A boy prevents fall of his book on the ground by pressing it against a vertical wall. If weight of his book is 10 kg and ${\mu }_s$  of the wall is 0.2. Find the minimum force needed by him in his attempt. $(given,g=10m{s}^{-2})$ (In Newton)

$m=10kgand{\mu }_s=0.2$The various forces acting on the book are shown in the figure. 
At equilibrium,
         

                                               
R=N          ….(i)

 ${\mu }_{s}R=mg\mathrm{....}\left(ii\right)$

Given,     

From Eq. (ii), we get

  $0.2R=10\times 10$

 $R=\frac{100}{0.2}=500N$

  Minimum force needed is 500N.