Answer:
\[u=10m/s\] \[v=-20m/s\] \[t=0.2\sec \] \[m=1/2kg\] When the defender strikes the ball, he is exerting the impulsive force on the foot ball. We know that, Impulse\[(1)=F\times t\] \[\Rightarrow \,\,mv-mu=F\times t\,\,\Rightarrow \]average force\[=F=\] \[\frac{mv-mu}{t}=m\left( \frac{v-u}{t} \right)\] ??(1) \[\therefore \]substituting the above values in (1), we have \[F=\frac{\frac{1}{2}(-20-10)}{0.2}=\frac{\frac{1}{2}(-30)}{0.2}=\frac{-15}{0.2}=-75\,\,N\] Therefore, average force exerted by the defender on the ball is 75N. (\[-ve\]sign indicates that this force is opposite to the initial direction of the ball.)
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