What goes up must come down. Gravity: it's not just a good idea, it's the law. It's also what puts the "coast" in roller coasters.
The physics are simple: roller coasters are powered by gravity. As the train of cars on
a coaster is pulled--purposely slowly--by a cable to the top of the first hill, potential
energy is introduced into the system. Once the train descends the hill, potential energy
is converted into kinetic energy. That's why even the most awe-inspiring coasters usually
start with the biggest drop--the train has to build up enough energy for the entire ride.
But speed isn't the only factor that makes or breaks a ride on a coaster. Positive gravitational forces, or g-forces, press you into the seat when you're at the bottom of a dip, and so-called negative g's create the sense of weightlessness, or "air time," as you're lifted against the safety bar over the peak of a hill.
The history of the roller coaster did not begin in a laboratory, though. In fact, it didn't start on a warm summer day at an amusement park either. Rather, the ride begins in the cold winter of 15th-century Russia.
THE RUSSIAN MOUNTAINS
The first "scream machines" were not out-of-control trains but sleds on
man-made 
ice-covered hills constructed
of cut lumber and tree trunks. Elaborate constructions often stretching several city
blocks, the 50 mile-per-hour rides earned the nickname "Flying Mountains."
Children and adults would make the tedious trek up 70 feet (five stories) of stairs and
climb into an ice-block sled outfitted with a straw seat. The trip back down lasted little
more than a few fleeting seconds.
"As you shoot along, a sort of ethereal intoxication takes hold of the senses, which is absolutely delightful," gushed one rider.
The fast-paced fun, and potential for high
profit, was brought to Paris in 1804 in the form of Les Montagnes Russes (Russian
Mountains). Small wheels were added to the sleds, but little attention was given to safety
measures. Oddly enough, the injuries that passengers suffered from runaway cars increased
attendance. Suddenly, thrill seekers had a new raison d'être.
Thirteen years later, Les Montagnes Russes
¦ Belleville (Belleville Mountains) and the Promenades Aeriennes (Aerial Walks) in Paris
improved on the original Russian Mountains, by adding locking wheels, continuous tracks,
and, eventually, cables that hoisted cars to the top of the hill. The history of roller
coasters was, well, downhill from there.
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