The shod Kenyan runner on the left strikes the ground with his heel, creating a rapid, large collision force, while the barefoot runner on the right lands on the ball of her foot, avoiding a high collision force. Credit: Benton et al.

To complete a recent study, a team of scientists left Boston and went halfway around the world, to the middle of Kenya. They wanted to find out more about barefoot running.

Sure, people can run barefoot anywhere. But the Rift Valley Province in Kenya has produced some of the most famous long-distance runners in history, and many of these athletes grew up not wearing shoes. With a video camera in hand, scientist Daniel Lieberman and his colleagues visited some of these runners to figure out what a difference shoes make.

It was a big difference, and not necessarily for the better. In particular, when a bare foot hits the ground, the blow is softer and the running motion smoother. This research suggests that running barefoot may have advantages over running with shoes on, though more studies are needed to determine whether or not barefoot running reduces the chance of injuries. Also, the team didn’t investigate whether there’s a difference for sprinting.

“One shouldn’t be scared of barefoot or minimal shoe running or think it odd,” Lieberman told Science News. “From an evolutionary perspective, it’s normal and, if done properly, it is very fun and comfortable. We evolved to run barefoot.”

Lieberman is an evolutionary biologist at Harvard University. An evolutionary biologist is a scientist who studies the way living creatures have changed over long periods of time. With his research, Lieberman wants to know why and how the human body works the way it does.

Previous studies have shown that when a person runs barefoot, she lands on the fronts or middles of the feet. Then the heel goes down. During this process, the weight of the body is at first on the front of the feet, then moves to the heel. Lieberman and his colleagues saw this motion firsthand in Kenya — the runners landed on the fronts of their feet.

When a person wears shoes, however, he tends to run so that his heels hit the ground first. The impact of the heel hitting the ground may be much more forceful than the impact of the front of the foot hitting the ground.

In the 1970s, shoe companies began selling running shoes that had cushioned soles. Those soles distributed the body weight through the foot and may have influenced the way people ran. Once runners started wearing these shoes, they could land on their heels and still be comfortable.

The researchers also studied barefoot runners in their laboratory in Boston. The goal was to measure the force with which a runner’s foot hits the ground. Force is calculated by multiplying the mass of an object — such as a human body — with its acceleration. By studying this force, the scientists could compare the impact of different running styles.

“A rear-foot strike is like someone hitting you on the foot with a hammer with about one and a half to three times your body weight. It would hurt without a shoe,” Lieberman told Science News. “A forefoot strike is like having no one hit you at all.”

Daniel Schmitt is an evolutionary anthropologist at Duke University. He told Science News that the new study by Lieberman and his colleagues is “really elegant and well done,” and that the finding is a clear, good example of the science behind different running styles.

Lieberman’s study explores the physics of running, which is a complex topic. Reed Ferber is a biochemist at the University of Calgary in Canada. The idea that barefoot running is better “is a massive assumption,” he told Science News. “Fundamentally, there are no studies out there that show barefoot running is less injurious.” In other words, don’t throw out those fancy running shoes just yet.

POWER WORDS (adapted from Yahoo! Kids Dictionary)

biological evolution The process of physical change in living things across generations.

biology The science of life and of living organisms, including their structure, function, growth, origin, evolution and distribution.

biochemistry The study of the chemical substances and vital processes occurring in living organisms.

force The capacity to do work or cause physical change.

Now, a scientist named Stephan Gekle has found that you can make air move faster than the speed of sound by doing a simple little trick: throw a rock in a pond.

View video | As a disc representing a stone plunges into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in the video, the air is filled with smoke for visibility) shoots thro

Stephan Gekle/Physical Review Letters 2010

Gekle is a scientist at the University of Twente in the Netherlands who studies the physics of fluids. Physics is the study of forces and motion, and Geckle investigates how forces act on liquids, like water. In a recent study, he and his colleagues showed that after a rock drops into a body of water, a tiny jet of air shoots upward faster than the speed of sound.

This isn’t the first time Gekle has explored what happens when a rock sinks through water. In an earlier study, he and his team showed that as a rock falls into a flat surface of water, like a pond, it carves out a tiny tube of air. This tube connects the sinking rock to the air above the pond. The tube doesn’t exist for very long, though — almost immediately, the surrounding water pushes on the sides. This pressure is stronger in the middle than at the ends. As a result, the tube looks like an hourglass, where the middle gets smaller and smaller as the water forces the air out.

There’s not room in the hourglass for water and air, so as the water comes in the air escapes upward — and fast. These tiny jets of air can blast faster than the speed of sound, Gekle found.

To measure these air jets is trickier than it may seem. Gekle and his colleagues had to do more than stand at the edge of a pond with stopwatches. A careful science experiment requires a scientist to take multiple measurements of the exact same thing, to check and double-check the results. In this case, it would have been almost impossible for Gekle and his colleagues to throw a rock in a pond in the same way over and over again.

Instead, the scientists created a lab experiment that acted like a rock falling through water: They dragged a circular disc down through water at the same speed, over and over again, and watched what happened.

But there was another difficulty: It’s hard to see and measure air. To solve that problem, the scientists filled the air above the water with smoke and illuminated the smoke with a laser, which made the moving air easier to see. (To make the smoke, Gekle said, they used a smoke machine like the ones that provide the dramatic effects seen onstage at theaters.)

Finally, because everything happens so fast when the rock moves through water, the scientists had to find a way to slow down time. As the disc moved through the water, the scientists took pictures with a camera that captured 15,000 frames every second. (That’s faster than most movie cameras.) After the experiment, the researchers could slow down the movie and, aided by computer simulations, calculate the speed of air as it blew out of the hourglass-shaped tube.

But there’s one aspect of supersonic air that Gekle and his team didn’t observe. When a jet exceeds the speed of sound, the air around it produces a noise like thunder, called a sonic boom. So far, however, Gekle says the tiny air jets aren’t making even a teeny, tiny boom — but the researchers will keep listening.

POWER WORDS (adapted from the Yahoo! Kids Dictionary)

speed of sound About 760 miles per hour, through air at sea level.

supersonic Faster than the speed of sound.

physics The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics and electromagnetism, as well as in modern fields including atomic and nuclear physics, solid-state physics, particle physics and plasma physics.

force The capacity to do work or cause physical change.

pressure Force applied uniformly over a surface, measured as force per unit of area.

Supersonic flows in action from Science News on Vimeo.

As a stone plows into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in this video, the air is filled with smoke for visibility) shoots through the shrinking opening at supersonic speeds.

Credit: Stephan Gekle/Physical Review Letters 2010

Going Deeper:

The Hyper-X recently broke the record for air-breathing jet planes when it traveled at a hypersonic speed of seven times the speed of sound. That’s about 5,000 miles per hour. At this speed, you’d get around the world—flying along the equator—in less than 5 hours.

Powered by its scramjet engines (shown in gold), the black, unmanned X-43A flew at a record speed for an air-breathing jet plane. The experimental plane is only 12 feet long.

NASA

The Hyper-X is an unmanned, experimental aircraft just 12 feet long. It achieves hypersonic speed using a special sort of engine known as a scramjet. It may sound like something from a comic book, but engineers have been experimenting with scramjets since the 1960s.

For an engine to burn fuel and produce energy, it needs oxygen. A jet engine, like those on passenger airplanes, gets oxygen from the air. A rocket engine typically goes faster but has to carry its own supply of oxygen. A scramjet engine goes as fast as a rocket, but it doesn’t have to carry its own oxygen supply.

A scramjet’s special design allows it to extract oxygen from the air that flows through the engine. And it does so without letting the fast-moving air put out the combustion flames. However, a scramjet engine works properly only at speeds greater than five times the speed of sound.

A booster rocket carried the Hyper-X to an altitude of about 100,000 feet for its test flight. The aircraft’s record-beating flight lasted just 11 seconds.

In the future, engineers predict, airplanes equipped with scramjet engines could transport cargo quickly and cheaply to the brink of space. Hypersonic airliners could carry passengers anywhere in the world in just a few hours.

Out of the three experimental Hyper-X aircraft built for NASA, only one is now left. The agency has plans for another, 11-second hypersonic flight, this time at 10 times the speed of sound.

Hang on tight!—S. McDonagh

Going Deeper:

Weiss, Peter. 2004. Soaring at hyperspeed: Long-sought technology finally propels a plane. Science News 165(April 3):213-214. Available at http://www.sciencenews.org/articles/20040403/fob6.asp .

You can learn more about NASA’s Hyper-X plane at oea.larc.nasa.gov/PAIS/FS-2003-07-77-LaRC.html and www.nasa.gov/missions/research/x43-main.html (NASA).