The Strange Activities of 19th Century Scientists - Losing Cats

In the course of the development of modern physics, there is one animal that is particularly loved by scientists. Whether as a pet or as an experimental subject, it is fascinating. This animal is the cat.

For example, the famous cat abuser Erwin Schrödinger, by judging the state of the cat in the box, raised questions about the superposition of life and death , which is the famous quantum mechanics thinking experiment - "Schrödinger's cat".

Starting in the 19th century, a strange activity emerged among some scientific circles: losing cats.

At that time, some famous theoretical physicists were trying to drop cats from different heights, such as James Clerk Maxwell, yes, the Maxwell who proposed Maxwell's equations.

When Maxwell was at Trinity College, Cambridge, he discovered that cats always land on their paws first after falling from a height, even if they fall with their backs facing the ground.

This "strange phenomenon" caught his attention because according to Newton's first law, ideally, a stationary object will remain stationary unless acted upon by an external force. However, the cat was not acted upon by any external force when it fell from a height, so how could it move in the air?

In order to find out the reason, Maxwell conducted several cat-dropping experiments. However, due to the limited observation ability of the human eye, he could only roughly guess that a cat's paws would not touch the ground first when it fell from a height of about less than 5 centimeters (later, scientists conducted further research and found that the height had to be 30 centimeters to ensure the safety of the cat).

It wasn't until decades later, with the advent of time-lapse photography, that scientists were able to break down the process of a cat falling in a more intuitive way.

In 1882, French scientist and photographer Etienne-Jules Marey invented a photographic gun that could capture 12 frames per second, resulting in the famous photo below.

Direction from right to left

Before this, the more common theory was that the cat used the hand of the person who threw it as a lever to complete the rollover.

But from this photo, it can be seen that the cat did not rotate when it began to fall, but somehow gained angular momentum during the free fall. Marey believed that the cat used the inertia of its own mass to flip its body.

First, the force generated by the contraction of the cat's spinal muscles acts on the front feet, which are shortened and pressed against the neck, so the moment of inertia of the front feet is very small. In addition, the hind legs are stretched during the fall and are almost perpendicular to the body axis, which generates another moment of inertia, but the direction of rotation is opposite to the former. In the second stage of the movement, the states of the front and back feet are opposite, and the inertia of the front feet provides a fulcrum for the rotation of the hind legs.

——Quoted from the abstract of Marey’s paper on Wikipedia

Although Marey's explanation is basically consistent with the posture he showed in the photo, many physicists still disagree with it. The main reason involves the concept of "angular momentum conservation" in physics.

The law of conservation of angular momentum is one of the universal laws of physics, reflecting the universal law of the motion of particles and systems of particles around a point or an axis; it reflects the universal law of the motion of particles and systems of particles around a fixed point (or axis) when they are not acted upon by external forces or when the resultant torque of the external forces on the point (or axis) is always equal to zero.

Therefore, if a cat wants to conserve angular momentum when turning over in the air, physicists at the time believed that the cat's hind legs should flip up when its front legs flip down, thereby canceling out each other's angular momentum . However, this was not the case in the photos taken by high-speed photography.

Later, someone suggested that the angular momentum generated by the cat's tail when the cat fell offset the angular momentum generated by the front body. This explanation is more consistent with Mare's photo.

Unfortunately, they were slapped in the face again later. Someone conducted an experiment using tailless cats and found that they could also complete the action, which shows that the tail is not of much help.

The mystery was not solved until 1969, when Stanford applied physicists Kane and Scheer published a paper entitled "A Dynamical Explanation of the Falling Cat Phenomenon."

They divided the cat into the front half and the back half, and used a cylinder as a model to explore it, creating a new physical model.

Simply put, when a cat falls from a height, the vestibular structure in the cat's inner ear will first sense its direction. Then they will bend their bodies into a hunched shape, so that the front and back parts of the body will rotate along different axes.

Together with the axis parallel to the cat's body, it forms the angular momentum of a vector triangle, which also obeys the law of conservation of angular momentum.

Interestingly, physicists at the time did not expect that the cat's structure was so special that it could achieve conservation of angular momentum by bending its waist. So they chose axes parallel and perpendicular to the cat to establish a coordinate system, which obviously violated the law of conservation of angular momentum.

Finally, don’t try to throw cats at random, as they may get hurt. According to a study published in the Journal of the American Veterinary Medical Association in 1987, the degree of injury to each cat increases with the height. After all, gravity is no joke.