A cat usually lands on its feet regardless of the position from which it is dropped. A slow-motion film of a cat falling shows that the upper half of its body twists in one direction while the lower half twists in the opposite direction. (See figure below.) Why does this type of rotation occur?

total angular momentum must be constant.

The rotation of a cat's body when falling occurs due to a physical phenomenon called the conservation of angular momentum. Angular momentum refers to the tendency of a rotating object to keep rotating at a constant speed unless acted upon by an external force.

When a cat falls, it starts rotating its body in order to change its orientation in mid-air and align itself in a feet-first position. The cat achieves this by manipulating its body segments.

To understand the mechanics of the rotation, consider the following:

1. Cat's Body Segments: A cat's body can be roughly divided into two segments, the upper and lower half. The upper half consists of the head, neck, and front legs, while the lower half comprises the hind legs, rear torso, and tail.

2. Twisting Movements: As the cat falls, it instinctively arches its back and extends its limbs outward. This action causes the upper half of the body to twist in one direction and the lower half in the opposite direction.

3. Conservation of Angular Momentum: When the upper half of the cat's body twists in one direction, it creates an angular momentum in that direction. According to the conservation of angular momentum, if one part of the body gains angular momentum in a specific direction, another part must gain angular momentum in the opposite direction to maintain the overall balance.

4. Newton's Third Law: The rotation of the upper and lower body segments in opposite directions is a result of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. As the upper half twists in one direction, it exerts a reactive force on the lower half, causing it to twist in the opposite direction.

By twisting its body segments in opposite directions, the cat is able to conserve angular momentum and perform a mid-air rotation, allowing it to land on its feet.

It's important to note that a cat relies on a combination of its highly flexible spine, a specialized inner ear structure called the vestibular system, and reflexes to perform this rotation accurately and maintain balance during a fall.