An object is kept on a smooth inclined plane of 1 in l. The horizontal acceleration to be imparted to the inclined plane so that the object is stationary relative to the incline is given by

See fig.From figuretan⁡θ=1/(l2−1)A horizontal acceleration imparted to the inclined plane has a component acos⁡θ down the plane.The object will appear stationary relative to the incline if acos⁡θ=gsin⁡θa=gtan⁡θ=g/l2−1.