Five physics lessons for Olympic athletes
The Olympics are a stirring demonstration of human physical achievement – and physics. Cathal O'Connell outlines five surprising physics lessons for Olympic athletes.
In the 200m sprint, the outside lane is actually the fastest (if not the best)
Surprisingly, the speed of a sprinter does not boil down to how quickly they take steps, more about how hard their feet hit the track to propel them forward.
A typical person can hit the ground with about 2,500 newtons of force, while a sprinter such as Usain Bolt can apply 4,500 newtons. The extra force propels Bolt much farther for a given stride.
To round a turn, the sprinter must use some of that propulsion to produce a centripetal or "turning" force, and this slows him or her down. The tighter the turn, the more braking occurs.
Physicists have calculated that this can lead to a significant difference in race times. For instance, a runner in lane 1 (the innermost lane) would do the 200-metre sprint in 19.72 seconds, while in lane 8 (the outermost lane) the same runner could do it in 19.60 seconds. So on paper, lane 8 is the fastest.
In actual races, though, lane 8 is hardly prized. Because the sprinter in lane 8 starts ahead of the rest, he can’t see where his competitors are – so is at a psychological disadvantage. The best lanes are in the middle, where the turns are not too tight, yet you can still see where your competitors are.
Modern high jumpers can clear a bar while their centre of mass passes beneath it
For most of the 20th century, the most prevalent high jump technique was to try to fling yourself over the bar face down. Then in 1968, the American Dick Fosbury – an unknown and somewhat comparatively ungifted athlete – cruised to gold using a radical new technique, which became known as the Fosbury flop.
As he leapt, Fosbury turned and arched his back to bend backwards over the bar.
In physics terms, the key to the flop is how it manipulated his centre of mass.
Your centre of mass is the average position of all the mass in your body. Standing upright, it’s somewhere around your midriff. For a given jumping force, your centre of mass will reach a certain height given by Newton’s laws of motion.
When he jumped, Fosbury could not lift his centre of mass as high as his competitors could, but he was able to clear a higher bar. This is because his back arching technique raised his body some 20 centimetres above his centre of mass – a huge advantage.
By the next Olympics in 1972, most high jumpers had switched to using the flop technique, raising the bar so much that Fosbury himself couldn’t even qualify.
Pole vaulting is much more about speed than strength
In physics terms, the pole vault is a lovely illustration of how energy can be transformed. In this case, the pole is a tool for converting kinetic energy (speed) into gravitational potential energy (height).
The vaulter builds up her kinetic energy by sprinting with the pole. Then she plants the pole, causing it to bend and here, her store of kinetic energy is transformed into elastic potential energy (just like in a coiled spring).
As the pole straightens out, it lifts her up over the bar, converting that elastic energy into gravitational potential energy (in other words, height). Ultimately, the faster the pole vaulter runs, the more kinetic energy she can build up, and the higher she can vault.
A discus can go farther if thrown into the wind
The discus is a sort of heavy frisbee which was thrown by ancient Greeks in the original Olympiad. And physicists have found that a discus can travel five or six metres further if it is thrown against the wind, rather than with it.
A discus has a rounded top and a flat bottom, a bit like an aerofoil wing. This shape creates lift as the discus flies through the air. The extra lift created by a headwind more than compensates for the increased drag.
The javelin is deliberately given a sub-optimal flight design
In 1984, the German Uwe Hohn set a record that may never be broken, throwing the javelin 104.8 metres.
Such an epic throw was actually somewhat dangerous, raising the risk of javelins overshooting the field and flying into the crowd. So in 1986, the javelin was redesigned to deliberately shorten throws.
Rather than simply make a heavier javelin, the International Association of Athletics Federations decided to push the centre of gravity around four centimetres forward along the javelin shaft.
This caused the javelin to pitch forward as it flew, which reduced lift, and so reduced the gliding distance. The new design also prevented the javelin from landing flat, which is much more difficult to score.
Since the redesign, the world record for the men’s javelin has remained shy of 100 metres – so spectators sitting in front of the javelin throw at Rio should have nothing to worry about.