A block of metal weighing $2\mathrm{ }\mathrm{kg}$ is resting on a frictionless plane. It is struck by a jet releasing water at a rate of $1 \mathrm{kg} {\mathrm{s}}^{-1}$ and at a speed of $5 \mathrm{m} {\mathrm{s}}^{-1}$. The initial acceleration of the block will be