A 55.7-kg skateboarder starts out with a speed of 1.69 m/s. He does 95.7 J of work on himself by pushing with his feet against the ground. In addition, friction does -278 J of work on him. In both cases, the forces doing the work are non-conservative. The final speed of the skateboarder is 6.86 m/s. (a) Calculate the change (PE - PEo) in the gravitational potential energy. (b) How much has the vertical height of the skater changed? Give the absolute value. (a) Number Units (b) Number i Units