Right!

• Since the system includes both the car and the ground, the force of kinetic friction is now internal. The work done by the external forces (e.g., gravity) is zero.
• Since the system is now closed, and there are no external forces doing work on it, the total energy should be conserved, so all the kinetic energy lost is converted into "dissipated" (thermal) energy. Some of it ends up in the car (here taken to be 3 units, just like in the first diagram), and the rest (1 unit here) goes into the roadway, which is now included in the system.
• Of course, the kinetic energy lost is the same as when the system was only the car, since the car is the only part of the system that has kinetic energy in the first place.

Go back if you want to look at the original diagram for the car only.

Go back if you want to look again at the choice of diagrams for this question.

Good job! You may go on to the next question.