A bullet of mass 20text{ g} moving with a velocity of 300text{ m/s} penetrates a wooden block and comes to rest after traversing a distance of 5text{…

By work-energy theorem, work done by the resistive force equals the change in kinetic energy: $F \cdot d = \frac{1}{2}mv^2$. Here, $m = 0.02\text{ kg}$, $v = 300\text{ m/s}$, $d = 0.05\text{ m}$. $F \times 0.05 = \frac{1}{2} \times 0.02 \times (300)^2 = 0.01 \times 90000 = 900\text{ J}$. Thus, $F = \frac{900}{0.05} = 18000\text{ N}$.