The work done to stop the bullet is equal to the variation of kinetic energy of the bullet:
[tex]W=K_f - K_i[/tex]
where Kf is the final kinetic energy of the bullet, while Ki is the initial kinetic energy.
The final kinetic energy is zero (because the final velocity of the bullet is zero), while its initial kinetic energy is
[tex]K_i = \frac{1}{2}mv_i^2 = \frac{1}{2}(0.010 kg)(400.0 m/s)^2 =800 J [/tex]
So, the work done to stop the bullet is
[tex]W=-K_i = -800 J[/tex]
with a negative sign because the direction of the force applied by the block of wood is opposite to the direction of motion of the bullet.
Now, we know that the work is the product between the force F and the distance d:
[tex]W=Fd[/tex]
Therefore, if we re-arrange the formula and we use d=3.00 cm=0.03 m, we can find the intensity of the force (neglecting the sign of the work):
[tex]F= \frac{W}{d}= \frac{800 J}{0.03 m}=2.67 \cdot 10^4 N[/tex]
