A stone of $1\mathit{ kg}$ is thrown with a velocity of $20\mathit{ m}/s$ across the frozen surface of a lake and comes to rest after travelling a distance of $50\mathit{ m}$. The force of friction between the stone and the ice is _______ $N$.

A stone of $1\mathit{ kg}$ is thrown with a velocity of $20\mathit{ m}/s$ across the frozen surface of a lake and comes to rest after travelling a distance of $50\mathit{ m}$. The Force of friction between the stone and the ice is$-4\mathit{ N}$.
Given:
Mass, $\mathit{m }= 1\mathit{ kg}$
Initial velocity, $\mathit{u }= 20\mathit{ m}/s$
The final velocity, $\mathit{v }= 0\mathit{ m}/s$
Distance travelled, $\mathit{s }= 50\mathit{ m}$
From the third equation of motion,
    $v^{2 }= u^2 + 2\mathit{as }$ $\Rightarrow \mathit{a }= \frac{v^{2 } - u^2}{2s}$ $\Rightarrow$      $=\mathit{ }\frac{0 - {\left(20 \right)}^2}{2 \times 50 }$
$\Rightarrow$      $= \frac{-400}{100}$
      $a=\mathit{ –}4\mathit{ m}/s^2$
We know,
Force of friction between the stone and the ice is $=\mathit{mass }\times \mathit{ acceleration}$.
                     $= 1\mathit{ kg }\times (-4)\mathit{ m}/s$²        $= -4\mathit{ N}$
Negative sign indicates that the direction of the force of friction is opposite to the direction of moving stone.