A laser sends out a pulse of 40 mJ that lasts 8.0 ns…

Step 1. The power. $P = \dfrac{U}{\tau} = \dfrac{0.040\ \text{J}}{8.0 \times 10^{-9}\ \text{s}} = 5.00\ \text{MW}$.

Step 2. The momentum. $p = \dfrac{U}{c} = \dfrac{0.040\ \text{J}}{3.00 \times 10^{8}\ \text{m/s}} = 1.33 \times 10^{-10}\ \text{kg m/s}$.

Step 3. The force. $F = \dfrac{P}{c} = \dfrac{5.000 \times 10^{6}\ \text{W}}{3.00 \times 10^{8}\ \text{m/s}} = 16.7\ \text{mN}$.