A block of metal weighing 2 kg is resting on a frictionless plane (as shown in figure). It is struck by a jet releasing water at a rate of $1 \mathrm{kgs}^{-1}$ and at a speed of $10 \mathrm{~ms}^{-1}$. Then, the initial acceleration of the block, in $\mathrm{ms}^{-2}$, will be