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Real World Physics Problems Newsletter - Cat Twist, Issue #11
September 30, 2014

Cat Righting Reflex

The cat righting reflex is where a cat can, upon falling, turn its body around if necessary and land on its feet. All cats have the ability to do this, and it is made possible due to the great flexibility of their backbone and a lack of functional collarbone. The figure below shows the sequence of twisting movements a cat undergoes when falling from an initial upside down position so that it lands on its feet. This amazing ability that cats have can be performed from a minimum falling height of about 12 inches.

cat righting reflex picture

Source: Wikipedia via StefanPohl

This righting reflex in cats begins to appear at 3–4 weeks of age, and it is perfected at 6–7 weeks.

The physics principle at work here is the conservation of angular momentum. Angular momentum is a quantity that is directly related to rotational motion. If a body has no external torque acting on it then its angular momentum remains unchanged (i.e. it is conserved). It would make sense then, that when a body, such as a cat, experiences free fall it cannot rotate by itself unless an external torque acts on it. But surprisingly, if the body twists a certain way it can still rotate, even though its angular momentum doesn't change. Cats fall with an initial angular momentum of zero and by twisting their bodies a certain way they can still land on their feet while their angular momentum remains at zero throughout the fall.

For those interested in the mathematics of the cat righting reflex I created a simplified solution which analyzes it. To see it click here:

The Wikipedia page also has a nice explanation of this phenomenon.

Here is a short video clip discussing cats and their righting reflex. If you want to skip directly to the part that shows a cat righting itself during a fall go to time 0:30.

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