The RWC Physics Blog V: Final Thoughts

The World Cup Final is just one and a half fitful sleeps away, so it must be time to wrap up this blog series (for now 🙂 ). There is a bundle of other topics that could be discussed, so I’m just going to list a few ideas (from high school physics, to interdisciplinary research, and to science fiction) that will hopefully get you thinking. They’re all areas where the skills we learn in physics classes can play a role. I reckon a good keen physicist could even get their teeth into some of the social science topics that have been highlighted by Massey University researchers.

So here goes, the final round-up:

  • The Julian Savea collision topic ended up going a little bit viral (“cultural”?) … well, at least it replicated a few times. Fans of that topic, and school teachers, will be happy to hear that English physicist John Biggins has gone retro and analysed the Lomu collisions from the 1995 World Cup semi-final. The question sheet here has been posted by Isaac Physics, which is a really good (and fast-growing) resource bank for secondary school physics.
  • The lineout is another rich source of mechanics and dynamics problems, if you need one. A player jumping for the ball has to reach as high as possible, as quickly as possible, while supported by two team-mates, one at the front and one at the back. The thrower needs to deliver the ball on the right trajectory so that it intersects with the lifted player at the top of his jump. And this has to happen at the right time!Lineout

Figure: Physicist’s impression of an All Black lineout.

  • Famously, the bounce of  rugby ball is seemingly irregular and unpredictable. However, it is possible to think about how the ball will bounce based on the angle at which is hits the ground. Of course, this problem is much simpler if the ball doesn’t deform during impact … but this isn’t the case. The collision is inelastic and the coefficient of restitution tells us about the ball’s energy loss.


Figure: The ellipsoidal ball (image from World Rugby Laws) might land vertically (red arrow) but so that the reaction force does not act through the centre of mass.

  • Following on from that link in the last point, there’s been far more slow-motion photography in sports coverage over the past few years, bringing new perspectives to high velocity ball sports like cricket and tennis. One important reason for cheaper and cheaper cameras is that researchers are continuously improving the efficiency of CCD detectors (which collect the light and form an electronic image).
  • It seems inevitable that rugby will come under increasing pressure to adopt technologies similar to the ‘hawkeye’ systems used in cricket and tennis. These systems use images from several cameras at different angles to triangulate the position of a ball. In rugby, the ball is sometimes hidden under a pile of bodies, so something similar to the magnetic sensors now used for goal line technologies in soccer may prove useful. Good old radio waves are the favoured means of communication for referees and their assistants, although ensuring the reliability of these transmissions is an interesting challenge – at least for those involved in club rugby.
  • Advances in materials science play a key role in modern rugby jerseys, as highlighted by Michelle Dickinson in her latest column.
  • The thrill of the chase: if a lucky Australian picks up an intercept on Sunday morning, someone like Ben Smith has the task of trying to chase them down. We’re lucky that ‘Ben from Accounts’ has good skills with a calculator, because it turns out that figuring out the best speed and direction for chasing a moving object is a complicated mathematical problem. Should he go towards the ball, or head towards the corner post? The latter gives him the best chance of stopping a try, but the former (or half way in between) might gain him more territory. What is the optimal running line for a flanker from the set piece? Where should the ref go to see the next phase most clearly?


Figure: As tweeted by @JayReeve.

  • For my colleagues down the hall at Te Punaha Matatini, I have to mention data science.There is a good reason that Steve Hansen and co. have laptops propped up in front of them while they’re watching the game – a huge amount of data can now be extracted from rugby games in order to analyse a team and their opposition. This is used by people like Wayne Smith (aka ‘the nutty professor‘) to form tactical strategies. Dealing effectively with large and complex data sets is a very modern problem, and one which can take some lessons from physics. Of course, physicists learn to deal with lots of data, and are skilled with numbers and computers. There are also concepts that might be transferred from fields such as condensed matter physics, in which we try to explain how simple properties of matter emerge from a large number of interacting bodies (like, really large, such as atoms or molecules).
  • Related to this, Trevor Lispcombe’s book (source note below) has an interesting section comparing tacklers to gas molecules (!). You can figure out the mean free path for a ball-carrier trying to run past opponents. When a team is defending close to their line, it’s as if the gas is compressed. The players are all confined to a relatively small space between the goal line and the ball. If they were gas molecules, we would say that the pressure is higher – the pressure might be reduced a little if someone is in the sin-bin! Trevor therefore thinks that it is easier to run through a defence when the ball is further from the line, which is food for thought.
  • … and finally, of course, time travel. I suggest we set the dial for Johannesburg, 1995.

A last thought is that rugby is a sport in which players and teams that rise to the top are persistently innovative, highly analytical, and base their analysis on a strong evidence base. This definitely how the All Blacks operate, and these are also key traits of high-quality science. As scientists in New Zealand, there are certainly things we can learn from our successes on the rugby pitch – if not from talkback radio!

You can leave questions / comments / suggestions here or @GeoffWillmott. Source note: some material is based on the book “The Physics of Rugby” by Trevor Davis Lipscombe (Nottingham University Press).



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