Suppose the skate boarder shown in problem 38 (cutnell & johnson, 9th ed) reaches a highest point of h=1.8 m above the right side of the semicircular ramp. He then makes an incomplete midair turn and ends up sliding down the right side of the ramp on his back. When the skateboarder reaches the bottom of the ramp, his speed is 6.4 m/s. The skateboarder's mass is 61 kg and the radius of the semicircular ramp is 2.7 m. What is the average frictional force exerted on the skateboarder by the ramp?