mass of the bullet, m = 0.02 kg
Initial speed of the bullet, u = 10 m/s
Final speed of the bullet, v = 0 m/s
Penetration, x = 10 cm = 0.10 m
Using the Newton’s law of motion, we get
v^2 = u^2 – 2ax
or, 0 = 100 – 2a*0.1
or, a = 100/0.2
or, a = 500 m/s^2
Thus, retardation a = 500 m/s^2
Now, when thickness of the wooden block x = 6 cm = 0.06 m
v^2 = u^2 – 2ax
or, v^2 = 100 – 2*500*0.06
or, v^2 = 40
or, v = 6.325 m/s
Kinetic energy of the bullet, KE = ½ mv^2
= ½*0.02*40 = 0.4 J