# How is momentum conserved here?

1. May 23, 2010

### zeromodz

If a car moving with a constant velocity hits a large brick wall. The car just stops once it hits the wall? Both objects are now at rest after the collision, where does all the momentum go? Also, does making the wall vibrate count as anything, or does the wall have to move to give it momentum?

2. May 23, 2010

### cjameshuff

You've partly hit on the answer yourself...into the planet the wall is anchored into (or more precisely, into the resulting system of planet+wall+car wreckage), carried by the vibrations from the impact. (The entire planet can't react instantly, the vibrations carry the momentum and spread it through the planet. Note that vibration *is* motion, just of component parts rather than the whole.)

Since the planet is so much larger than the car, there's no noticeable change. Also note that the car used traction against the planet to accelerate in the first place...by smashing into the wall, it's basically just returning some momentum it borrowed while accelerating.

3. May 23, 2010

### Staff: Mentor

Since the total momentum is always zero, where does the kinetic energy go....? Some goes into permanently deforming the car, some is converted the heat.