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

Answer 1

When you ascend the first hill of a roller coaster, you will like hear a continuous series of loud "clicking" sounds. This is the result of a device called a "Chain Dog," which is a roll-back safety device, part of which is on the lift hill itself and part of which is on the coaster train.

If you look between the tracks on the lift hill, you will see a strip or two of metal that have a series of raises and dips. On the bottom of each coaster car, there are one-way hinged metal flaps that drag along this strip of raised metal, sliding over the raises and falling into the dips. In the event that the chain fails--or worse, breaks--the one-way hinged metal flaps will fall into the dips, the coaster train will back up very slightly, and the metal flaps will then lock into place in one of these dips. This device is called a "Chain Dog," and it is what prevents a roller coaster from rolling backward down the lift hill.