The last force is the kinetic friction force—it's parallel to the surface. It is this frictional force that slows down the car (since there is no other horizontal force).

Friction: Crash Course Physics #6 Episode 6 | 10m 27s | CC In today's episode, Shini tells us about Static and Kinetic friction. Aired 05/05/2016 | Rating NR From Problems playing video? Report a ...

No matter what that Intro to Physics book says, you can't calculate the "work done by friction." Nope. Doesn't work. Don't even try it.

The "friction coefficient," or the resistance to relative motion between the blocks, was surprisingly small at 0.08, the scientists point out.