Solving the Mystery of Drop Out Ride: Find Rev/s Needed for Floor Drop Out

[habs.fan]

Homework Statement

In a ride called Drop Out, riders are spun in horizontal circles of radius 5.5m, which forces them to the outer wall. When they are spinning fast enough, the floor drops out, and they are suspended by friction. The coefficient of static friction is 0.28, how many rev/s must the ride achieve before the floor is allowed to drop out?

Homework Equations

a_{}c = rv^2

The Attempt at a Solution

I'm sort of lost for what to do, I have an FBD with G (down) and Friction (up), I also know the centripetal acceleration will be along the radius, towards the center of the circle, but I do not know how to solve this one.
Thanks in advance.

 

[Dick]

No, a=v^2/r. If you know the centripetal acceleration, then you know the normal force. You then can relate the frictional force to the normal force, et voila. Please continue.

 

[habs.fan]

ok so...ac = v^2/5.5
v is unknown though, so we don't really know the centripetal acceleration?

 

[Dick]

You know the radius, and v can be expressed in terms of the angular velocity. Leave it unknown. That's what you want to solve for.