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How Roller Coasters Work
by Tom Harris

If you're studying physics, there are few more exhilarating classrooms than a roller coaster. Roller coasters are driven almost entirely by basic inertial, gravitational and centripetal forces, all manipulated in the service of a great ride. Amusement parks keep upping the ante, building faster and more complex roller coasters, but the fundamental principles at work remain the same.

 Photo courtesy Coaster CentralThe Pepsi Max Big One, at Blackpool Pleasure Beach in Blackpool, England, has a winding track that is over 5,000 feet (1,524 meters) long.

In this edition of HowStuffWorks, we'll examine the principles that keep coaster cars flying around their tracks. We'll also look at the hardware that keeps everything running, as well as the forces that make the ride so much fun.

What Goes Up Must Come Down
At first glance, a roller coaster is something like a passenger train. It consists of a series of connected cars that move on tracks. But unlike a passenger train, a roller coaster has no engine or power source of its own. For most of the ride, a roller coaster is moved only by the forces of inertia and gravity. The only exertion of energy occurs at the very beginning of the ride, when the coaster train is pulled up the first hill (called the lift hill).

The purpose of this initial ascent is to build up a sort of reservoir of potential energy. The concept of potential energy, often referred to as energy of position, is very simple: As the coaster gets higher in the air, there is a greater distance gravity can pull it down. You experience this phenomenon all the time -- think about driving your car, riding your bike or pulling your sled to the top of a big hill. The potential energy you build going up the hill can be released as kinetic energy, energy of motion, as soon as you start coasting down the hill.

A roller coaster's energy is constantly changing between potential and kinetic energy. At the top of the first lift hill (a), there is maximum potential energy because the train is as high as it gets. As the train starts down the hill, this potential energy is converted into kinetic energy -- the train speeds up. At the bottom of the hill (b), there is maximum kinetic energy and little potential energy. The kinetic energy propels the train up the second hill (c), building up the potential-energy level. As the train enters the loop-the-loop (d), it has a lot of kinetic energy and not much potential energy. The potential-energy level builds as the train speeds to the top of the loop (e), but it is soon converted back to kinetic energy as the train leaves the loop (f).

The same thing happens in a roller coaster. When the coaster is released at the top of the first hill, gravity takes over. Gravity applies a constant downward force on the cars. The coaster tracks serve to channel this force -- they control the way the coaster cars fall. If the tracks slope down, gravity pulls the front of the car toward the ground, so it accelerates. If the tracks tilt up, gravity applies a downward force on the back of the coaster, so it decelerates.

Since an object in motion tends to stay in motion (Newton's first law of motion), the car will maintain a forward velocity even when it is moving up the track, opposite the force of gravity. When the coaster ascends one of the smaller hills that follows the initial lift hill, its kinetic energy changes back to potential energy. In this way, the course of the track is constantly converting energy from kinetic to potential and back again. This fluctuation in acceleration is what makes roller coasters so much fun.

 Photo courtesy Coaster CentralThe Pepsi Max Big One, at Blackpool Pleasure Beach: This first hill drops the train 205 ft (62 m) at 74 mph (119 kph).

In most roller coasters, the hills decrease in height as you move along the track. This is necessary because the total energy reservoir built up in the lift hill is gradually lost to friction between the train and the track, as well as between the train and the air. When the train coasts to the end of the track, the energy reservoir is almost completely empty. At this point, the train either comes to a stop or is sent up the lift hill for another ride.

At its most basic level, this is all a roller coaster is -- a machine that uses gravity and inertia to send a train along a winding track. In the next sections, we'll look at the different components that make this machine work.

Staying on Track
In the last section, we looked at the basic elements of a roller coaster. The coaster cars ride along on a long, winding track. The track begins with a steep ascent, which builds up a reservoir of potential energy in the coaster car. The rest of the track's hills, valleys, twists and turns serve to change the built-up energy back and forth between potential energy and kinetic energy. As the train moves, it gradually loses energy to friction until it reaches the end of the ride.

 Photo courtesy Coaster CentralThe Giant Dipper at Santa Cruz Beach Boardwalk, Santa Cruz, CA: This classic wooden coaster was built in 1924.

There are two major types of roller coasters, distinguished mainly by their track structure. The tracks of wooden roller coasters are something like traditional railroad tracks. The metal wheels of the coaster train roll on a flat metal strip, measuring 4 to 6 inches (10 to 15 cm) across. This strip is bolted to a sturdy running track made of laminated wood. In most coasters, the car wheels have the same flanged design as the wheels of a train -- the inner part of the wheel has a wide lip that keeps the car from rolling off the side of the track. The car also has another set of wheels (or sometimes just a safety bar) that runs underneath the track. This keeps the cars from flying up into the air.

Wooden coaster tracks are braced by wooden crossties and diagonal support beams. The entire track structure rests on an intricate lattice of wooden or steel beams, just like the beam framework that supports a house or skyscraper. With these materials, designers can combine the hills, twists and turns into an infinite variety of course layouts. They can even flip the train upside down (though this is rare in modern wooden coasters). But, since the track and support structure are so cumbersome, a wooden track is fairly inflexible. This makes it difficult to construct complex twists and turns. In wooden coasters, the exhilarating motion is mainly up and down.

 Photo courtesy Coaster CentralThe Dragon Khan, a tubular steel roller coaster at Universal Studios Port Aventura in Salou, Spain: The 4,165 feet (1,269 meters) of track in this coaster is twisted into several loops, twists and corkscrews.

The world of roller coasters changed radically with the introduction of tubular steel tracks in the 1950s. As the name suggests, tubular steel tracks consist of a pair of long steel tubes. These tubes are supported by a sturdy, lightweight superstructure made out of slightly larger steel tubes or beams. In tubular steel coasters, the train wheels are typically made from polyurethane or nylon. In addition to the traditional wheels that sit right on top of the steel track, the cars have wheels that run along the bottom of the tube and wheels that run along the sides. This design keeps the car securely anchored to the track, which is absolutely essential when the train runs through the coaster's twists and turns. The train cars in tubular steel coasters may rest on top of the track, like the wheels in a traditional wooden coaster, or they may attach to the track at the top of the car, like in a ski-lift. In suspended coasters, the hanging trains swing from a pivoted joint, adding an additional side-to-side motion. In an inverted coaster, the hanging train is rigidly attached to the track, which gives the designer more precise control of how the cars move.

A tubular steel track isn't laid out in small pieces like a wooden track. It is prefabricated in large, curved segments, like the steel girders in a skyscraper are prefabricated. The steel manufacturing process allows for a smoothly curving track that tilts the coaster train in all directions. On a wooden roller coaster, the ride is punctuated by the rattling sensation of the coaster rolling over the joints that connect the pieces of the wooden track. In a tubular steel coaster, the track pieces are perfectly welded together, making for an incredibly smooth ride. As any coaster enthusiast will tell you, each sensation has its own distinctive charm.

In the next section, we'll look at the coaster components that get the train rolling and bring it to a stop.

Starting and Stopping
As we saw in the first section, the roller coaster train has no motor of its own: For most of the ride, the train is moved by gravity and momentum. To build up this momentum, you need to lift the train to the top of the first hill or you need to give it a powerful launch.

 Photo courtesy Coaster CentralThe first hill of the Pepsi Max Big One at Blackpool Pleasure Beach: The train descends down the first hill at a 65-degree angle.

The traditional lifting mechanism is a long length of chain (or chains) running up the hill under the track. The chain is fastened in a loop, which is wound around a gear at the top of the hill and another one at the bottom of the hill. The gear at the bottom of the hill is turned by a simple motor. This turns the chain loop so that it continually moves up the hill like a long conveyer belt. The coaster cars grip onto the chain with several chain dogs, sturdy hinged hooks. When the train rolls to the bottom of the hill, the dogs catches onto the chain links. Once the chain dog is hooked, the chain simply pulls the train to the top of the hill. At the summit, the chain dog is released and the train starts its descent down the hill.

In some newer coaster designs, the train is set in motion by a catapult launch. There are several sorts of catapult launches, but they all do basically the same thing. Instead of dragging the train up a hill to build up potential energy, these systems start the train off by building up a good amount of kinetic energy in a short amount of time.

One popular catapult systems is the linear-induction motors. A linear-induction motor uses electromagnets to build two magnetic fields -- one on the track and one on the bottom of the train -- that are attracted to each other. The motor moves the magnetic field on the track, pulling the train along behind it at a high rate of speed. The main advantages of this system are its speed, efficiency, durability, precision and controllability.

Another popular system uses dozens of rotating wheels to launch the train up the lift hill. The wheels are arranged in two adjacent rows along the track. The wheels grip the bottom (or top) of the train between them, pushing the train forward.

Like any train, a roller coaster needs a brake system so it can stop precisely at the end of the ride or in an emergency. In roller coasters, the brakes aren't built into the train itself; they're built into the track. This system is very simple. A series of clamps are positioned at the end of the track and at a few other braking points. A central computer operates a hydraulic system that closes these clamps when the train needs to stop. The clamps close in on vertical metal fins running under the train, and this friction gradually slows the train down.

In the next section, we'll look at the various sensations you feel during a roller coaster ride to see what causes them and why they are so enjoyable.

Lighter than Air
In the last few sections, we looked at the forces and machinery that send roller coasters rocketing around elaborate courses. As you move over the hills, valleys and loops of the track, the forces on you seem to change constantly, pulling you in all directions. But why is this rollicking movement so enjoyable (or, for some people, so nauseating)?

 Photo courtesy Coaster CentralThe Boomerang at Knott's Berry Farm in Buena Park, CA: This coaster features many boomerang sections, where the track turns the train one way and then snaps it back the other way.

To understand the sensations you feel in a roller coaster, let's look at the basic forces at work on your body. Wherever you are on Earth, gravity is pulling you down toward the ground. But the force you actually notice isn't this downward pull -- it's the upward pressure of the ground underneath you. The ground stops your descent to the center of the planet. It pushes up on your feet, which pushes up on the bones in your legs, which pushes up on your ribcage and so on. This is the feeling of weight. At every point on a roller-coaster ride, gravity is pulling you straight down.

The other force acting on you is acceleration. When you are riding in a coaster car that is travelling at a constant speed, you only feel the downward force of gravity. But as the car speeds up or slows down, you feel pressed against your seat or the restraining bar. You feel this force because your inertia is separate from that of the coaster car. Newton's first law of motion states that an object in motion tends to stay in motion. That is, your body will keep going at the same speed in the same direction unless some other force acts on you to change that speed or direction. When the coaster speeds up, the seat in the cart pushes you forward, accelerating your motion. When the cart slows down, your body naturally wants to keep going at its original speed. The harness in front of you accelerates your body backward, slowing you down.

Your body feels acceleration in a funny way. When a coaster car is speeding up, the actual force acting on you is the seat pushing your body forward. But, because of your body's inertia, you feel a force in front of you, pushing you into the seat. You always feel the push of acceleration coming from the opposite direction of the actual force accelerating you. This force (for simplicity's sake, we'll call it the acceleration force) feels exactly the same as the force of gravity that pulls you toward the Earth. In fact, acceleration forces are measured in g-forces, where 1 g is equal to the force of acceleration due to gravity near the Earth's surface (9.8 m/s2, or 32 ft/s2).

A roller coaster takes advantage of this similarity. It is constantly changing its acceleration and its position to the ground, making the forces of gravity and acceleration interact in many interesting ways. When you plummet down a steep hill, gravity pulls you down while the acceleration force seems to be pulling you up. At a certain rate of acceleration, these opposite forces balance each other out, making you feel a sensation of weightlessness -- the same sensation a skydiver feels in free-fall. If the coaster accelerates downward fast enough, the upward acceleration force exceeds the downward force of gravity, making you feel like you're being pulled upward. If you're accelerating up a steep hill, the acceleration force and gravity are pulling in roughly the same direction, making you feel much heavier than normal. If you were to sit on a scale during a roller coaster ride, you would see your "weight" change radically from point to point on the track.

 Photo courtesy Coaster CentralRiders hang upside down on the Riddler's Revenge, a stand-up coaster at Six Flags Magic Mountain in Valencia, CA. The coaster track, which climbs as high as 156 ft (48 m) above the ground, is 4,370-ft (1,332-m) long.

This is the real heart-thumping thrill of a roller coaster -- your apparent weight is constantly changing intensity and direction. This has a strange effect on your body because your body is not completely solid -- it is composed of many different, loosely connected parts. When your body is accelerated, each part of your body is accelerated individually. The seat pushes on your back, the muscles in your back push on some organs and those organs push on other organs. This is why you feel the ride with your whole body. Everything inside is being pushed around. Normally, all the parts of your body are pushing on each other because of the constant force of gravity. But in the "free-fall" state of plummeting down a hill, there is hardly any net force acting on you. In this case, the various pieces of your body are not pushing on each other as much. They are all, essentially, weightless, each falling individually inside your body. This is what gives you that unique sinking feeling in your stomach -- your stomach is suddenly very light because there is less force pushing on it. The same thing happens when you drive down a dip in the road in your car or descend in an elevator moving at high speed.

On a roller coaster, this full-body sensation is complemented by all sorts of visual cues -- the upside down turns, dizzying heights and passing structures. Visual cues are an important part of the ride because they tell you that you are going fast. Your body can't feel velocity at all; it can only feel change in velocity (acceleration). The only reasons you know that you are moving quickly on a coaster is that the support structure is whipping past you at top speed, and the air is rushing in your face. Roller-coaster designers make sure to create plenty of tight fits and near misses to make you feel like you're rocketing through the structure at out-of-control speeds.

One of the most exciting elements in modern coasters is the loop-the-loop. These structures turn the whole world upside down for a few seconds. As you go around, your inertia not only produces an exciting acceleration force, but it also keeps you in the seat when you're upside down. In the next section, we'll see how this loop-the-loop works.

Feeling Loopy
A roller coaster loop-the-loop is a sort of centrifuge, just like a merry-go-round. In a merry-go-round, the spinning platform pushes you out in a straight line away from the platform. The constraining bar at the edge of the merry-go-round stops you from following this path -- it is constantly accelerating you toward the center of the platform.

 Photo courtesy Coaster CentralA teardrop-shaped vertical loop in the Canyon Blaster at the Adventure Dome in Las Vegas, NV

The loop-the-loop in a roller coaster acts exactly the same way as a merry-go-round. As you approach the loop, your inertial velocity is straight ahead of you. But the track keeps the coaster car, and therefore your body, from traveling along this straight path. The force of your acceleration pushes you from the coaster-car floor, and your inertia pushes you into the car floor. Your own outward inertia creates a sort of false gravity that stays fixed at the bottom of the car even when you're upside down. You need a safety harness for security, but in most loop-the-loops, you would stay in the car whether you had a harness or not.

As you move around the loop, the net force acting on your body is constantly changing. At the very bottom of the loop, the acceleration force is pushing you down in the same direction as gravity. Since both forces push you in the same direction, you feel especially heavy at this point. As you move straight up the loop, gravity is pulling you into your seat while the acceleration force is pushing you into the floor. You feel the gravity pulling you into your seat, but (if your eyes are still open) you can see that the ground is no longer where it should be.

At the top of the loop, when you're completely upside down, gravity is pulling you out of your seat, toward the ground, but the stronger acceleration force is pushing you into your seat, toward the sky. Since the two forces pushing you in opposite directions are nearly equal, your body feels very light. As in the sharp descent, you are almost weightless for the brief moment when you are at the top of the loop. As you come out of the loop and level out, you become heavy again.

 Photo courtesy Coaster CentralA loop in the Viper at Six Flags Magic Mountain in Valencia, CA: The Viper, which opened in 1990, cost \$8 million to build.

The loop-the-loop is amazing because it crams so much into such a short length of track. The varying forces put your body through the whole range of sensations in a matter of seconds. While these forces are shaking up all the parts of your body, your eyes see the entire world flip upside down. To many coaster riders, this moment at the top of the loop, when you're light as a feather and all you can see is the sky, is the best part of the whole ride.

In a loop-the-loop, the intensity of the acceleration force is determined by two factors: the speed of the train and the angle of the turn. As the train enters the loop, it has maximum kinetic energy -- that is, it is moving at top speed. At the top of the loop, gravity has slowed the train down somewhat, so it has more potential energy and less kinetic energy -- it is moving at reduced speed.

Originally, roller-coaster designers made circle-shaped loops. In this design, the angle of the turn is constant all the way around. In order to build an acceleration force strong enough to push the train into the track at the top of the loop, they had to send the train into the loop at a fairly high rate of speed (so it would still be going pretty fast at the top of the loop). Greater speed meant a much greater force on the rider as he entered the loop, which could be fairly uncomfortable.

The teardrop design makes it much easier to balance these forces. The turn is much sharper at the very top of the loop than it is along the sides. This way, you can send the train through the loop fast enough that it has an adequate acceleration force at the top of the loop, while the teardrop shape creates a reduced acceleration force along the sides. This gives you the force you need to keep everything running, without applying too much force where it might be dangerous.

Coasting Through History
Roller coasters have a long, fascinating history. The direct ancestors of roller coasters were monumental ice slides, popular in Russia in the 16th and 17th centuries. Ice slides had a fairly simple construction -- they consisted of a long, steep wooden slide covered in ice. Riders walked up a ladder or set of stairs to the top of the slide, as high as 70 ft (21 m) up. At the top, they climbed into sleds made out of wood or blocks of ice and shot down the slope. At the base of the slide, the sleds would crash-land in a sand pile.

Coaster historians diverge on the exact evolution of these ice slides into actual rolling carts. The most widespread account is that a few entrepreneurial Frenchmen imported the ice slide idea to France. For all but a few months out of the year, the warmer climate of France would melt the ice, so the French started building waxed slides instead. To help the sleds move down these slides, they added wheels. In 1817, the Russes a Belleville (Russian Mountains of Belleville) became the first roller coaster where the train was attached to the track (in this case, the train axle fit into a carved groove). The French continued to expand on this idea, coming up with more complex track layouts, with multiple cars and all sorts of twists and turns.

The first American roller coaster was the Mauch Chunk Switchback Railway, built in the mountains of Pennsylvania in the mid-1800s. The track was originally built to provide an easy way to send coal to the railway 18 miles (29 km) down the mountain. When the track was first built, a crew at the bottom of the mountain would attach the cart to a team of mules after emptying the load, and the mules would drag it back up to the top. To make the system more efficient, the railway designers eventually added more carts, a return track and a pulley system powered by large steam engines at the top of the mountain.

Soon after these improvements were made, the railway built a new tunnel that brought the freight trains much closer to the coal mine. Now useless for coal-mining, the switchback railway was reconfigured as a "scenic tour." For one dollar, tourists got a leisurely ride up to the top of the mountain, where they might enjoy a meal at the restaurant, a rest in the hotel or a walk in the woods, followed by a wild, bumpy ride straight down the mountain. The ride was soon a resounding success, attracting thousands of tourists every year.

 Photo courtesy Coaster CentralThe Giant Dipper at Santa Cruz Beach Boardwalk, CA: The Santa Cruz Beach Boardwalk is one of the few remaining boardwalk-style amusement parks from the 1920s, the glory days of roller coasters.

Over the next 30 years, these scenic rides continued to thrive and were joined by wooden roller coasters similar to the ones we know today. These coasters were the main attraction at popular amusement parks throughout the United States, such as Kennywood Park in Pennsylvania and the many parks of Coney Island in New York. By the 1920s, roller coasters were in full swing, with some 2,000 rides in operation around the country.

With the Great Depression and World War II, roller-coaster production declined, but a second roller-coaster boom in the 1970s and early 1980s revitalized the amusement-park industry. This era introduced a slew of innovative tubular steel coasters. Some of the most popular ride variations -- such as the curving corkscrew track -- saw their heyday around this time.

In the 1990s and 2000s, the amusement-park industry has experienced another coaster boom of sorts. New catapult launching techniques, hanging-train designs and other technological developments have opened up a world of options for designers. In recent years, designers have introduced coasters that have you lying flat against the train car so you feel as if you are flying and coasters that shoot you down long stretches of spiraled track. In the next 15 years, we are sure to see many faster, taller and more twisted rides popping up in amusement parks around the world. Roller coasters provide a unique, full-body experience that attracts thrill-seekers from hundreds of miles away.

For much more information on roller coasters and related topics, check out the links on the next page.