Beer mats prevent unattractive condensation rings on tables. They are, nevertheless, occasionally employed as projectiles. Typically, with little success: the cardboard coaster leaves its path, spins off to the side, and falls to the ground after only a short time. But why is this the case?
This subject has now been investigated by physicists from the Helmholtz Institute for Radiation and Nuclear Physics and the Argelander Institute for Astronomy at the University of Bonn. According to them, the beer mat’s behavior is unavoidable, at least when using the standard throwing technique: after 0.45 seconds at most, it will begin to drift off. After just 0.24 seconds, playing cards go astray, and CDs after 0.8 seconds.
The combination of gravity, lift, and the conservation of angular momentum is the cause for this: the mat tips backwards quickly after being thrown due to gravity. This provides it a similar angle of attack to a landing plane. The airflow is lifted because of this angle. “However, the lifting force is applied in the front third of the mat, rather than in the center,” explains PhD student Johann Ostmeyer, who devised the study’s concept.
The spherical cardboard would generally flip over as a result of this. And it does, but only if it is thrown in an unusual manner. “A beer mat is typically rotated when tossed, similar to a frisbee,” says Christoph Schürmann of the University of Bonn’s Argelander Institute for Astronomy. “It turns into a spinning top this way.” This spin keeps the flight stable and prevents it from flipping over. Instead, the lifting effort causes the mat to sag to one side, to the right if rotated counterclockwise, and to the left if spun clockwise.
A machine that throws beer mats
Simultaneously, it straightens out, no longer parallel to the ground but rather standing upright in the air like a turning wheel. The mat has a backspin in this posture, and if it were to stand up like a wheel on the ground, it would return to its initial spot. It quickly loses height and falls to the earth while in flight. This is a process that all flat, round objects go through.
The study’s concept came during a trip to Munich by the University of Bonn’s physics show team. Hundreds of people are usually enthralled by the event’s interesting physical experiments. The participants questioned why floating beer mats behaved the way they did while visiting a bar together.
When they returned, the physicists took a methodical approach to the problem, designing a beer mat throwing machine and filming the flights with a high-speed camera. This allowed them to see if their theoretical predictions matched their observations in the field. “The project has no applicability,” says Prof. Dr. Carsten Urbach of the Helmholtz Institute of Radiation and Nuclear Physics, which is part of the University of Bonn’s Department of Physics and Astronomy. “The difficulty, however, is obvious to both laypeople and physicists. It also brilliantly demonstrates the complete process by which natural scientists acquire information, from observation to theory and experimental testing, to adjustment and future progress.”
Playing cards have been known to travel up to 60 meters
Beer mats, by the way, travel the most steadily and thus the farthest when they rotate very quickly, a trick mastered by Rick Smith Jr., the world’s best playing card thrower, who has a record throwing distance of over 60 meters. Fast rotating beer mats, on the other hand, do not go straight for more than 0.45 seconds. “Those who want to throw really far and precisely should place the mat in a vertical position and apply backward rotation,” Ostmeyer explains — and then warns about possible injuries in the same breath.
It’s not without reason that the publication concludes with a cautious apology: “Our heartfelt apologies to everyone injured by a beer mat, whether due to incorrect aim or because we pushed others to do dumb experiments.”