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Why a cat always lands on its feet
Why a cat always lands on its feet
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Phụ đề (62)
0:00
This is the first live cat recorded on film, and it’s being dropped to solve a physics problem:
0:05
Why do cats always land on their feet?
0:09
It’s a question that was driving 19th-century scientists nuts.
0:12
Until one of them used an unexpected tool to solve the mystery: a camera.
0:21
Étienne-Jules Marey was an obsessive scientist and inventor who analyzed how things moved.
0:28
And he started experimenting with photography at a time when the medium was mostly used
0:32
to document static subjects.
0:34
But his goal was to capture motion.
0:39
And he did that by building on a basic principle of photography: exposing a photosensitive
0:43
material to light and then covering it in darkness.
0:46
So his way of creating this darkness and light was to have a disc with slots in it.
0:53
By controlling the light as the subject moved across the frame, Marey was able to record
0:57
movement onto a single glass plate.
0:59
Essentially, all he does is block that light intermittently.
1:04
A slot from the disc opens, and then there’s darkness as the man moves, opens, darkness.
1:13
This technique is called chronophotography, and the results show something human eyes
1:17
will never see on their own: individual stages of motion.
1:22
A couple of years later, Kodak introduced celluloid film and Marey updated his slot
1:26
camera in a crucial way.
1:28
He swapped the single glass plate with a roll of film that moved in between exposures.
1:33
So, light: an image is made.
1:36
Darkness, the film moves on.
1:37
Light, an image — so it’s a movie camera, is what it is.
1:42
Marey made a lot of films for research purposes, and even tried dropping other animals to see
1:47
if they’d land on their feet, specifically this rabbit and this chicken.
1:52
Which brings us back to the cat.
1:55
It seems to be able to right itself by flipping in the air without pushing off anything first,
2:00
which would contradict the law of conservation of angular momentum.
2:03
Sounds scary, but stick with me here.
2:05
One of Newton’s laws of motion says that something in motion can’t just stop itself
2:10
unless an opposing force acts upon it.
2:12
Basically, you can’t just change direction midair, Wile E. Coyote style.
2:16
But to the naked eye, it looks like a cat can.
2:19
Most people assumed the cat was “cheating” by kicking off the hands of the person dropping
2:23
it, but Marey’s film showed what’s actually happening.
2:26
The first few frames prove right away that the cat doesn’t start its rotation from
2:30
a kick.
2:31
But what it does do is arch its back.
2:33
And by arching its back, it’s divided its body into a front part and a back part, and
2:38
the two parts can work independently.
2:40
You know how a figure skater pulls their arms in to rotate faster?
2:43
That’s what’s happening here too.
2:45
Early in the rotation, the cat pulls its front legs in and leaves the back splayed out so
2:50
the front half can rotate quickly while the back half stays relatively still.
2:54
Then halfway through, it does the opposite.
2:56
Front legs stretched out, back ones tucked in to flip the other half of its body around.
3:01
And you notice by the time the cat is landing, all four legs are stretched out as far as
3:06
they can be, which means slow rotation.
3:09
So the cat has rotated itself, but not overall; the two halves are working in opposite ways.
3:16
It uses the inertia of its own bodyweight to spin each side.
3:20
And because the two spins operate separately in opposing directions, they cancel each other out.
3:25
So Newton’s law isn’t broken.
3:29
Marey published his findings in Nature in 1894, breaking down the falling cat problem
3:33
for the first time.
3:35
His work remains an early example of using photography for scientific discovery.
3:39
What does photography do for science?
3:42
It records something and it makes it permanent so you can analyze it later, or so you can
3:47
share it.
3:48
But what Marey did was show something that the eye could not possibly see — ever.
3:57
You might have seen another famous early example of motion photography: In 1878, Eadweard Muybridge
4:03
used 12 cameras connected to tripwires to prove that a horse lifts all four feet off
4:07
the ground at some point in a gallop.
