Physics question on kinematics

G123 SG

A metal ball is dropped from rest over a bed of sand. It hits the sand bed one second later and makes an impression of maximum depth 8.0mm in the sand. Air resistance is negligible. On hitting the sand, what is the average deceleration of the ball?

Can anyone tell me why can't I use s=ut + 1/2at^2 ? I put these values inside: s=8.0mm, u=0, a=9.81, t=1s. Ans is 6.0 x 10^3 m/s^2

8light SG

That formula works only for a trajectory which has constant acceleration. In this question, your acceleration changes when the ball hits the sand. You have to apply the formula separately to the two different motions - falling through air and decelerating on the sand.