Answer
The minimum speed is 9.2 m/s.
Work Step by Step
If the centripetal acceleration is at least equal to $g$, then the people on the ride will not fall out. Therefore:
$\frac{v^2}{r} = g$
$v = \sqrt{gr} = \sqrt{(9.80 ~m/s^2)(8.6~m)}$
$v = 9.2 ~m/s$
The minimum speed is 9.2 m/s.
