When a ball is dropped into a lake from a height $4.9 \mathrm{m}$ above the water level, it hits the water with a velocity $\mathrm{v}$ and then sinks to the bottom with the constant velocity $\mathrm{v}$. It reaches the bottom of the lake $4.0 \mathrm{s}$ after it is dropped. The approximate depth of the lake is :