Why do not the roller coaster cars roll backwards back down first hill?

The answer is friction. Yes, gravity tries to pull the roller coaster cars back down the first hill, but the cars are firmly held on the track and do not move because the friction between the wheels of the cars and the track is greater than the force of gravity pulling the cars back down the hill. Friction will prevent roller coaster train from rolling backwards at any point along the coaster track, not only the first hill.

If you do some research, you can find that there is a specific equation to determine the amount of force (frictional force) that is needed to keep the train from rolling backward at the highest peak of the track:

$$F_f=mgsinθ$$

* Ff=frictional force

* m=mass of train

* g=gravitational field strength (9.81 m/s2)

* θ=maximum roller coaster track angle