At what minimum speed must a roller coaster be traveling when upside down at the top of a circle (fig. 5-34) so that the passengers will not fall out? assume a radius of curvature of 9.6 m.

At top, 2 forces act on passengers: normal force, force of gravity. The two act downward in the direction of the center of the circle. 
To answer the problem we would be using this formula which is apply N2L: 
Fnet = ma 
mg + N = mv^2/r 
minimum speed occurs when N = 0 
mg = mv^2/r 
v = sqrt (rg) 
= √94.09
v = 9.7 m/s