retarding force impacting a stationary object

JamesTheEng

Please can you check and if i have gone wrong tell me?

1. The problem statement, all variables and given/known data
A mass of 150 kg is dropped onto a pile of mass 1,200kg from a height of 2.5m and drives it 7cm into the ground. calculate (a) the intial velocity of pile and driving weight immediately after impact, (b) the average reisting force exerted by the ground.
150kg
gravity 9.81
1200kg
2.5m
7cm-0.07m


2. Relevant equations
m1u1+m2u2=m1v1+m2v2
V²=u²+2as
F=ma
Mg-r=MA


3. The attempt at a solution
Here is my attempt not the best.

Intial velocity of pile and driving weight after impact
m1u1+m2u2=m1v1+m2v2
(1200x7)+(150x0)=(1200+150)
8400=1700

(a)v=3.8m/s combired velocity after impact

caculating deccelration

V=0m/s combined mass

V²=u²+2as

0=14.44+(2xax0.07)
0=14.44+0.14a
a=-103m/s²

Resitance

F=MA
Mg-R=MA

(b) (1700x9.8)-R=(1700x-103)
16660-R=-175100
16660+175100=R
R=191700N

gneill

I can't follow any of that. It doesn't look like any kind of math that I recognize Where does 3.8m/s pop out of?