A charge of $4.0 \mu \mathrm{C}$ is moving with a velocity of $4.0 \times 10^6 \mathrm{~ms}^{-1}$ along the positive $y$ axis under a magnetic field $\vec{B}$ of strength $(2 \hat{k}) \mathrm{T}$. The force acting on the charge is $x \hat{i} N$. The value of $x$ is __________.