The cat lands so smoothly and gracefully that it can be explained by this law of physics!

It is said that "cats have nine lives", but when a cat is old, sick or encounters an accident, it will also "die". Although the so-called "nine lives" is just a legend, it reflects from the side that cats are extremely tenacious.

We often see cats climbing high trees or running on narrow walls. We may wonder if cats are afraid of heights? If they fall, can they survive? The fact is that cats can land perfectly on their feet when they fall from a height, which makes them less likely to get hurt (of course, there is a limit to how high they can fall) . This phenomenon can be explained scientifically by the law of conservation of angular momentum in physics!

Cat (Image source: pixabay.com)

Why can a cat land on its feet when it falls?

The interesting phenomenon that cats always land on their feet first when falling from a height attracted attention decades ago.

In the 1990s, a fast-moving camera invented by French scientist Marey recorded the process of a cat falling from a height. As shown in the figure below, after falling, the cat can perform free rotation in the air, its front legs tend to shorten and close to the neck, and the hind legs stretch perpendicular to the body, and finally the body can be turned 180 degrees, and the feet successfully land.

The process of a cat falling from a height (Image source: Reference [1])

In fact, the movement of a cat landing on its feet can be explained by the law of conservation of angular momentum in physics.

Angular momentum is a measure of rotational motion. In linear motion, we use momentum to describe the motion of an object; in rotational motion, we use angular momentum to describe it. Angular momentum can be seen as the "rotational momentum" of an object when it is rotating. Its size is positively correlated with the radius and speed of rotation.

The law of conservation of angular momentum means that in a system, when an object rotates without external forces, the total amount of angular momentum remains constant. For example, when a figure skater on ice spins and jumps, if he draws his arms together, the speed of rotation will increase, and if he opens his arms, the speed of rotation will decrease. However, due to the law of conservation of angular momentum, the total angular momentum remains constant.

Let's look at the process of the cat falling to the ground. It can also be considered that the law of conservation of angular momentum will keep its rotation direction and speed unchanged. That is, once the cat falls and starts rotating, it will continue to rotate and the direction of rotation will remain unchanged until an external force acts.

Since the postures of the front and back legs of a cat are inconsistent when it falls and rotates, the process of a cat's falling and rotating can be analyzed by breaking down the front and back halves. Scientists call the cat's rotating behavior when it falls to the ground a twisting rotation in which the front and back halves rotate in opposite directions.

When a cat falls from a height, based on its body characteristics, its front limbs will shrink, and the radius of rotation will decrease. Due to the conservation of angular momentum, the rotation speed of the front half of the body will increase, making it more likely to rotate. The cat's hind limbs will stretch, that is, the radius of rotation will increase, so the rotation speed is relatively low, and the rotation behavior occurs more slowly.

Finally, assuming the front half of the cat rotates clockwise, the back half rotates counterclockwise. The sum of the angular momentum generated by the two parts will always remain constant to maintain the conservation of the overall angular momentum. The cat can then change the moment of inertia of the front and back parts of the body by twisting the waist and spine differently. For example, when the cat bends the front body, its moment of inertia decreases, so the rotation speed increases; the moment of inertia of the back body increases, and the rotation speed decreases.

When the front half of the cat rotates to the appropriate position, it will quickly return to its original position to stop rotating. The back half will do the same. Due to the coordinated action of the front and back parts of the cat, the whole body of the cat will adjust to the appropriate direction.

Of course, the cat's leg jumping ability must be added in the end. The cat's leg joints are flexible, and when it is close to the ground, the cat can use its legs to bounce to reduce the impact of landing.

In addition, the reason why cats fall down is also related to their body structure and quick reaction ability. More and more people like to pet cats, and one of the reasons why cats are so popular is that they have very soft bodies, which are comfortable and relaxing to pet. Cats have five more vertebrae on their spine than humans, which ensures that they can twist their bodies more flexibly to adjust their descending posture when rotating at high altitudes.

At the same time, cats react very quickly because their reflex nerves are very developed. In the early stage of falling, cats can sense and judge their own position, and use their developed nerve reflexes to achieve the resetting reflex when flipping in the air, that is, the reflex behavior used to restore the normal position when the body is in an abnormal state. All of the above ensure the posture and safety of the cat when landing.

Cat falls to the ground (Image source: pixabay.com)

It doesn’t matter if you don’t quite understand all the above. After all, we cat owners just need to know that cats are very powerful, especially powerful, and super powerful…

Some of cats' skills are easy to understand, and we can also learn a thing or two.

What other special skills do cats have?

1Superb hearing

Cats are well-known for their ability to catch mice. This skill is inseparable from their super hearing. The number of muscles in a cat's ear is five times that of a human's, and the structure is complex. Cats can hear three times the range of sound waves that humans can hear, and can hear ultrasonic and infrasonic waves that humans cannot hear. At the same time, their ability to receive sound is very strong, and they can accurately locate sounds even from far away. Therefore, even if the mouse is far away, it can be accurately captured by the cat.

Cat and mouse (Image source: pixabay.com)

2Super night vision function

In the dark, we will find that cats have big and bright eyes. Due to the structure of cats' eyes, they have super night vision. The photoreceptors inside the eyeball affect vision. Photoreceptors are mainly composed of cones and rods. Among them, cones determine vision in light conditions. Rods determine vision in dark conditions. The ratio of rods to cones in cats is more than six times that of humans, so cats have stronger night vision.

Cat eyes (Image source: pixabay.com)

You never thought that cats have so many unique skills! By the way, a healthy adult domestic cat can run at a speed of more than 40km/h, which is comparable to the sprinter Bolt!

Editor’s Note:

Although cats are very strong, you should never throw them down from a height just for the sake of experimenting. They may still get hurt or even die. You can also find a soft place such as a sofa or mattress, hold the cat with its belly facing up, lower it a little and let go. The cat will still fall flat on its face because the height is too low for the cat to react.

References

[1] hotographs of a Tumbling Cat. Nature 51, 80–81 (1894). https://doi.org/10.1038/051080a0. [2] How does a cat fall on its feet? The New Scientist, vol. 7, no. 189 (1960), pp. 1647-9. [3] Wang Yanan. Can cats really break the law of conservation of angular momentum? [J]. Aviation Knowledge, 2022(9):1. [4] Tang Yonglian. Cones, rods and visual environment [J]. China Optical Science and Technology Magazine, 2003, 000(006):55-56.

Planning and production

Author: Denovo Popular Science Writer

Reviewer: Huang Chengming, Researcher at the Institute of Zoology, Chinese Academy of Sciences

Editor: Ding Zong