## 08 August 2012

### Field Hockey Semifinals

Field hockey in the London Olympics has now reached the semifinal stage for both men and women.  Both of my pre-Olympics picks, Australian men and Argentine women, are still alive.  The women's semifinal is on Wednesday; the men's semifinal is on Thursday.  There is so much interesting physics in the sport of field hockey that I couldn't possibly do justice to the entire sport in a single blog post.  I'll instead focus on one aspect of the stick.

I spent my sabbatical year working with Matt Carré in his Sports Engineering Research Group at the University of Sheffield in England.  One of Matt's research areas concerns field hockey sticks.  Check out the graph below that he sent me before the Olympics started (click on the image for a larger view).
The graph comes from the PhD thesis of one of Matt's students, Mark McHutchon.  The COR on the vertical axis is the coefficient of restitution, which helps measure the efficiency of energy transfer in the collision between ball and stick.  If COR = 1, no energy is lost during the collision, but that's completely unrealistic.  By "lost" I mean that the energy that goes into a sound wave (we hear the collision!), heat generated in the ball and stick, and some wave energy associated with stick oscillation cannot be returned.  Global energy amount remains the same, of course, but the energy associated with the ball and stick goes down after the collision.

The parameter on the horizontal axis is a measure of the stick's stiffness; moving left to right means a stiffer stick.  Note that the data suggest that more efficient transfers of energy from stick to ball are associated with stiffer sticks.  That means that if a player really wishes to smash the ball hard and have it leave the stick with a large speed, he or she should employ a stiff stick.

Ice hockey sticks are not quite so stiff, but of course ice hockey is quite different from field hockey.  The control players feel from their sticks is manifested in different ways in the two types of hockey.  In fact, when taking shots on goal, look for field hockey players to use the edges of their sticks instead of the sticks' faces.  That technique creates an effectively stiffer stick, which means better energy transfer, as the above graph shows.  Improving a stick's "second moment of area," which, loosely put, is connected to a stick's resistance to bending, is one way manufacturer's enhance stick design.

Field hockey can be a tough sport.  Pay close attention to those shots on goal -- they happen fast!  Some of the best players can whop the ball such that it leaves their sticks at around 120 km/hr (33.3 m/s or 74.6 mph).  Now that's fast!