Physicists show that flying beer coasters will flip 0.45 seconds into flight - Ars Technica

Many a pub crawler has engaged in the time-honored tradition of throwing beer mats—those round cardboard coasters that are ubiquitous in bars—as if the mats were frisbees, often competing to see who can throw one the farthest. But unlike frisbees, beer mats tend to flip in the air and fly with a backspin.

Now physicists at the University of Bonn have come up with a theoretical model to explain the phenomenon, according to a new paper submitted to the physics arXiv preprint server.

Thanks to their natural curiosity, physicists are fascinated by the physics of beer, and lead author Johann Ostmeyer is no exception.

A couple of years ago, he became intrigued by the physics of so-called "beer tapping": a common prank where the perpetrator, holding an open bottle of beer, finds a target in the bar holding another open bottle.

As they rise to the surface, those bubbles get bigger and speed up, eventually creating the shower of foam that makes beer tapping such a popular pub prank.

Ostmeyer submitted his own analysis on the beer tapping issue to the arXiv last year, specifically focusing on why the lower beer bottle foams over and the upper one does not.

The initial tap creates a low pressure in the lower bottle, and the CO2 bubbles expand and collapse into fragments.

Thus, you don't get the same rapid growth cloud turning your entire beer into foam.

Now Ostmeyer, inspired by a 2017 trip to Munich for a German physics outreach demonstration show, has turned his attention to the pressing question of beer-mat flight trajectories. Beer mats (at least the round variety) are basically thin, flat disks with a given radius and mass.

But beer mats don't have the frisbee's distinctive airfoil-like edge, and that affects their aerodynamics.

Toss a beer mat like a frisbee, and the rotation will initially stabilize it.

Every good theoretical prediction must be experimentally tested, of course, and randomly tossing beer mats around in the local pub wouldn't be scientifically rigorous.

The researchers placed the launcher on a table, then placed a beer mat between the treadmills and launched the mats at various speeds, marking where they landed on the floor.