The portion of a toboggan run shown is contained in a vertical plane.

The portion of a toboggan run shown is contained in a vertical plane. Section AB and CD have radii of curvature as indicated, and section BC is straight and forms an angle of 20 deg with the horizontal. Knowing that the coefficient of kinetic friction between a sled and the run is 0.10 and that the speed of the sled is 25 ft/s at B, determine the tangential component of the acceleration of the sled a) just before it reaches B, b) just after it passes C.