It’s a common sight in bars around the world: usually inspired by some sudsy stout, pub mates challenge each other to a game of tossing beer coasters – those round, thin mats placed under glasses and bottles to protect surfaces from moisture – to see who can hit a target with the most accuracy.
But beer coasters have a tendency to flip and spin in reverse, causing trajectories to alter and veer off course. Trying to compensate for this aerodynamic anomaly takes dexterity, forethought and perhaps a few more—or maybe less—beers.
Now, a team of physicists from the University of Bonn knows why beer coasters don’t sail smoothly through the air. In a new paper published in the European Physics Journal Plus, the researchers have unveiled a theoretical model explaining the discs’ teetering, reports Jennifer Ouellette of Ars Technica.
While on a trip to Munich, the team observed beer coasters—or mats, as they are called in Europe—had a tendency to invert shortly after launch. The team found that beer mats will flip over at about 0.45 seconds into flight. If tossed by a righthanded person, the coasters will spin clockwise and break to the right—and vice versa for lefties.
That’s because the discs are not aerodynamically designed like Frisbees. The curved rim on a Frisbee acts as an airfoil, which generates lift almost like an airplane wing. For beer mats, however, gravity takes hold soon after becoming airborne, greatly affecting its lift and drag. Lacking an airfoil, the coaster twists from horizontal to vertical, quickly negating any chance at maintaining a Frisbee-like trajectory.
“The wing form of a frisbee allows it to remain stable for a much longer time when thrown professionally,” the authors write in the study. “The reason is that frisbees have their aerodynamic center very near to their center of mass and, thus, experience much less torque.”
“The lifting force is not applied in the center of the mat, but rather in the front third,” Ostmeyer explains in a University of Bonn press release.
To test their mathematical theory, the researchers built a mat launcher—mainly because humans are wholly unreliable at repeatedly tossing coasters correctly, a condition that might be influenced by beer consumption as well.
Their testing proved the flight of a coaster will invariably break down at about a half second after release. They also found similar results for playing cards, which will invert after 0.24 seconds, and CDs, which will last to about 0.8 seconds before flipping. The much-heavier discus used in the Olympics will last about 16 seconds.
This is not the first time Ostmeyer and his drinking buddies—um, fellow researchers—have tested beer-related phenomenon. A couple of years ago, they examined the physics behind “beer tapping,” where a prankster taps the top of another’s beer bottle with the bottom of their own, reports Donald Conway of Insider Voice. The result is an outpouring of suds from the unsuspecting target’s bottle. In a study published on the preprint server arXiv, the researchers discovered that a series of shock waves cause the beer to bubble and erupt.
In the latest research, Ostmeyer contends it is not easy to send a beer coaster flying straight. To do so, the tosser must release it in a vertical position while applying reverse spin—an extremely challenging exercise, especially in the confines of a pub or bar where collateral damage is likely to take a toll.
“Our sincere apologies to everyone hit by a beer mat, be it through inaccurate aim or due to us instigating others to perform silly experiments,” Ostmeyer says in a statement.