The Physics of Feline Flexibility

Image Courtesy: kouyuzu via Flickr

When cats fall, they always seem to land on their feet. Scientists and cat owners alike revel in the feline flipping phenomenon, known as ‘cat-turning’ or the ‘cat-righting reflex’.  

Physicist James Clerk Maxwell sought answers, notoriously dropping cats while studying at Cambridge. Previously, French mathematician Antoine Parent believed cats turn around ‘the center of buoyancy’. Here, the Newtonian argument suggests a misalignment between the downward force of gravity and the upward force of buoyancy on an object (such as a cat) causes the object to rotate towards a happy equilibrium. Essentially, a quick arch of the cat’s spine would initiate the necessary pivot. But Parent’s hypothesis falls short, because, as many physics students have gleefully read before, in this problem buoyancy is negligible. 

Whilst Maxwell knew something special was occurring, the creatures’ lightning reflexes were too quick for the human eye to discern. Fortunately, the invention of the camera could capture what the human eye could not. Unfortunately, caught in the frame, ‘cat-turning’ seemed to violate the very laws of physics. In 1894, scientist and photographer Étienne-Jules Marey’s cat-dropping photos, twelve frames every second, showed how a cat twists to stick the landing. Marey’s evidence caused a ruckus in the physics field. Angular momentum is a conserved quantity, meaning, without net torque (rotational force), it cannot change. So how does a cat manage to fall, feet up, and then rotate to a stop, paws down? 

Here, a vital assumption clouded the understanding of the ‘cat-righting reflex’. Angular momentum was understood through the motion of rigid bodies such as planetary bodies, but cats are certainly not rigid (not to be confused with the ‘liquid cat’ meme). Instead, they have lengthening limbs, bending vertebrae, and reflexes to match. By extending their back legs and protracting their front legs, cats effectively increase their  ‘moment of inertia’ in the back and decrease this quantity in the front, causing a prominent forward spin and a smaller backwards bend. Then, cats stick out their front limbs and bring their back limbs towards the body, completing the rotation and landing— nine lives intact. 

How does a cat know when the time is ‘right’? Inner ear sensors, called ‘otoliths’ signal both relative position to the ground and acceleration to the animal. The behavioural response may not be present at birth; instead developing over time, as shown in ‘zero-gravity’ experiments. Cats’ special skeletal structure, including 53 slender vertebrae, ‘elastic cushioning discs’ between bones, and a stress-diffusing spine allows felines to masterfully contort. Ultimately, ‘cat-turning’ reveals the heart of physics. Laws describing the beautiful motion of planetary bodies began to break down at the drop of a cat. Because, after all, physics is a language to describe our world. Stories are meant to be retold, just as rules are meant to be rewritten—  a ‘righting’ process much longer than a cat’s.