Question

What is the minimum speed of a roller coaster at the top of a 39.0 meter tall vertical loop if the passengers are "weightless" at that point?

Answer 1

In the top point of the motion the centripetal acceleration should be equal to the gravitation acceletation:
g = V2/R
Thus
V = √(gR) = √(10*39) = 19.75 m/s