| It is the name given to a force like friction, tension, gravity, or the normal force when that force causes an object to move in a circle. When the roller coaster is in a vertical loop, the direction of the centripetal force is always changing to ensure that the force vector always points to the center of the circular track. Pay particular attention to how this is phrased. Although the force is always toward the center, the direction is always changing since in the circle, the force may be toward the left or the right or up, but still point toward the center. In the vertical loop, this centripetal force can be either the gravitational force, the normal force of the track on the car or a combination of the two. When it is a combination of the two, you must add the forces as vectors. At the bottom of the circle, the normal force points toward the center of the circle (upward) while the gravitational force points downward. The vector sum of these two forces must be toward the center of the circle. You can therefore conclude that the normal force is larger than the gravitational force. The normal force corresponds to your apparent weight, as it did in the elevator activity. This is why you feel as if you weigh more at the bottom of the loop of the roller coaster. At the top of the loop-the-loop, the gravitational force and the normal force both act downward, toward the center of the loop. The sum of these two vectors provides the required centripetal force. |
How much of the normal force is required will depend on the mass and velocity of the car. The red force vector represents the force of gravity or weight of the cars. Both weight vectors are identical because the weights of the roller coaster car are identical at the top and bottom. The black vector represents the normal force of the track on the car. The sum of the normal force plus the weight must be equal to the required centripetal force. At the top, the normal force is small since the weight contributes to the centripetal force. |
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