Conservation of mechanical energy

Question

Is the mechanical energy after the collision conserved or not conserved? Explain.

Answer 1

No. In order for mechanical energy to be conserved, you have to have an elastic collision. This is not an elastic collision because the cars stick together. Mechanical energy is only conserved when the forces acting on the objects in the system are conservative forces, like gravity, the (ideal) spring force, or the electric force. An elastic collision is caused by either the electric force or an idealized spring force on a microscopic scale as the colliding objects deform but then snap back into shape; the only way the objects could stick together would be by deforming and not snapping back into shape (like lumps of clay) or if they were magnetically attracted (magnetism is not a conservative force).