Principle of conservation of momentum

[Masafi]

In a test laboratory, a car is crashed into a concrete wall and comes to rest. There is no damage to the wall. Explain of how the principle of conservation of momentum applies to this situation.

Answer:
The Earth plus car recoils with same momentum as the car had

Why "the Earth" and not just the car?

 

[dacruick]

I'm not sure I understand your question. But we assume that the concrete wall is attached indefinitely to the earth. So the Earth as a closed system must have a finite amount of momentum. All that is saying that after the collision, all the car's momentum was transferred into everything on earth . since the mass of the Earth is enormous in comparison to the car, the acceleration of the earth(and everything on it) is negligible.