Imagine me holding an apple above the ground and at rest
with respect to it. I let go of the apple. The apple starts
to fall immediately and so gains downwards momentum.
On the other hand, the earth does not know to move upwards
right away. It needs to wait for the information to get to
it which is delayed by the speed of light. Consequently,
the earth's motion up is just a fraction delayed.
So, briefly, the apple has downwards momentum while the
earth does not yet have upwards momentum. (a) Momentum is
not conserved?
(b) Since the apple is moving longer than the earth, it
will gain a greater change in momentum than the earth. In
inelastic collisions the combined object moves in the
direction that the object with the greater momentum had
before the collision. So, the apple can give the earth a
net velocity?