The acceleration of circular motion is limited by acceleration due to gravity. Two common scenarios are roller coaster on top of the loop and a car on top of a curved bridge or a hump. At such situations normal reaction force is zero .There will be a feeling of weightlessness for the passenger in the roller coaster or the car . The minimum acceleration of a roller coaster inside a loop can only be g ,the centripetal acceleration must be greater than g , the speed must be greater than (square root of rg ). A car going over a bridge also has a maximum acceleration of g and therefore the centripetal acceleration must be smaller than g, the speed must not be greater that (square root of rg) .The speed (square root of rg) is the min. speed and also the max. speed depending on the direction the normal reaction is acting.
link : unsw clips showing how a car becomes airborne, click airborne automobiles.
