an amusement park ride consists of a large vertical wheel of radius r that rotates counterclockwise on a horizontal axis through its center, as shown above. the cars on the wheel move at a constant speed v. points a and d represent the position of a car at the highest and lowest point of the ride, respectively. a person of weight fg sits upright on a seat in one of the cars. as the seat passes point a, the seat exerts a normal force with magnitude 0.8fg on the person. while passing point a, the person releases a small rock of mass m, which falls to the ground without hitting anything. question what is the normal force exerted on the rider when passing point d?