Physics - W.E.P. Question with Solution | TestHub

The initial kinetic energy of the ball is $K_i=0$. The final kinetic energy is $K_f=\frac{1}{2}m{v}_f^2=\frac{1}{2}\cdot 2\cdot (10{)}^2=100\text{ J}$. The work done by gravity is $W_g=mgh=2\cdot 10\cdot 20=400\text{ J}$. Let the constant air resistance force be $f_{air}$. The work done by air resistance is $W_{air}=-f_{air}\cdot h=-f_{air}\cdot 20$. According to the Work-Kinetic Energy Theorem, $W_{net}=\Delta K=K_f-K_i$. So, $W_g+W_{air}=K_f-0$. Substituting the values, $400-20f_{air}=100$. This gives $20f_{air}=300$, so $f_{air}=\frac{300}{20}=15\text{ N}$.