An open tank 10 m long and 2 m deep is filled upto 1.5 m height of oil of specific gravity 0.82. The tank is uniformly accelerated along its length from rest to a speed of 20 m/sec horizontally. The shortest time (in sec) in which the speed may be attained without spilling any oil is: $\left[\mathrm{g}=10\mathrm{m}/{\sec }^2\right]$