Problem 8: A skateboarder with mass m, = 47 kg is standing at the top of a ramp which is h, = 4.4 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= 6.4 m/s. Part (a) Write an expression for the work, Wr, done by the friction force between the ramp and the skateboarder in terms of the variables given in the problem statement. Expression : W = Select from the variables below to write your expression. Note that all variables may not be required. cos(0), sin(0), a, ß, n, Q, 0, d, g, hy, mg, P, t, Vf, Wf Part (b) The ramp makes an angle 0 with the ground, where 0 = 30°. Write an expression for the magnitude of the friction force, f,, between the ramp and the skateboarder. Expression : F%= Select from the variables below to write your expression. Note that all variables may not be required. cos(0), sin(0), a, ß, x, Q, 0, d, g, hy, mg, P, t, Vf, Wf Part (c) When the skateboarder reaches the bottom of the ramp, he continues moving with the speed vfonto a flat surface covered with grass. The friction between the grass and the skateboarder brings him to a complete stop after 5.00 m. Calculate the magnitude of the friction force, F, grass in newtons, between the skateboarder and the grass. Numeric Anumeric value is expected and not an expression. F grass