A block of mass  m=0.1 kg is kept on a rough horizontal surface of friction coefficient $\mathrm{\mu }=0.2$. A time varying horizontal force $\mathrm{F}=\mathrm{kt}$ is applied on the block; where  $\mathrm{k}=1.04\mathrm{N}/\mathrm{s}$ and t is time in second. The rate of change of angle of friction just before the block starts slipping is