Solution:
Here, $m=1 \mathrm{~kg}, u=20 \mathrm{~m} \mathrm{~s}^{-1}$
$v=0, \quad s=50 \mathrm{~m}, \quad F=?$
From, $v^{2}-u^{2}=2 a s$,
$0-(20)^{2}=2 a \times 50=100 a$
or $a=\frac{-400}{100}=-4 \mathrm{~m} \mathrm{~s}^{-2}$
$\therefore \quad F=m a$,
$F=1(-4)=-4 \mathrm{~N}$
Negative sign indicates that force of friction is opposing the motion of the ball.