A 57.4-kg skateboarder starts out with a speed of 2.04 m/s. He does 104 J of work on himself by pushing with his feet against the ground. In addition, friction does -212 J of work on him. In both cases, the forces doing the work are non-conservative. The final speed of the skateboarder is 7.85 m/s.
(a) Calculate the change (PEf - PE0) in the gravitational potential energy.
(b) How much has the vertical height of the skater changed? Give absolute value.