Velocity versus time graph of a ball of mass

Here, $\mathrm{m}=50 \mathrm{~g}=5 \times 10^{-2} \mathrm{~kg}$

$u=80 \mathrm{~m} \mathrm{~s}^{-1}, v=0, t=8 \mathrm{~s}$

$\therefore$ Acceleration, $a=\frac{v-u}{t}=\frac{-80}{8}=-10 \mathrm{~m} \mathrm{~s}^{-2}$

Force, $\mathrm{F}=m a=-5 \times 10^{-2} \times 10=-0.5 \mathrm{~N}$

This force is known as retarding force or the frictional force of the floor on the ball.