A bullet of mass 0 . 1kg moving horizontally with speed 400m s ^ -...

Let the mass of the bullet be $m$ and mass of the block be $M$. It is given that $V = 400m{s}^{- 1}$. By conservation of momentum,

$mV = ( m + M ) v$

$⇒ 0 . 1 \times 400 = ( 0 . 1 + 3 . 9 ) v$
$⇒ v = 10m{s}^{- 1}$

The force acting on the block

$f = \mum ' g ⇒ a = \mug$

Using $v{'}^{2} - {v}^{2} = - 2aS$

$⇒$ ${0}^{2} - {10}^{2} = 2 ( - \mug ) ( S )$
$⇒ \mu = \dfrac{100}{2 \times 10 \times 20} = 0 . 25$