(a) From acting on block are shown in adjoining figure
As the block does not move, hence
$F\cos{60{^\circ} = f = \mu N = \mu\left( Mg + F\sin 60{^\circ} \right)}$
[image:5_image197.emf]
$\therefore F\frac{1}{2} = \frac{1}{2\sqrt{3}}\left( \sqrt{3} \times 10 + F \bullet \frac{\sqrt{3}}{2} \right)$
On simplification, we get $F = 20$ N