Cats and Physics

An illustration of Newton's First Law:


"A cat at rest tends to stay at rest."

It is well known that "cats always land on their feet". This sequence of photos show how they make use of conservation of angular momentum while falling.

(1) The cat brings in her front legs while leaving her back legs extended. This makes the moment of inertia of the front part of her body much smaller than the moment of inertia of the back part of her body.

(2) She then turns the front part of her body to face downward. To conserve angular momentum, the back part of her body turns in the opposite direction, but much less, because its moment of inertia is larger.

(3) When the front part of her body is facing downward, she extends her front legs and pulls in her back legs - so that the moment of inertia of the front and back parts of her body are now relatively large and small. She can now turn the back part of her body to face downward. The front part of her body will turn in the opposite direction, but by a much smaller amount, since its moment of inertia is larger.

(4) When both parts of her body are facing downward, she is ready to land on her feet!

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