It’s a common sight in bars around the world: people challenging each other to a game of tossing beer coasters — those round, thin mats placed under glasses and bottles to protect surfaces from moisture. The goal is to see who can hit a target with the most accuracy. Unfortunately, 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. The team found that beer mats will flip over at about 0.45 seconds into flight. If tossed by a right-handed 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 coasters, 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.
