Study shows why beer mats do not fly in a straight line
Physicists analyzed an as-yet barely researched phenomenon

Date:
July 27, 2021
Source:
University of Bonn
Summary:
Anyone who has ever failed to throw a beer mat into a hat
should take note: physicists have discovered why this task is so
difficult. However, their study also suggests how to significantly
increase accuracy and range.



FULL STORY ==========================================================================
Beer mats protect tables from unsightly condensation rings. However,
they are sometimes also misused as projectiles. Usually with little
success: after just a short time, the cardboard coaster leaves its path,
spins off to the side, and falls to the ground. But why is that so?

========================================================================== Physicists at the Helmholtz Institute of Radiation and Nuclear Physics and
the Argelander Institute for Astronomy at the University of Bonn have now investigated this question. According to them, the behavior of the beer
mat is inevitable, at least when employing the usual throwing technique:
it unavoidably begins to drift off after 0.45 seconds at most. Playing
cards go awry after just 0.24 seconds, CDs after 0.8 seconds.

The reason for this is the interaction between gravity, lift, and the conservation of angular momentum: the mat tips backwards shortly after
being thrown due to gravity. This gives it an angle of attack, similar
to a landing aircraft. This angle creates lift in the airflow. "However,
the lifting force is not applied in the center of the mat, but rather
in the front third," explains PhD student Johann Ostmeyer, who came up
with the idea for the study.

This would normally soon make the round cardboard flip over. And it
actually does -- but only if it is thrown in a rather unconventional
manner. "A beer mat is usually rotated when thrown, similar to a frisbee,"
says Ostmeyer's colleague Christoph Schu"rmann from the Argelander
Institute for Astronomy at the University of Bonn. "This turns it into a
kind of spinning top." This rotation stabilizes the flight and prevents flipping over. Instead, the lifting force causes the mat to drift off to
the side -- to the right, if it is rotated counterclockwise; otherwise
to the left.

Beer mat throwing machine designed At the same time, it straightens up --
so it is no longer parallel with the ground but instead stands upright in
the air like a rotating wheel. In this position, the mat has a backspin
-- if it were to actually stand up like a wheel on the ground, it would
thus travel back to its starting point. While in flight, it quickly
loses height and falls to the ground. This process is characteristic of
all flat, round objects.



==========================================================================
The idea of the study arose during an excursion by the physics show team
from the University of Bonn to Munich. The event regularly captivates
several hundred visitors with its fascinating physical experiments. While
the participants were visiting a bar together, they wondered why flying
beer mats behave the way that they do.

On their return, the physicists tackled this question systematically:
They specially designed a beer mat throwing machine and recorded
the flights with a high-speed camera. This allowed them to verify
whether their theoretical predictions corresponded to their practical observations. "There is no application for the project," explains
Prof. Dr. Carsten Urbach from the Helmholtz Institute of Radiation and
Nuclear Physics, an institute of the Department of Physics and Astronomy
at the University of Bonn. "However, the problem is clear for laypeople
and physicists alike. And it wonderfully illustrates the entire process by which the natural sciences acquire knowledge -- from the observation to
the theory and its experimental testing, right through to its adjustment
and further development." Playing cards travel distances of up to 60
meters Incidentally, beer mats travel most stably, and thus the furthest,
if they rotate very quickly -- a trick that has also been mastered by
the world's best playing card thrower Rick Smith Jr., whose record
throwing distance is over 60 meters. However, quickly rotating beer
mats do not travel 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," explains Ostmeyer -- and then,
in the same breath, warns about possible injuries.

It is not without reason that there is a precautionary apology at the end
of the publication: "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." The study was supported with funds from the German Research Foundation (DFG) and the National Natural Science Foundation
of China (NSFC).

Video: https://www.youtube.com/watch?v=a2iCosVA8AU&t=49s ========================================================================== Story Source: Materials provided by University_of_Bonn. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Johann Ostmeyer, Christoph Schu"rmann, Carsten Urbach. Beer mats
make bad
frisbees. The European Physical Journal Plus, 2021; 136 (7) DOI:
10.1140/ epjp/s13360-021-01732-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210727171557.htm

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