A skateboarder with mass ms = 36 kg is standing at the top of a ramp which is hy = 3.4 m above the ground. The skateboarder then jumps on his skateboard and descends down the ramp. His speed at the bottom of the ramp is vf = 6.1 m/s. Write an expression for the work, Wf, done by the friction force between the ramp and the skateboarder in terms of the variables given in the problem statement.