The center of mass is a very interesting concept. The center of mass of an object is a point in the object where its average mass is located. At this point the mass of the object on one side of the center of mass equals the mass of the object on the other side, so in principle the center of mass of an object is also its point of equilibrium. In other words, if you could place the object on top of a pivot at its center of mass, it would balance perfectly. The center of mass of symmetrical objects such as a bowling ball is at its center. However, the center of mass of an object such as a broom (an asymmetrical object) is on a section of the handle closest to the brush. The principle of the center of mass is a useful one in physics as it simplifies calculations regarding the trajectory of objects in space, and it has also been used in the arts to achieve great displays of balance.
For today’s post I will focus on a very interesting property regarding the center of mass, and this is that the center of mass of an object can lie outside the object! Consider an object like a donut. The center of mass of a donut lies exactly in its middle, in other words in thin air! The same happens with the center of mass of other objects like boomerangs. Not only can the center of mass lie outside of an object, it can also shift if the object experiences changes in the distribution of its mass. Case in point: the human body. Many of the movements you perform on a regular basis such as lifting your arms, leaning, kneeling, sitting etc. change the center of mass of your body. In fact, some movements such as bending over can bring the center of mass outside your body. This property was instrumental for Dick Fosbury to obtain a medal at the 1968 Mexican Olympics in the high jump competition, and it changed forever the jumping style athletes use in this discipline.
Back in 1963 when Dick Fosbury was in high school, most athletes performed the high jump employing jumping techniques like the straddle or the western roll that involved the center of mass of the body passing over the bar. Not being particularly successful with these techniques, Forsbudy experimented with different ways of jumping over the bar. One day he tried jumping over the bar facing away from it and bending his body into an arch as he passed over the bar. Not only did he clear the bar effortlessly, but he stunned himself by improving his personal record by 6 inches that same day! Over the next few years, as he transitioned from high school to college, Fosbury kept on training and improving his newly discovered technique which by then was being dubbed the Fosbury Flop. Even though Fosbury managed to gain a slot in the United States high jump team that was to compete in the 1968 Olympic in Mexico, his style of jumping was still considered a curiosity. But that soon changed.
Pitted against opponents that employed the traditional straddle methods, Fosbury performed impeccably in one jump after another and cleared the bar in his first attempt every single time. Finally, when the bar was raised to 2.20 meters only 3 jumpers succeeded: Fosbury, his teammate Ed Caruthers, and a soviet jumper, Valentin Gavrilov. Gavrilov failed to clear the bar when it was raised to 2.22 meters whereas Fosburry and Caruthers succeeded and went on to attempt to clear the bar at 2.24 meters, which at the time was an Olympic record height. Whereas Caruthers failed in his attempt, Fosbury sailed over the bar to win the gold medal.
In subsequent years only one jumper, Vladimir Yashchenko from the Soviet Union in the in the 1978 Olympics, succeeded in breaking a record using the straddle technique. The Fosbury Flop spread like wildfire throughout the sport of high jumping and was improved upon by many athletes. Today it is used universally and the record holder is Cuba’s Javier Sotomayor with an impressive 2.45 meters.
So what is so remarkable about the Fosbury Flop technique?
When Dick Fosbury devised this high-jumping style, he unknowingly exploited the ability of the human body to displace its center of mass towards the outside. In the physics of the high-jump, the launch speed that the jumper achieves when they jump will determine how far their center of mass can be propelled in the air. By bending the body backwards into an arch as it goes over the bar, the center of mass of the body goes below the bar as opposed to above the bar as happens with other techniques. This allows the athlete using the Fosbury Flop to spend less energy to achieve a given height and gives them an edge over athletes employing other jumping techniques.
Thus Dick Fosbury’s win at the 1968 Mexican Olympics was not only a truly out of body gold medal (at least as far as the center of mass is concerned), but also an example of how exploiting scientific principles can improve athletic performance.
Image by AlanSiegrist used under an Attribution-Share Alike 4.0 International license.