The Physics Involved in Throwing a Discus

The shape of a discus resembles the airfoil of an airplane wing, which gains greater lift as wind speed increases, according to aerodynamic engineers. As a result, you can throw a discus farther against the wind than with it, according to a 2000 report by the University of California Davis. Computer simulations and test flights also show that the gyroscopic spin of a discus, atmospheric conditions and altitude affect discus flight in various ways.
Aircraft gain greater lift in cold air because its slower-moving molecules are closer together than warm air, making it denser. Cold air gives any aerodynamically shaped object, such as a discus, more support to stay aloft. Research at the University of Texas Institute for Geophysics reported in the “American Journal of Physics” that a discus travels about 5 inches farther on a cold winter day at 32 degrees than on a hot summer day at 104 degrees. Air is also denser at sea level than high altitudes. A discus travels 7.5 inches farther in Rome, Italy, 120 feet above sea level, than in Mexico City, 7,300 feet above sea level.
A discus spins as it leaves an athlete’s hand. This gyroscopic, spinning motion stabilizes the discus in flight. The faster it spins, the greater its angular momentum and the more its gyroscopic action resists tilting or changing its spin axis. The cross-section of a discus is wing-like, so spin holds its aerodynamic shape pointed into the wind. This maintains lift and prolongs flight time.
Baseball pitchers and football quarterbacks know their throws will lose speed when they throw into oncoming winds. However, a discus thrower combines the physics of aerodynamic lift and gyroscopic stability with a headwind to gain an advantage. The Texas researchers documented that a discus thrown into a 20-mph headwind can fly up to 25 feet farther than a discus thrown with that wind.
The characteristic spinning windup and delivery of a discus thrower creates great speed at release. While a baseball pitcher has, at most, only a 180-degree arc through which to accelerate the ball before release, the discus thrower has two full spins. In addition, as also practiced by figure skaters, drawing the arms inward while spinning preserves angular momentum by increasing spin velocity. Careful timing increases acceleration just before re-extending for release. The best discus throw combines physical strength with complex physics.


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