a 55 kg skateboarder starts out with a speed of 1.8 m/s. he does +80 J of work on himself by pushing with his feet against the ground. in addition, the 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 6 m/s. Calculate the change in the gravitational potential energy. How much has the vertical height of the skater changed, and is the skater above or below the starting point?

Answer 1

Initial total energy-friction force=final energy.

Let the final energy be KE only, set PE to zero, after all, that point is arbritary.

1/2 m 1.8^2+mgh-frictionwork=1/2 m 6^2
solve for h

1/2 m 1.8^2-265=1/2 m*26

solve for h. If h is negative, then the skater is above the starting point.