27 July 2012

Flight of the discus: history and physics of Olympic throws

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Olympic track and field has a handful of throwing events - discus, shot put, hammer and javelin - that can be traced back to ancient Greece and ... well, Scotland.
Olympic games gold medal throws

The first Olympic gold medal winner in discus had never thrown the object before the games in which he won gold. Credit: iStockphoto

Every four years, athletics gets its moment in the Olympic sun. Most of the events involve running, but mixed in are a handful of throwing events: hammer, shot put, javelin, and discus.

Throwing, of course, is one of the most natural of sports. Put two young boys and a pile of rocks together, and most likely, rocks will soon be flying. But Olympians don’t throw anything quite so ordinary. Somewhere between children’s rock piles and the London Olympics, a lot of history intervened – in the process producing at least two throwing implements that might have more in common with airplane wings than rocks.

Olympic throw: Discus

The most iconic of these is the discus. Dating back at least to 708 BC in ancient Greece, it’s not changed much in 2700 years, other than standardization of its weight, size, and tapering thickness.

It’s actually that shape that lets top contenders hurl the discus as far as they do (over 70m for both men and women). Geoffrey Spedding, an aerospace and mechanical engineering professor at the University of Southern California, describes it as a “spin-stabilised oblate spheroid” – a technical mouthful that means it’s designed so that, if thrown at the proper slightly backward tilt, airflow will create greater pressure on the bottom/forward side than the top. This gives it lift, extending its trajectory almost as though defying gravity. The fact it’s also spinning produces a gyroscopic effect that keeps it from tilting to the side and dropping prematurely.

The effect is strong enough that a 1981 paper in the American Journal of Physics found that it actually goes more than 10% farther in a 10m/s headwind than calm air. Anything that creates denser air, such as cold temperatures, humidity, or low elevation also increases the distance of the discus’s flight.

I nearly learned all this the hard way a few years ago when I was asked to officiate discus at a college meet in a cold day of blustery rain, when one of the women unleashed a line-drive throw that travelled an amazing distance on a super-flat trajectory that never got more than a meter off the ground. Standing on the sideline, I barely had time to dodge.

Wild throws are nothing new. In the first modern Olympics in Athens, Greece 1896, the American thrower, Robert Garrett, also had some troubles with his aim.

Garrett’s primary sport was the shot put, but before leaving for Greece a history teacher had urged him to give the discus a try. There was just one problem: his university didn’t own a discus he could use for practice. Undeterred, he asked a blacksmith to make one for him, based on ancient Greek sculptures.

The blacksmith, of course, made it out of iron. The resulting disc weighed 13 or 14 kilos. Garrett reportedly gave it one attempt and decided to stick with the shot put.

When he got to Athens, however, he discovered that the real discus was a lot lighter (today it’s weight is pegged at 2 kilos for men, less for women). With no time to practice, he decided to enter the competition anyway.

His first throw went wild, nearly crashing into the spectators according to at least one account. So did his second. The Greeks thought him a buffoon. Garrett shrugged, tried again, got the coordination and aerodynamics right . . . and scored gold.

Olympic throw: Shot Put

Shot put is the only other throwing event to date all the way back to 1896. But there’s nothing Greek about it. Or American, for that matter, even if Garret was to win that event as well. (He also scored silver medals in long jump and high jump.)

In 1984, on holiday in Scotland, I stumbled upon the village of Newtonmore, where kilted strong men were tossing the caber, struggling to lift a 110-kilo “stone of heroes”, and heaving an impressive rock unimaginatively called the “64-pound stone.”

I had, quite by accident, found the ancestors of today’s shot put.

How far back in history the Scots were heaving heavy stones isn’t clear. It might date back nearly as far as the ancient Greek Olympics. The modern shot put, however, dates to the invention of gunpowder – and cannonballs, which were a lot easier to throw than big rocks.

From a physics point of view, the shot put is a lot less interesting than the discus. When you throw it, Spedding says, “you get a ballistic trajectory in the traditional ‘cannonball’ mode.” In other words, the ideal throw launches at about a 45-degree angle (compared to 30 degrees for the discus), which is part of why shot putters use the classic off-the-shoulder throw. The spinning approach is simply a way of getting a bit more speed into the shot before they let it fly.

Olympic throw: Hammer Throw

Another event undreamed of by the ancient Greeks was the hammer throw.

I first discovered it in the 2008 U.S. Olympic Trials. As luck would have it, I found myself watching it with one of America’s top discus throwers, whom I took the opportunity to ask why an event that involves a ball on a chain is called a ‘hammer’ throw.

“It comes from Scotland, where they threw an object that looked more like a hammer,” he said. “Maybe it was a sledgehammer.”

But it turns out that the hammer’s path to the Olympics actually predates sledgehammers, originating in Ireland, possibly as many as 3,500 to 3,800 years ago. There, in ancient celebrations known as the Taileann Games, Irish strong men threw something that looked remarkably like today’s hammer: a weight attached to a rope. (Or maybe it was a chariot wheel on an axle. Accounts vary.) Whatever the origin, the event crossed the Irish Sea and mutated into a sledgehammer toss – a throw at which, 500 years ago, England’s King Henry the VIII was reputedly quite good.

Aerodynamically, however, it’s nothing special. “It’s just a different-shaped shot put,” says Spedding. “I don’t see how there could be anything exploitable by having the chain there.”

What the chain does do is to allow athletes to spin the hammer (which weighs the same as a shot for both men and women) a lot more vigorously before letting it fly. A well-thrown hammer can go 80m, nearly four times farther than a well-put shot.

Olympic throw: Javelin

Not so alien to the ancient Greeks was the javelin. The javelin is simply a spear; and spear throwing was an obvious competition in an era when spears were important weapons.

Today’s javelins are hollow, with very precise rules about length and center of mass. Like the discus, they have aerodynamics that allow them to soar – farther, in fact, than any other Olympic implement.

What makes them work, Spedding says, is what javelin throwers call the ‘angle of attack’ – the difference between the javelin’s tilt when it leaves the thrower’s hand, and its direction of flight. That is, the javelin may travel on an arcing trajectory, but it begins the flight with its tip pointing upward.

This, Spedding says, causes air to flow across the curved underside of the javelin, building up pressure on the front (underneath) side, while reducing it on the upper side. This creates drag, but it’s drag that makes the javelin want to rise as it flies, until it runs out of speed and falls.

“Optimum launch angle is 30° but the javelin itself has an optimum angle of 37°,” John McLester and Peter St. Pierre wrote in the 2008 book, Applied Biomechanics: Concepts and Connections. “If the angle of attack is less than optimal,” lift will not be maximized; and if it is too high, the javelin will stall in mid-flight (because more drag is produced than lift).”

All of this was effective enough that officials were forced to redesign the javelin in 1986 when the world-record throw reached 104.80 meters – a dangerous distance given the fact the javelin is traditionally thrown on the infield of the track, where too long a throw could hit a runner on the far curve.

One possibility would have been simply to make the javelin heaver (it currently weighs 800g for men, 600 for women) but instead, officials decided to require that its weight be redistributed so that its centre of gravity is 4cm forward from its midpoint.

What that does, Spedding says, is to shift the centre of lift backward from the centre of mass. This means that the same aerodynamic forces that give the javelin lift also cause it to rotate forward, shortening the throw by causing it to nose-dive.

As a side-effect, this also means the javelin is more likely to hit and stick in the ground: a boon to officials trying to determine precisely where and how it hit. (To score, javelins must land point-first.) It’s also an added safety benefit by reducing the javelin’s tendency to skip on landing.

Richard A. Lovett is a writer in Portland, Oregon.
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