When a collision occurs in an isolated system, the total momentum of the system of objects is conserved. Provided that there are no net external forces acting upon the objects, the momentum of all objects before the collision equals the momentum of all objects after the collision. If there are only two objects involved in the collision, then the momentum change of the individual objects are equal in magnitude and opposite in direction. Certain collisions are referred to as elastic collisions.
Elastic collisions are collisions in which both momentum and kinetic energy are conserved.
The total system kinetic energy before the collision equals the total system kinetic energy after the collision. If total kinetic energy is not conserved, then the collision is referred to as an inelastic collision. In the collision between the truck and the car, total system momentum is conserved. The total system momentum is conserved. The total kinetic energy before the collision is not equal to the total kinetic energy after the collision.