Wikipedia:Reference desk/Archives/Science/2019 May 30

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May 30

Gyroscope video

Is there a simple explanation of why the gyro doesn't fall about 27:15 to 30 minutes into [https://www.youtube.com/watch?v=OpCEJxO6V9g this video}. (I'm sure there is one if you account for all of the forces.) Bubba73 ^{You talkin' to me?} 03:26, 30 May 2019 (UTC)

The gyroscope demo, which is equivalent to a spinning top remaining upright (or at a fixed angle from perfectly upright!) and a spinning bicycle wheel on a rope (classic physics demo) are all examples of precession. That article has some math and some links to external explanations. Googling for explain gyroscopic precession will get lots of hits at varying levels of detail, and likewise those search terms on YouTube. DMacks (talk) 05:34, 30 May 2019 (UTC)

The explanation is related to angular momentum - it's really quite simple! Spinning objects like to keep spinning, and they like to keep spinning on exactly the same axis of rotation.

This is a true effect - no magic, no gimmick - it's equally as real as the fact that the weight of the object pulls the object toward Earth's center.

Just as you expect an object to fall downward if you release it under its own weight, you should also expect a spinning object to try to keep itself aligned to the axis on which it spins. This is its natural behavior.

There exists some rate of spinning where the effective force is larger than the object's own weight. When you get the object spinning that fast, it has the effective property of "rigidity in space" (cf. PHAK §8-15) - it doesn't want to fall off the table - and that is a real, actual force that can be larger than its own weight. It's not any different than if you carried the object yourself: it still wants to fall because of gravity; but something else is holding it upright, balanced against its own weight.

The only "not-simple" part is how we calculate the exact direction and magnitude of this force. This calculation requires a little bit of vector algebra. The simplified summary is that the spinning object "induces" a force at a right-angle to its axis of rotation. If you like to follow the math, you can learn about the vector cross-product, and our article section on angular momentum and torque. This is the set of equations we usually teach to high-school physics students.

Personally, I prefer to think about gyros by studying potential energy, which requires less arithmetic and a lot more mathematics. A treatment of this is presented in Marion & Thornton, Chapter 8 Centrifugal Energy and the Effective Potential, but you need to be pretty fast at your calculus to follow it.

Never doubt the importance of conservation of angular momentum - it's a more fundamental property in our universe than gravity itself - and it's the only thing that keeps our Earth from getting sucked into our own sun under the pull of gravity!

Nimur (talk) 12:33, 30 May 2019 (UTC)

It isn't just love that makes the world go round. Robert McClenon (talk) 23:00, 30 May 2019 (UTC)

Origin of Cell Membrane

I have a question about the origins of life. Is there a missing link that is more complex than molecules and less complex than a membrane-bound cell? My understanding is that the simplest known organisms that are unmistakably alive are membrane-bound bacteria and archaea. RNA and DNA are capable of self-replication, but viruses are not truly alive because they do not replicate except in a living host, and they are not organized at a level above the molecular level. What is there that is organized at a higher level than the (macro)molecular level that is below the cellular level? Is there a form that resembles a cell membrane before it is wrapped around the cell? Robert McClenon (talk) 23:00, 30 May 2019 (UTC)

See Micelle and liposome for some possibilities. Rmhermen (talk) 00:30, 31 May 2019 (UTC)

Thank you, User:Rmhermen - I see that various sorts of micelles do exist in nature. The article on micelle shows a liposome as a more complex structure formed from a micelle. I don't see any mention of liposomes existing in nature, so the missing link to the membrane-bound cell may be some sort of liposome that existed under previous conditions.

It seems that that the membrane is the least well-understood step in the origin of life. The chemicals all happen. Then once you get the first bacteria, and the bacteria start swallowing each other, then there are eukaryotes, and after that, there is the Cambrian explosion. No real missing links except the micelle upgrading itself into a liposome and then into a cell membrane. Or you can postulate panspermia, which doesn't answer anything because it is turtles all the way down. Robert McClenon (talk) 01:43, 31 May 2019 (UTC)

It's conceivable that RNA "life" could have existed that was self-replicating without a membrane, though it's not completely clear how. There has been a considerable effort to find an RNA sequence or set of sequences that can copy themselves without the aid of another living thing. These efforts have not been successful, but some incredible activities have been demonstrated. There are now RNA sequences with limited RNA-dependent RNA-replication activity (typically these only go a short distance and cannot utilize all four common ribonucleotides). There are also RNA sequences that are capable of recognizing partial self-copies and ligating them into whole copies [1]. It is also notable that many more molecules than just lipids can form cell-like compartments, and so the very first cell membranes could have been made of protein or something else (see coacervate). Someguy1221 (talk) 02:12, 31 May 2019 (UTC)

I've noticed that there are a number of cases where cell membranes precipitate hydroxylapatite or vice versa [2]. This mineral is also implicated in one-pot pentose formation [3]. Thus, while lacking proof, I am very suspicious that the chemistry of life could have begun on this surface, generating both calcium-bound chelators (like the Krebs cycle ketoacids) and phosphate-bound sugars and nucleotides. Wnt (talk) 04:59, 31 May 2019 (UTC)

See also here and here. Count Iblis (talk) 05:32, 31 May 2019 (UTC)

I would personally prefer that there not be answer, that there not be any explanation of how the cell membrane came into being, but that is just my belief system. Robert McClenon (talk) 04:52, 3 June 2019 (UTC)

Well, certainly glad you are able to separate what you want from what actually exists --Lgriot (talk)