A box is placed on the floor of a truck moving with an acceleration of $7m/s^{-2}$. If the coefficient of kinetic friction between the box and surface of the truck is $0.5$ . Then the acceleration of the box relative to the truck is------ (in $m/s^2$ )

$\left[\text{ Take }\mathrm{g}=10{\mathrm{ms}}^{-2}\right]$