$(10\backslash \%)$ Problem $10$: A skateboarder with massms$=53$kgis standing at the top of a ramp which ishy$=4\mathrm{.}8$mabove the ground. The skateboarder then jumps on his skateboard and descends down the ramp. His speed at the bottom of the ramp isvf$=6\mathrm{.}7$m$/$s$.$

A$.$ The ramp makes an angle$$$$with the ground, where$$$=30.$ Write an expression for the magnitude of the friction force,$$fr$,$ between the ramp and the skateboarder.

B$.$ When the skateboarder reaches the bottom of the ramp, he continues moving with the speed$$vf$$onto a flat surface covered with grass. The friction between the grass and the skateboarder brings him to a complete stop after $5\mathrm{.}00$ m$.$ Calculate the magnitude of the friction force,$$Fgrass$$in newtons, between the skateboarder and the grass.