Question

A bullet initially moving with a velocity 20ms strikes a target and come to rest after penetrating a distance 10cm in target. Calculate the retardation caused by the target.

Answer 1

average speed in block= 10m/s
time to go .1 m n block= .1/10m/s=.01sec

force*time= changemomentum
and change in momentum is massbullet*20m/s

so you need mass bullet

Answer 2

See previous solution (under related questions):
http://www.jiskha.com/display.cgi?id=1337537113