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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 bottom of a roller coaster loop, the passengers are entering a turn. They are beginning to move along a part of a circle (as pictured by the black dashed line) and they are at the bottom of that circle. The center of the circle is above the riders. There must be a net force directed toward this center in order to move along the turn. Think about how the normal force and the gravity force compare to each other in order to meet this centripetal force requirement. 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. The normal force results from the contact between the riders and their seats. The seats are below the riders and push upward on the riders. This upward normal force must be sufficiently large to meet the centripetal force requirement. The net force must be upward (refer to Dig That Diagram section). The only other force acting upon the riders is gravity. Gravity acts downward and away from the center of the circle. In order to have a net upward (towards the center) force, the normal force must be bigger than the force of gravity.
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