Netizens test: Can cats always land on all fours? Go home and try it!

There was a very popular test video on the Internet before, and I believe many people have seen it:

Image source network

Turn the cat over and hold it horizontally on the bed or sofa, then suddenly let go and let it fall freely. Basically, as long as the height is not too low, most cats can turn over and land steadily on all fours , which is amazing.

This also makes people very curious: How do cats do it?

In fact, scientists have been exploring the answer to this question more than a hundred years ago.

A cat that defies the laws of physics?

Regarding the myths or legends about cats, the saying "cats have nine lives" has appeared among the people a long time ago. This saying may come from the cat's strong jumping ability and their ability to jump from high places without any injury.

Cats love to jump off the roof. Image source: pixabay

In the 19th century, scientists discovered an even more amazing ability of cats: when people caught cats and let them fall freely on their backs, they would always land on their feet.

This makes people very confused. The cat is clearly falling upside down, so why does its body flip over when it lands? However, the whole process happens so quickly that it is difficult for the human eye to observe the cat's behavior in the air.

Yes, this challenge existed more than a hundred years ago. Image source: gloubik

However, a common speculation at the time was that the cat might have obtained the kinetic energy needed to flip its body from the person's hand while falling, because according to the law of conservation of angular momentum, an object that does not rotate itself cannot suddenly rotate without external influence.

In 1894, French scientist Etienne-Jules-Marais used a chronograph gun to take images of many animals. This chronograph gun could take 12 frames per second, allowing people to observe animals in multiple stages of movement within one second.

Marey captured a pelican in flight. Photo credit: Etienne Jules Marey

Of course, he also filmed the cat falling upside down. The images showed that, contrary to people's previous speculation, the cat was not affected by external force from humans, but fell directly down, and then began to rotate in the air with itself as the axis until its body returned to the correct direction and finally landed on all fours.

Cat's falling motion. Image credit: Etienne-Jules-Marais

Obviously, the cat's movement in the air seems to violate the laws of physics. How is this possible?

Maxwell and the Cat

This phenomenon soon attracted the attention of many scientists, including the famous physicist James Clerk Maxwell, who designed a number of experiments to drop cats from different heights and observe their movements.

It turns out that cats have very strange bodies. From the perspective of mathematical models, the body of an animal can be regarded as a cylinder and can rotate as a whole. But if you look closely at the body state of a cat when it falls, you will find that the upper and lower parts of the cat are also rotating, and they are rotating in two different directions. At this time, the conservation of angular momentum is preserved, and its change is zero.

Schematic diagram of the cat's aerial rotation model

Based on this observation, Maxwell proposed the "figure skater" explanation, in which a cat could create an internal torque by twisting its body muscles as it falls , just as a figure skater tightens his arms to increase his angular velocity as he spins his body. At this point, the cat's front paws would contract while the upper body rotates rapidly about itself.

Once the cat's upper body has rotated into the correct position (with its head facing the ground), the cat's lower body also begins to rotate, but in the opposite direction of the upper body - extending its hind legs and facing in the opposite direction to create the largest possible moment of inertia.

In this way, the cat's hind legs also caught up with its front paws, and now all four limbs of the cat are facing downwards, and the cat landed successfully!

Image source: Dreamstime.com

It is worth mentioning that cats are able to perform this special "air rotation" movement thanks to their special and flexible spine, so this ability is innate for cats.

Scientists call this behavior the cat's righting reflex. Just like a person's knee-jerk reflex, a cat's body will reflexively return to its normal state (limbs facing down) as long as it is in an abnormal state.

What is the best posture for a cat to land?

Generally speaking, when a massive object falls from a higher place, the impact force it receives is greater. This is determined by the laws of physics. However, for cats, this "iron law" seems to have been challenged.

In the 1980s, two veterinarians in New York City conducted a study in which they collected information about 132 cases of cats falling from buildings between June and November 1984. The cats had a 90 percent survival rate.

However, when veterinarians checked the cat's specific injuries, they found a strange phenomenon. Although the cat's injury severity increased with increasing height, it actually decreased from the 7th floor upwards.

In other words: a cat falling from the 11th floor may be less injured than a cat falling from the 6th floor!

As we all know, the higher the floor an object falls from, the longer it is accelerated by the earth's gravity, and the faster it will be until it finally hits the ground. The sudden impact will convert the animal's kinetic energy into other forms of energy, leading to fractures, chest collapse, and even death on the spot. Therefore, falling from a higher floor should cause more serious injuries than falling from a lower floor.

This kitten fell from a 26-story building and broke one of its front paws. Source: SPCA

At this moment, the laws of physics seem to have broken down for the cat.

But another possibility is that people ignore air resistance when thinking about the free fall of a cat. In fact, the cat will experience resistance when falling through the air, which will slow down its fall.

Therefore, the cat will be affected by two opposing forces when falling, namely gravity and air resistance (friction). Gravity is easy to calculate, which is the product of the cat's mass and the acceleration due to gravity, but air resistance depends on multiple factors such as cross-sectional area, drag coefficient, air density and falling speed.

When the cat first starts to fall, its initial velocity is 0. At this time, only the acceleration caused by gravity acts on the cat. As the speed increases, the cat will feel the air resistance that is opposite to gravity.

At the same time, the cat's righting reflex kicks in, and they instinctively adjust their limbs downward to land on all fours. But this posture causes the limbs to be subjected to a huge impact, which can cause serious damage because the cat's weight is unevenly distributed. This also explains why cats can still obey the laws of physics below the seventh floor.

But on higher floors, veterinarians believe that the cat will feel resistance during the fall, reducing its sense of weightlessness, so it can relax its body and not stretch its legs straight, and the weight is more evenly distributed, giving it a greater chance of survival.

Of course, a cat’s actual experience during the falling process may be more complicated, so for the cat’s safety, it is best not to put them in a dangerous situation.

References:

Liu Yanzhu. Rotational motion of a free-falling cat[J]. Chinese Journal of Mechanics, 1982(04):80-85.DOI:10.6052/0459-1879-1982-4-1982-046.

How Does a Cat Always Land on its feet?

http://helix.gatech.edu/Classes/ME3760/1998Q3/Projects/Nguyen/

Cat righting reflex

Why Do Cats Land on Their Feet? Physics Explains

https://www.scientificamerican.com/article/why-do-cats-land-on-their-feet-physics-explains/

A simple model for the falling cat problem

https://iopscience.iop.org/article/10.1088/1361-6404/aaac06

Author: Fish

Winner of the Silver Award for Outstanding Science Popularization Works of China Science Writers Association

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Editor: Dong Xiaoxian