newton's third law: motion of connected particles.?

Question

From the instant that a pile driver of mass 2000 kg comes into contact with a pile of mass 600 kg,they move vertically downwards together,with common deceleration 0.8 m/s^2 until they come to rest.Make separate sketches showing the forces acting on the pile-driver and on the pile.
Find the magnitudes of all the forces exerted on the pile
a) when it is moving
b)after it has come to rest

Answer 1

No, there's enough info here. The driver will not be in contact with the soil, so only the pile has the resistance on it to stop the penetration. (Part b is a clue for something I might have forgotten otherwise: weight.) The mass and acceleration can give you the net force acting on the 2 particles.
Fnet = (m1+m2)*a
Fnet acts upward. The components are weight downward and the resistance of the soil upward.
Fnet = Fres - W
where W = (m1+m2)*g
Calculate out Fnet and W and then you can get Fres.

So forces on the pile:
(m1+m2)*g downward and Fres upward. For the pile, it's as if the 2 are really one object.

Forces on the pile driver: Need to repeat the Fnet analysis for the driver only. Call it Fnet2 when looking at the driver. The driver is sitting on the pile once contact is made. Call the support that the pile is providing Fs. And call the driver's weight W2. So
Fnet2 = m2*a = Fs - W2
where W2 = m2*g

b) Downward: the pile's weight + the driver's weight
Upward: the soil's support which equals the pile's weight + the driver's weight.

Answer 2

you left out some important info, like intial velocities.