Wednesday, February 18, 2015

Birds in a Lorry

Does the weight of an air-tight box containing a bird change when it starts flying?

I remember discussing this question with friends and teachers since I first encountered it nearly 25 years ago. It is a classic problem in mechanics, for which "intuition" can be misleading.

The standard answer to this riddle (for example on this reddit thread) suggests that over long periods, a time-average of the weight of the box+bird does not depend on whether the bird is flying or not, due to Newton's third law. There will be (fluctuations of opposite signs) around the average weight, depending on the direction in which the birds are flapping their wings.

Earlier this year, scientists from Stanford provided experimental confirmation of this long-lived conjecture by building an ultra-sensitive experimental platform that helped them measure these tiny transient forces. See "Birds in a lorry riddle finally solved by Stanford University", and videos such as "How a bird can become weightless".

While it is always nice to have concrete physical evidence for a theory, I think the popular media overplayed the insight that this particular experiment offered. Don't get me wrong - development of the force platform to measure tiny forces is a wonderful advance in its own right. But to claim that the "riddle was finally solved" suggests that it was unsolved before this paper came out.

In a certain sense, the experiment "merely" confirmed a rich body of theory, and fairly detailed and compelling computational fluid dynamics calculations. This previous body of work used multiple techniques and experimental inputs, and I think evidence for the standard explanation was robust. As the authors state in their introduction, their primary contribution was the device, not the resolution of the riddle.
... these calculations are based on indirect measurements of variables that need to be differentiated or integrated numerically to calculate force, which introduces numerical error. A non-intrusive, real-time, direct force measurement method (similar to the instrumented tether) does not exist for studying free locomotion in fluids. ... Here, we present an aerodynamic force platform (AFP) that enables such measurements in fluids. 

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