The Physics Behind Your Favorite Roller Coasters

Roller coasters may be vomit- and tear-inducing thrill machines, but they're also fascinating examples complex physics at work.

Getting a string of cars through a knot of drops, flips, rolls, and launches requires teams of mechanical engineers analyzing concepts like forces, acceleration, and energy. To get an idea of the science behind our favorite rides, we spoke to Jeffrey Rhoads, a professor at Purdue's School of Mechanical Engineering and creator of the university's roller coaster dynamics class.

Completing the Circuit

Let's start with the basics. Roller coasters, like everything else, must obey the law of conservation of energy, meaning the train can only go as fast and as far as the amount of stored (potential) energy allows.

Potential energy usually comes from lifting the train up a hill with a chain or cable. As a train travels down a hill, the potential energy turns into moving (kinetic) energy; the faster the train goes, the more kinetic energy it has.

The kinetic energy turns back into potential energy as the cars ascend subsequent hills. Because the cars necessarily lose some energy through forces like friction and air drag, the highest point on a traditional coaster (think: Six Flags Magic Mountain's Goliath or Twisted Colossus rides) is almost always the first hill. If there's another major drop coming higher than the first, the designers add more lifts (think: the big drop at the end of Disney's Splash Mountain).

Some coasters drop further than 90 degrees, curving inward at the top of the lift hill, like on Valravn in Cedar Point. The physics at play are the same, but Rhoads says these drops can offer a more acute feeling of weightlessness.

Other coasters, like Six Flags Great Adventure's Kingda Ka or Cedar Point's Top Thrill Dragster, store their energy in launchers, fluid or air pressure-powered pinball plungers, or in electromagnets built into the track and cars. Launch coasters don't require gigantic lift hills (which saves a lot of space), and offer a different kind of anticipatory thrill. "Large parks want a variety of rider experiences and launch coasters are a great way to change the feel," says Rhoads.

Loops, Flips and Turns

Engineers generate thrill through acceleration—basically changing riders' velocity in highly engineered, unnatural ways. Coaster engineers call upon Newton's laws of motion to get riders to feel the combined forces of gravity and acceleration, which produces an exciting, unusual body feel. Loops, corkscrews, and tight turns force riders' bodies vertically and horizontally in calculated ways.

Ever wonder why loops are teardrop shaped, rather than circular? "The challenge is designing the transitions into and out of the loop," Rhoads says. "You need to make sure that you're not inducing jerk," or changes in acceleration that can lead to whiplash. Anything moving in a circular motion experiences another kind of acceleration called centripetal acceleration, which increases the faster the car goes, or the smaller the circle is. A circular loop would cause a jolt from the sudden addition of the centripetal acceleration. A teardrop shape controls that acceleration, easing the rider through the loop and preventing jerk.

And then there are rolls, which can disorient riders in several ways. Inline twists are rolls that rotate trains around the track, but heartline rolls try to rotate riders around their chests. Colossus in Thorpe Park (above) is the best example of heartline rolls at work—the 90-second ride boasts 10 inversions, including four consecutive heartline rolls. "We'll see more [coasters with] multiple rolls in series one after another," said Rhoads, "because it creates a tremendous amount of disorientation."

Wood Versus Steel

Wooden coasters can't accommodate loops very well, so they're often less disorienting than their steel counterparts. So why is it that some riders prefer them? "People... like the anticipation, the rickety-ness of them that amps them up a little bit. They want to feel like the structure is moving underneath them," says Rhoads. "Steel coasters are almost the exact opposite. It's like driving an antique vehicle versus driving the newest sports car."

Wooden coasters tend not to have loops or rolls, because it would take far too much wood to support the force of a heavy roller coaster train. Hades 360 at Mt. Olympus in Wisconsin supports a roll on wooden tracks with steel scaffolding.

Next-Generation Coasters

There are only so many ways you can fling people around in little carts by sending them up, down, and upside down. Some ride builders create compartments that roll independently from the cars, circling axes perpendicular to the track, which adds more flips without needing more loops. You can really see this on The Joker at Six Flag's Great Adventure (below).

Roller coaster experiences are more than just the sum of their accelerations, though. Other builders are adding lights, smoke, sending coasters underground, and adding "head" and "foot choppers," close-but-not-too-close bars that provide an extra element of thrill and/or terror. "That's the trajectory we're going to follow for a while," said Rhoads. "Bigger and faster won't be possible for much longer."