The cable on an elevator of mass m snaps, and two safety mechanisms come into play. Emergency brakes apply a constant friction force ƒ for the entire drop. Also, at the bottom of the elevator shaft is a vertical cushioning spring (initially uncompressed) having spring constant k. The elevator drops from a distance h above the spring. At the point where the spring is compressed a distance d (before the maximum compression, as the elevator is still dropping), find an expression for the speed.