In an amusement-park ride, riders stand with their backs against the wall of a spinning vertical cylinder. The floor falls away and the riders are held up by friction. The acceleration of gravity is 9.81 m/s 2 . If the radius of the cylinder is 3 m, find the minimum number of revolutions per minute necessary when the coefficient of static friction between a rider and the wall is 0.7. Answer in units of rev/min