A block of mass 40 kg slides over a surface, when a mass of 4 kg is suspended through an inextensible massless string passing over frictionless pulley as shown below. The coefficient of kinetic friction between the surface and block is 0.02. The acceleration of block is. $\left(\right.$ Given $\left.\mathrm{g}=10 \mathrm{~ms}^{-2}\right)$