Olympic Physics: The Discus

Editor’s Note: The summer Olympics draw viewers to sports that they otherwise ignore. We marvel as athletes ranging from divers to pole vaulters turn power and speed into athletic artistry. Speed (velocity) and power (force) are also key elements in physics. This is the third in a series of guest posts by Dr. Larry Silverberg, an NC State professor of mechanical and aerospace engineering, who works in the area of engineering dynamics. He talked to NC State coaches Tom Wood (track and field), Jon Choboy (tennis), and Josh Karshen (diving) about the dynamics involved in their sports.

Track and field is about the individual – the challenge that one places on one’s self. Every year, world competitions are held, but the public really only pays attention once every 4 years – during the Olympics.

The discus, shot put, javelin, and the hammer events are all similar with respect to dynamics and training. This is why many will compete in more than one of these events. But in any of these sports, movement and strength turn into grace and beauty for the athlete – and the spectator.

Let’s take a look at the discus event. The goal: to throw the discus as far as possible. The discus looks much like a thickened Frisbee. The men’s discus weighs two kilograms (4.4 pounds), while the women’s discus weighs one kilogram (2.2 pounds). The athlete must stay within a circle that is 2.5 meters in diameter, and starts out with both legs planted. The athlete begins to spin, first on the right leg, then on the left. This builds up speed, or velocity, which can be translated into momentum for the discus. When the left leg lands the body turns and transfers its angular momentum to the discus and the athlete then releases it.

The release speed, release angle, spin and height of release are the key parameters. The release angle is between 37 and 42 degrees. The flight path and the angle of release are matched so the discus doesn’t wobble. Ideally, the athlete wants to release the discus as high as possible within that window between 37 and 42 degrees.

Once the discus is in the air it continues to spin. The spin helps the discuss catch and ride the wind to go farther. If the athlete released the discus at the right point, the discus will land in the cone-shaped throwing sector. If the discus lands outside that sector, the throw won’t count.

There are different types of discuses. Skilled throwers use a discus that has its mass distributed towards the outer edge of the disc. This makes it harder to spin but, once spinning, it makes the spin continue for a longer period of time – which helps it catch the wind. The skilled discus thrower can also account for head winds – throwing the discus with a flatter trajectory.

In track and field, the United State has been strong in the men’s and women’s sprint events, the men’s shot put and hurdle, and is growing stronger in the woman’s discus after winning the gold in 2008. Women’s discus begins Aug. 3, while men’s discus begins Aug. 6.