I can't figure this out, I could use an explanation too!

A speeding truck travels in the positive x direction. It hits a car traveling in the positive y direction. The vehicles are deformed and travel together at an angle θ with the positive x–axis. What can you say about the kinetic energy of the system after the collision?

1)Kinetic energy of the system is conserved.

2)Kinetic energy of the system is partially used up in the deformation of the vehicles.

3)Kinetic energy of the system is unrelated to the deformation of the vehicles.

4)Kinetic energy of the system is completely transformed to heat energy.

The collision described in the problem is a type of collision which is called "perfectly inelastic collision". In a perfectly inelastic collision, only the total momentum of the two objects colliding is conserved, while the total kinetic energy is not. In fact, in a perfectly inelastic collision the two objects stick and move together after the collision: since the two objects stick together, part ofthe total kinetic energy is converted into heat due to the frictions between the surfaces of the two objects.

Therefore, the correct answer is

2)Kinetic energy of the system is partially used up in the deformation of the vehicles.