In a popular amusement park ride, a rotating cylinder of radius R = 3.10 m is set in rotation at an angular speed of 4.20 rad/s, as in the figure shown below. The floor then drops away, leaving the riders suspended against the wall in a vertical position. What minimum coefficient of friction between a rider's clothing and the wall is needed to keep the rider from slipping? Hint: Recall that the magnitude of the maximum force of static friction is equal to ?sn, where n is the normal force—in this case, the force causing the centripetal acceleration.