A plank fitted with a gun is moving on a horizontal surface with speed of 4 m/s along the positive x-axis. The z-axis is in vertically upward direction. The mass of the plank including the mass of the gun is 50 kg. When the plank reaches the origin, a shell of mass 10 kg is fired at an angle of 60∘ with the positive x-axis with a speed of v=20 m/s with respect to the gun in x-z plane. Find the position vector of the shell at t=2 s after firing it. Take g=9.8 m/s2 .