+33614 Assuming the rock is near enough to the surface of the Earth that we can take its acceleration to be g (=9.81m/s2)' then we can get its velocity at 5m from vrock = sqrt(2*9.81*5)m/s, and its momentum as mrock * vrock (since it starts from rest).
To get Earth's change in velocity use mEarth*vEarth = -mrock*vrock
