Question

What type of impact will cause the metal in the car to "splash" (eliminating all hope of a simple conservation of momentum calculation) should the New Horizons interplanetary space probe hit your car?

Answer 1

The only thing which prevents linear momentum from being conserved is an external force. If there are no forces other than those exerted by the car and the probe (or pieces thereof), linear momentum is conserved. For example, if the collision happens in empty space with the probe of mass M moving with velocity V in the frame where the car is at rest, the total momentum of all the

pieces will be MV after the collision or any time during the collision. What will not be conserved is the kinetic energyMV2.