The discus has a long tradition in sports, first appearing in the ancient games in 708 B.C. In those days, stone and bronze disks were used. The size and weight of the disk varied. In 1896, the discus was an event in the first modern Olympic games. At the same time the sport was enjoyed in Scandinavia and the United States. It wasn't until 1907, however, that the event was standardized. Today, the men's discus weighs 2 kg (4.4 lbs) and measures 22 cm (8.66 in) in diameter. The women's discus weighs 1 kg (2.2 lbs) and has a diameter of 18.2 cm (7.2 in ). The discus is thrown from a circle which is 2.5 meters (8.2 feet) in diameter.

The throwing style has also changed. Orginally, the thrower stood in one place, only moving his arms. Later, the nordic swinging style of throwing was used. In 1926, the current throwing style was introduced. This style involves turning and skipping before release. This style was first used by Clarence Houser of the United States.

Aerodynamic Forces

While many things have changed, in the sport of discus throwing, one thing has stayed the same. The discus is greatly influenced by aerodynamic forces. In fact, the discus can go greater distances if thrown into a moderate headwind. To find out why this is true we must look at the shape of the discus.

By examining the shape of the discus, we notice that both the upper and lower surface have the same shape. If the discus is angled slightly up as it is thrown, into a moderate headwind, this creates a higher pressure on the lower surface than on the upper surface. Hence, the production of lift. However, if the angle of the discus is too steep, more than 26 degrees, there is a loss of lift.

So, how does a moderate headwind translate to a greater distance for a discus thrower? The headwind increases the speed of the air flowing over the discus. Lift is increased which results in longer flight time. Of course, to get this increase in distance (sometimes up to 5 meters farther) an athlete must be able to throw the discus at just the right (correct) angle to the wind.

Stability

Lift is not the only force operating on a surface. There is also drag or friction. These forces cause the surface to twist or turn, if not anchored (held) or otherwise steadied. It is similar to the lift felt by your hand when you hold it outside the window of a moving car. Your flattened hand experiences very strong forces acting either up or down depending on which way it is angled (giving it a positive or negative angle of attack). You also feel the friction. Notice how your hand wants to twist towards a broadside, (open palm facing the wind). This twisting motion is caused by a torque (the application of a force which tends to rotate or twist an object) being applied to your hand. Most surfaces that generate lift also experience this torque. The torque you experience is also felf by the discus. The torque will cause a "pitch up moment" which will cause the discus to stall (fall to the ground). But, we never see this happen at a track meet. This is because the discus is thrown with a spin. This spin is a counter force that tends to steady the discus. The torque motion created by lift and friction is cancelled by enough spin.

A simple way to experience this phenomena (event) for yourself is to hold a spinning gyroscope in your hand; try to change the orientation (placement) of the axis. You should notice the gyroscope resisting the movement. In other words, the spin "stabilizes" the motion.

Almost any object, flying through the air, can benefit from spin. Heavier objects need less spin for stability than lighter objects. A frisbee, for example, needs a high rate of spin to stay stable, a discus needs less.

Summary

The discus is greatly influenced by the forces of aerodynamics. While drag has some importance, lift dictates (controls) the distance of the discus flight.

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Last modified: Fri Jul 10 16:27:17 PDT 1998

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