 steps per minute, what total work he does he do each day? the acceleration of gravity is 9.8 m/s 2 . answer in units of j.