Dear student,
Given: Initial velocity, u = 20 m/s ; Final velocity, v = 0 m/s and s = 10 cm = 0.1 m
We use the third equation of motion: v2 = u2 + 2as
→ 0 = (20)2 + 2 × a × 0.1
→ 400 = – 0.2a
→a = –2000 m/s2
This is the acceleration of the bullet.
The retardation is the negative value of the acceleration.
Thus, the retardation of the bullet is 2000 m/s2
Regards!!