Final answer:
At the top of a roller coaster loop, the force exerted on a 65 kg rider by the seat is downward, created by both the gravitational force and the centripetal force necessary for circular motion.
Step-by-step explanation:
The direction of the force exerted on a 65 kg rider by the roller-coaster seat at the top of the loop is downward, towards the center of the loop. When a roller coaster is at the top of a loop, the force exerted by the seat on the rider is the sum of the gravitational force and the centripetal force required to keep the rider moving in a circular path. Since the roller coaster is designed with a smaller radius of curvature at the top of the loop, this ensures that the centripetal force is greater than the gravitational force, thus keeping the rider pressed firmly into the seat. The downward acceleration of the roller coaster car is given as 1.50 g, which indicates that the net acceleration is directed downwards, causing a greater force on the rider than just their weight alone.