The trajectory of point C is a quarter circle, so the tangent direction of the trajectory is the velocity direction (note that it is not the same as the direction of the shaft), and then judge the included angle between the direction of the shaft and the velocity direction (+( 1/2 π-2 times the known angle), and of course, use cos at last, so don't care about the sign. Finally, the answer is based on the component of the velocity along the axis at any point on the axis.