A smooth inclined plane of length l, having an inclination θ with horizontal is inside a lift which is moving down will retardation a. The time taken by a body to slide down the inclined plane from rest will be

Using s=ut+12at2, we have L=012(g+a)sin⁡θt2∴t=2L(g+a)sin⁡θ1/2On inclined plane, g sin θ is replaced by (g + a) sin θ as the lift is moving down with retardation a (this is equivalent to moving up with acceleration a).