How Zip Lines Work

Unlike multiple universes or string theory, the physics of zip lines is actually pretty intuitive, if you understand some basic principles.

First is that a zip line needs to descend on a slope. Gravity and inertia dictate that we'll be yanked unceremoniously toward the ground unless an opposing force is acting upon us to stop that motion. So here's a hint: If someone is trying to get you to zip line from, say, the bottom of an empty pool to his or her roof, you can politely decline and smile smugly because you know that just won't work.

So, you've got a line tied at a slope. Now, how the heck do you slide down it fast enough for it to be -- well -- fun? The answer lies in the pulley that attaches to the rope or cable. A pulley is a wheel with a grooved rim known as a sheave, and the wheel turns as it travels along. The advantage of a pulley is that it causes much less friction than pulling your jacket over the wire and trying to slide down, like the action star in our intro. Less friction equals more speed.

Along with the pulley and the line, which will be anchored at two spots, zip lines use some device that lets the rider take advantage of the pulley. This could include a harness or seat that attaches to the pulley by a carabiner, allowing you to travel down the line.

As you're flying down a zip line, reveling in the wonders of gravity, at some point you will no doubt wonder how, exactly, this ride ends. Professionally designed zip lines, thankfully, do have braking mechanisms. Some are active (where the participant or the instructor is manually in charge of the brake), and some are passive (where the "brake" is usually a gentle upward slope at the end of the ride, slowing down the rider and eventually bringing him or her to a stop).

Steve Gustafson, president of the board of directors of the Professional Ropes Course Association and owner of Experience Based Learning (a canopy tour company), says that he recommends zip lines with a passive braking system: "The person with the least experience and least knowledge is the guest. So why put them in control of their descent at 35 miles per hour? You're asking for problems."

In the next section, we'll see who decided that going down a zip line at 35 miles per hour (56.3 kilometers per hour) was a good idea in the first place.

While there's not much historical record of when the first zip line came along, there is ample evidence that people living in mountainous regions -- particularly the Himalayas and the Alps -- strung up zip lines quite early in their culture to both traverse dangerous country and to receive and carry supplies more efficiently.

Of course, mountain climbers have also been partial to the zip line for some time. While a Tyrolean traverse is a common mountaineering practice (shimmying across a line to cross between two steep points, sometimes without a pulley at all), the zip line one-upped the Tyrolean by using the gravity of the slope to make the process a little quicker.

On a more contemporary note, it might surprise you to know that the people who popularized zip lining were those reckless thrill seekers known as wildlife biologists. Indeed, it was biologists studying densely forested areas that needed a way to unobtrusively snake into -- or above -- a forest canopy.

It was this unlikely reason that zip-line tours -- also referred to as canopy tours -- sprung up as a recreational activity in the rainforest of Costa Rica under the banner of eco-friendly tourism. Promising the adventure of flying like a bird through a natural habitat, these zip-line rides have become a huge industry in the adventure tourism trade of that region.

Zip lines have also become a large part of ropes challenge courses, designed to teach teamwork and provide recreational activities through team-building and problem solving, as there are generally high-fives all around when a high-speed trip down a cable on a frighteningly simple machine ends in all participants still breathing.

Spreading in popularity, zip lines have sprung up throughout the United States and internationally. In fact, according to the Association of Challenge Course Technology, there are more than 150 courses in the United States alone [source: Allen]. Later, we'll check out some of the most extreme and intriguing zip lines in the world. But before we do ... safety first.

The first rule of zip line safety is to trust the professionals. While any kid with an elementary grasp of gravity and a pulley can make a zip line in his or her backyard, a professional accreditation by either the Professional Ropes Course Association (PRCA) or the Association for Challenge Course Technology (ACCT) ensures a regulated experience.

Both of these organizations are now accredited by the American National Standards Institute (ANSI) to develop industry standards and regulations. The European Ropes Course Association regulates zip lines in Europe, and accredits courses for safety. Some governments (Australia and Germany), as well as some U.S. states, regulate zip-line courses.

Steve Gustafson, president of the PRCA board of directors, says that when you're checking out a zip-line adventure, ask the operators about their accreditation with the PRCA or the ACCT, and their insurance policy -- commonly worth $1 million.

Now, let's get to the nitty gritty of what we're really all thinking about before we step into nothingness: What the heck is going to happen if the cable breaks? And keep in mind that 250 pounds (113 kilograms) is generally the weight limit for any one zip-line rider (depending on the company and particular zip line itself), lest you think that only waifs and sprites are gliding down a line.

As Gustafson points out, a cable "doesn't catastrophically fail overnight." He says that what we think of as one cable is actually seven smaller cables wound together, and each of those seven cables is actually made up of 19 even smaller cables. These smaller filaments show signs of wear before a line snaps suddenly, and staff members are expected to inspect lines daily for signs of wear.

Some companies will use two smaller cables with two pulleys. Gustafson argues that having two small cables -- not to mention twice the pulleys, the carabiners and other equipment -- increases the risk for both human error and unequal distribution on the ropes.

However, you don't have to immediately abandon your zip-line adventure if you're on a course with a double-cable system. Gustafson recommends you make sure to ask if the cables work together, as opposed to one being a "back-up."

Gustafson says to picture it like car tires; if the tires on the right-hand side have lower pressure than the tires on the left, the wear will be different. It's the same with ropes -- a balanced system makes a safer system.

Now that you've received safety instructions, consider yourself prepared to learn about some zip-line trips that have gone very, very wrong.

While most zip lines in the United States that are regulated by the PRCA or ACCT have standardized ratings and training, it doesn't necessarily mean that flying through the air on a wire many, many feet above the ground doesn't present hazards.

One of the first recorded accidents to occur on a zip line was in 1739, when Robert Cadman of Shrewsbury, England, attempted to zip line from the top of St. Mary's Church in Shrewsbury on a rope fastened to a tree, 250 meters (820.2 feet) below. Unfortunately, Cadman's rope broke, and he plunged to his death [source: French].

The recent popularity of zip lines has also caused more attention to be drawn to zip-line accidents. In March 2008, Barbara Sue Fojtasek fell more than 30 feet (9.1 meters) to her death on the Honduran island of Roatan [source: Potter]. Fojtasek was suspended in the air with an instructor when the primary line broke. A guide apparently failed to correctly set up the safety line, which is in place in case of a primary line malfunction.

Also in 2008, a seventh-grader in Tennessee died from a zip-line accident after staff members at the church retreat he was attending failed to move a ladder that blocked the line [source: Stambaugh]. Although the boy was wearing a helmet, he sustained a fractured skull and vertebrae, as well as brain and other injuries from the high-speed crash into the ladder.

This accident, which left the boy in a coma for four days before he eventually died, also points out that zip-line accidents aren't merely mechanical in nature. Humans run these operations, and people do make errors in judgment or simple mistakes.

Both of these more recent accidents speak to a larger problem. While many large-scale operations in the United States are standardized and regulated, many smaller operations -- or operations outside the United States -- are largely self-regulated.

As we said before, beware the zip lines that aren't professionally accredited, and those do-it-yourself zip lines that seem easy enough to attempt. Instead, read on to discover some professional courses that are a safer -- and way more exhilarating -- ride.

Now that we know how zip lines work, why they developed, and the safety and accident risks, consider yourself ready for the zip-line trips you can take. These aren't the backyard lines your neighbor Jimmy rigged up; these are some serious hair-raising adventures for both fun and, as we'll see, work.

Costa Rica is an extremely popular destination for zip-line (or canopy) tours. Soaring above and through the rainforest, the industry has become one of Costa Rica's biggest draws.

Sun City, South Africa, boasts the world's longest and fastest zip line. Descending into a canyon, it's 280 meters (918.6 feet) high and 1,900 meters (6,234 feet) long -- roughly 1.9 kilometers (1.2 miles) [source: Zip 2000]. It supposedly reaches speeds of more than 150 kilometers per hour (93.2 miles per hour). But is it the scariest?

Consider St. Augustine Alligator Farm in St. Augustine, Fla., where one can fly blissfully through the air over the private enclosure of one 600-pound (272.2-kilogram), 13-foot (3.9 meter) Malayan Gharial crocodile named Mr. T. [source: Berry].

If you're looking for the ultimate zip line ride in North America, Coolest Stuff on the Planet blogger Amanda Arnold speaks highly of (if highly means she has a healthy fear and will forever avoid) New York Zip Line Adventures at Hunter Mountain, where the 3,200-foot (975-meter) zip line reaches 50 miles per hour (80 kilometers per hour), over a 600-foot (3.9-meter) valley. Depending what kind of person you are, that may or may not be worse than a 600-pound alligator.

And then there are the fun, utilitarian uses of contemporary zip lines.

Like some awesome school kids in Columbia who zip line their way from their isolated village to school every day. At a brisk speeds over 35 miles per hour (56 kilometers per hour), with a 1,300-foot (396-meter) drop into the canyon, these kids will pretty much beat everyone else's "walking both ways uphill in the snow to school" stories.

And then there's the real work of transporting contraband across national borders. In 2008, a smuggling gang in China allegedly rigged a 980-foot (298-meter) cable from a high rise in China into a village house in Hong Kong. Best of all? The gang supposedly used a crossbow to shoot the cable across the heavily guarded border -- putting to shame any mouth-breathing action heroes we've ever seen.

If you aren't already running to your nearest (safest) zip line, follow the links on the next page to learn way more about zip lines, the physics that makes the ride fun and other articles that will get your heart racing.