

A Physics Lesson: Why Cats Land on Their Feet - andrewhillman
http://www.mentalfloss.com/blogs/archives/136804

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pilif
Oh boy. Linking to this opened a real can of worms for my productivity. The
guy in the video had a whole channel of similar content and then goes ahead to
link to even more content in the fields of chemistry and math.

Thank you for submitting this. You made my weekend!

Also, it's really cool to see videos like this that go such great lengths at
explaining difficult stuff in such an interesting way, providing enough detail
to get you started, but little enough so that you have to look it up in detail
if you really want to know.

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richworks
ever since I saw his video on the hen head stabilization technique, I've been
an ardent follower of his videos. They are very informative..

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acqq
Chicken Head Tracking:

<http://www.youtube.com/watch?v=_dPlkFPowCc>

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otneusnocuh
I think it's cos they don't want to get hurt landing on their back.

Now _how_ they do it is a different question all together.

Ps, awesome video
[http://www.youtube.com/watch?feature=player_embedded&v=R...](http://www.youtube.com/watch?feature=player_embedded&v=RtWbpyjJqrU)
, article is pretty useless.

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shasta
One nitpick is that he never answers his own question about how this doesn't
violate conservation of angular momentum. The simple answer is that the
angular momentum is zero before and after the cat flips over and is thus
conserved. There is no conservation of angular orientation principle.

Practice question: imagine you're floating in space and facing away from the
earth. How would you turn around to face it?

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shabble
Isn't this essentially the same basic principle as rotating yourself in an
office-chair without touching anything[1]? I'm not sure how the friction of
the seat bearing or air resistance contributes, but my understanding is it's
negligible.

[1] Usually by making sudden lateral movements with outstretched limbs whilst
bringing them inward, much the same as the cat.

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jrockway
Can the URL be rewritten to link directly to the video? I don't think the blog
in the middle adds anything.

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tzs
It would be interesting to compare long tailed vs. short tailed cats on
natural falls.

In the video, the cats started their falls with no angular momentum, and so
they finish their falls with no angular momentum. The problem the cat has to
solve is to get itself in an upright position.

In natural falls, they won't always start with no angular momentum. For
example, consider a cat walking along a branch and a gust of wind shakes the
branch and the cat tips off. There's a good chance the cat will get angular
momentum in that kind of accident.

In that situation, merely getting upright is not sufficient as the initial
angular momentum remains. There are several approaches that cat could take to
deal with this.

1\. Wait until near the ground to do the "get upright" operation, so that
there isn't time between that and landing for the rotation to take the cat too
far out of position.

2\. Get upright, and then whenever the rotation takes the cat too far out of
position, repeat the uprighting operation.

3\. Get upright, and then use a constant counterrotation of the tail to
maintain the upright position.

I would guess that the long tailed cats would do better on natural falls that
are high enough to need angular momentum management.

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seivan
Is it wrong of me to see #2 and #3 to be the same? The constant
counterrotation is basically the same thing as constantly trying to get to the
upright position?

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tzs
By the uprighting operation in #2 I mean the operation that involves using
extension and contraction of the legs to give the front and rear different
moments of inertia, so the cat can first bring one half of the body upright
and then the other half.

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Udo
It's actually a very neat trick, kind of similar to the way ice dancers speed
up or slow down their rotation. Why aren't we using this instead of (or in
addition to) flywheels for attitude control in spacecraft? It seems a cat-like
mechanism of shifting angular momentum around could help reduce fuel costs by
lessening the need to load/unload energy through maneuvering thrusters.

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andrewcooke
well flywheels _are_ the same physics.

but i think what you're asking is why do we have fairly compact flywheels
rather than longer, more extended "tails"? and the answer is probably that it
makes more engineering sense. you can increase the angular momentum stored in
something by making it "bigger" or by spinning it faster. i imagine it's a lot
easier to spin a small flywheel to thousands of rpm than to make a "tail"
(spinning at a very low speed) thousands of times larger.

also - unless i missed it - this video doesn't really explain all of it. by
itself a cat's tail isn't nearly large enough or rotating fast enough to spin
a cat's body. the cat is also very "intelligently" (presumably "designed"
through natural selection) doing much of the rotation while in a U shape. that
reduces the net angular momentum considerably (the two "arms" of the U
effectively cancel).

and, from what i remember elsewhere, they also reduce the amount of rotation
needed by rotating to only get one pair of legs perfectly aligned. as long as
those touch down first there is a little time (i admit i am not sure it is
sufficient) to then rotate the other legs while "holding on" to the ground.

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shardling
>also - unless i missed it - this video doesn't really explain all of it. by
itself a cat's tail isn't nearly large enough or rotating fast enough to spin
a cat's body.

There's a whole section where they discuss how it's not the tail at all, and
even bobtail cats can land on their feet. So you definitely missed it!

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andrewcooke
uff sorry. unfortunately i can no longer edit that post.

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EzGraphs
For the higher math involved: <http://count.ucsc.edu/~rmont/papers/cat.PDF>

For the humor value: <http://www.mentalfloss.com/blogs/archives/87351>

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trout
This lead me to my new favorite uncylopedia article:
[http://uncyclopedia.wikia.com/wiki/Murphy%27s_law_applicatio...](http://uncyclopedia.wikia.com/wiki/Murphy%27s_law_application_for_antigravitatory_cats)

I lost it here "The cat and gluing dispositive will actually come together,
but the cat spins so fast due to the invariance in the center of gravity that,
following the law of time travel, it goes backward in time.

This has led to another theory stating that at the beginning of time there is
an infinite number of antigravitatory cats, and therefore, causing the
creation of the universe in the first place, due to the excessive
gravitational forces produced by the infinite number of cats, they cause the
big-bang. "

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strictfp
The first animated gif delivers unprecedented comical value.

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EvaPeron
As a physics buff, I'm sure this is fascinating, but I just could not bring
myself to watch the video - I love my cats too darn much, lol.

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div
A lot of cats can be fussy / scared when people try to make them do things,
but the cat in the video didn't seem to mind at all.

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Udo
At first I was skeptical as well, but it turned out there is no reason to
worry. He picked the right cat for the job. She was totally cooperative and it
seems like she thought it was a game of some sort. It's obvious from the video
that the cat was neither frightened nor angry.

In fact towards the end of the video, another cat makes a brief appearance
(the guy just wanted to present her to the camera, no throwing or falling
involved at all) and it serves as a good demonstration what it looks like when
they're not in the mood for human shenanigans.

