HOW quickly would Usain Bolt have run the 100 metres at the Beijing Olympics if he hadn't slowed in celebration before the finish line? A team of physicists have calculated an answer: the Jamaican gold medallist could have slashed his time from 9.69 seconds to 9.55 seconds.
In the final 20 metres of the race on 16 August, Bolt extended his arms and thumped his chest in celebration, slowing himself a little. But he still broke the world record he himself had set in May.
Hans Eriksen of the Institute of Theoretical Astrophysics at the University of Oslo, Norway, and his colleagues, who normally spend their time pondering deep mysteries of the cosmos, wondered whether a few simple calculations might reveal how fast Bolt could have run.
So, in their spare time, they used television footage to measure Bolt's positions, speeds and accelerations as well as those of the runner-up, Richard Thompson. Their measurements show both Bolt and Thompson decelerating in the final 2 seconds of the race, with a larger drop in speed for Bolt.
The team then calculated what Bolt's time would have been had he only slowed down as much as Thompson, arriving at an answer of 9.61 seconds. But this may be too conservative, given how much Bolt outperformed Thompson in the first 8 seconds of the race. So the team also tried assuming, somewhat arbitrarily, that Bolt would have decelerated less than Thompson, by 0.5 metres per second squared in the final 2 seconds, which they say would have given Bolt a time of 9.55 seconds.
It is difficult to get precise measurements from ordinary TV footage, which records at 30 frames per second or less, says Matthew Bundle of the University of Wyoming in Laramie, who studies human locomotion. More precise human motion studies rely on cameras operating at 125 to 250 frames per second, or on tracking motion with a series of light beams which get tripped as people go by, he says. However, he thinks the team's numbers are reasonable.
"We don't mean to say that this is the final and ultimate result," says Eriksen. "Instead, it's a fun application of simple physics, and we've done the best we can." The researchers have submitted their findings to the American Journal of Physics (www.arxiv.org/abs/0809.0209).