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Homework Help: Principle of conservation of momentum

  1. Jan 25, 2010 #1
    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.

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

    Why "the Earth" and not just the car?
  2. jcsd
  3. Jan 25, 2010 #2
    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 transfered 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.
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