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A Physics Lesson: Why Cats Land on Their Feet

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Re: A Physics Lesson: Why Cats Land on Their Feet

#11

Post by pvarangot »

Udo wrote: post #3

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.

Differential control of spinning wheels is used to control attitude in spacecraft. Wertz's book describes the attitude control of the SEASAT satellites that used two canted reaction wheels in the pitch/yaw axis for momentum bias, and actuated them differentially to correct pointing errors. Read the chapter on momentum bias.

Re: A Physics Lesson: Why Cats Land on Their Feet

#13

Post by andrewcooke »

Udo wrote: post #3

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.

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.

Re: A Physics Lesson: Why Cats Land on Their Feet

#14

Post by mileswu »

div wrote: post #6
EvaPeron wrote: post #5

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.

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.

I believe the cat might be a Ragdoll, renowned for being really docile and letting you do things to it[1].

[1] http://en.wikipedia.org/wiki/Ragdoll

Re: A Physics Lesson: Why Cats Land on Their Feet

#15

Post by photon137 »

andrewcooke wrote: post #13
Udo wrote: post #3

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.

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"…

Oh, I can chip in here regarding big vs small flywheels. If you look at the purpose of a flywheel - ie storing kinetic energy - a bigger/smaller wheel can serve the same purpose. The reason we don't have "large" flywheels is twofold:

1. size (obvious) 2. Torsional/shear stress - most materials are bad at tolerating shear stress. A bigger flywheel would require immense amounts of torque to get it moving. Generating a higher torque would impose huge amounts of torsional stress on the shaft driving the wheel.

Re: A Physics Lesson: Why Cats Land on Their Feet

#17

Post by shardling »

andrewcooke wrote: post #13
Udo wrote: post #3

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.

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"…

>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!

Re: A Physics Lesson: Why Cats Land on Their Feet

#18

Post by richworks »

pilif wrote: post #9

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 d…

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..

Re: A Physics Lesson: Why Cats Land on Their Feet

#19

Post by [deleted] »

pilif wrote: post #9

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 d…

[deleted]

Re: A Physics Lesson: Why Cats Land on Their Feet

#20

Post by 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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