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Conservation of momentum in Car Vs. Wall

  1. Sep 28, 2009 #1
    If a car crashes into a solid unbreakable wall, how is momentum conserved (I know it must be). To me, it seems that the car will just "stop" and the wall will not move. If momentum is to be conserved, the car should either move through the wall, (which doesn't happen) or the wall should move away (this doesn't happen either).
  2. jcsd
  3. Sep 28, 2009 #2
    Momentum is not conserved in this problem because of the oversimplification. The momentum will mostly transfer into the air and wall as heat and sound. So, it is conserved, you just need to take the entire Earth and it's atmosphere as well.
  4. Sep 28, 2009 #3


    Staff: Mentor

    The conservation of momentum applies to an isolated system. If your system is subject to some non-negligible net external force then its overall momentum will change by Newton's second law.

    In this case the car-wall system is acted on by a net external force (the contact force of the wall with the ground). That force becomes very large during the collision. If the force were not there (e.g. wall on slippery ice) then the wall would indeed move.
  5. Sep 28, 2009 #4
    you're somewhat confused with conservation of energy. The engergy will end up as heat (mostly heating the car itself) and sound. Almost all of the momentum will end moving the earth by a tiny bit.
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