science & techThe Public Domain Review1894
Étienne-Jules Marey's 1894 photographs showed a falling cat turns in mid-air without pushing off anything
A cat dropped upside down always turns over and lands on its feet, which seemed to break the rule that a body falling freely cannot change its spin. In 1894 the French physiologist Étienne-Jules Marey used rapid chronophotography, taking about 12 frames per second, to show that the cat had no rotation at the start of its fall. Marey proposed that the cat twists its forelegs and hindlegs in turn, and uses the inertia of one half of its body as a fulcrum against the other. Many physicists still doubted him, and the mechanism was not fully explained until Kane and Scher's 1969 paper.
A body in free fall has zero total angular momentum, so it cannot begin to spin on its own. The cat avoids this by moving parts of its body in sequence: it tucks its forelegs and extends its hindlegs, rotates the front half one way and the back half the other, then reverses the pose, so the net rotation adds up to a turn.
The falling cat problem became a classic example of how a body can change its orientation without any external torque, by moving its parts in sequence. The same principle is used by divers and gymnasts, and by spacecraft that reorient without fuel.
Pure rotation without a pushing surface is studied in other settings too. Astronauts and divers rotate by changing their body shape, and the same idea explains why a falling cat's spin can be understood from simple conservation laws.
James Clerk Maxwell, the physicist who unified electricity and magnetism, was reportedly known at Cambridge for dropping cats to study how they turn.
background
The falling cat problem is a problem that consists of explaining the underlying physics behind the observation of the cat righting reflex.
Although amusing and trivial to pose, the solution of the problem is not as straightforward as its statement would suggest. The apparent contradiction with the law of conservation of angular momentum is resolved because the cat is not a rigid body, but instead is permitted to change its shape during the fall owing to the cat's flexible backbone and non-functional collar-bone. The behavior of the cat is thus typical of the mechanics of deformable bodies.
Several explanations have been proposed for this phenomenon since the late 19th century:
Cats rely on conservation of angular momentum.
The rotation angle of the front body is larger than that of the rear body.
The dynamics of the falling cat have been explained using the Udwadia–Kalaba equation.
From Wikipedia: Falling cat problem (CC BY-SA 4.0) ↗
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