A fairground ride consists of a large drum (radius 4m). People stand around the edge of the drum, which begins to rotate about a vertical axis. When the drum is rotating fast enough, the floor drops away, but participants do not fall: they feel that they are pressed against the wall of the drum, and are held in place by friction. If the coefficient of friction on the wall is = 0.4, how long should the drum take to complete one revolution so that the participants are just prevented from slipping down? Draw a diagram showing all the forces acting on each person, and find the speed at which they are moving.