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Centripetal Force Requirement:
Circular motion requires an inward force. To travel along the curved path of a circle, there must be a force directed centripetally. Any object or thing could supply the force as long as it is directed toward the center of the circular path.
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At the top of a roller coaster loop, the passengers are moving along a curved path. They are traveling along a part of a circle (as pictured by the black dashed line) and they are at the top of that circle. The center of the circle is below the riders. There must be a net force directed towards this center in order for the riders to continue along the turn. Constructing a free body diagram with the two individual forces shown will assist in answering this question. See Minds On Time section.
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You will need to find two answers that describe the normal force acting upon the riders. One answer will describe the direction and the other answer will describe the magnitude of the normal force. This is a unique situation in that the riders are upside down and the seats are above them. As such, the seats push downward on the riders. This means that there are two downward forces acting upon the riders at the top of the loop - gravity and normal force. The net force must be downward (refer to Dig That Diagram section). Since both individual forces are acting downward, the strength of the normal force will depend upon the acceleration. After all, gravity can supply as much as 9.8 m/s/s of downward acceleration. The normal force must supply the remaining force that is needed to sustain any acceleration greater than the gravitational value.
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