Final answer:
In a typical collision, momentum is conserved as long as there are no external forces acting on the system.
Step-by-step explanation:
A quantity which is conserved in a typical collision of a car and a truck is momentum. This is because in physics, momentum is always conserved in a collision, provided there are no external forces acting on the objects involved. On the other hand, kinetic energy may not be conserved in a collision if it is inelastic. For instance, during an inelastic collision, objects can stick together, converting some of their kinetic energy into other forms of energy such as heat or sound; as a result, total kinetic energy is not conserved. However, the mass and velocity of individual objects change independently and are not conserved quantities for the system.