Need help on this problem please!! I cant figure it out!! The problem is A 54.0 kg. skateboarder starts out with a speed of 1.90 m/s. He does +80.0 J of work on himself by pushing with his feet against the ground. In addition, friction does -265 J of work on him. In both cases, the forces doing the work are nonconservative. The final speed of the skateboarder is 5.90 m/s. (a) Calculate the change (PE = PEf - PE0) in the gravitational potential energy? J (b) How much has the vertical height of the skater changed? m (c) Is the skater above or below the starting point? above the starting point or below the starting point ?