Wikipedia:Reference desk/Archives/Science/2022 April 2

= April 2 =

Are there "natural freefall pose(s)"?
If you used a vertical wind tunnel to levitate a nude guy (or lady) and they went limp at random poses, orientations and spin axis velocities, directions and placements would they end up in a small set of stable poses? If there was a tunnel big enough to avoid getting too close to the edge (which there might not be). If the center of pressure is not at the center of mass of a simple object the COM tends to end up "down" but humans are more complicated, many joints and cheeks etc to potentially flap around in turbulence. Sagittarian Milky Way (talk) 03:27, 2 April 2022 (UTC)
 * The situation you describe is akin to "skydiving" aka free-fall parachuting, and in fact military (and possibly civilian) parachutists sometimes train in vertical wind tunnels (mentioned for example in the article High-altitude military parachuting), though not usually in the nude. I have also seen online videos of such devices used at recreational facilities, and know of instances of nude free-fall parachuting; mid-air 'coupling' may or may not have been attempted.
 * I am not a parachutist myself, but my father has parachuted in a military context: my understanding is that if one does not actively control one's attitude with the correct limb positions, there is a strong possibility of going into a spin so violent as to render one helpless and possibly unconscious. This would be a bad thing in a wind tunnel, and even worse if skydiving. {The poster formerly known as 87.81.230.195} 90.209.233.48 (talk) 04:03, 2 April 2022 (UTC)
 * So the natural state of a falling human is to spin indefinitely? I heard experts have skydove standing up for speed and maybe knife hands straight up too and they fell ≥60% faster than spread eagle, are they actively countering spin too? Sagittarian Milky Way (talk) 05:06, 2 April 2022 (UTC)
 * Limbs being somewhat irregularly shaped, when in a spread-eagle posture each will contribute some torque with respect to the falling body's centre of mass. Since these will not sum up to exactly zero, this leads to spinning. The torque in a diving posture will be minimal. Tumbling can result from a random limp posture. --Lambiam 11:12, 2 April 2022 (UTC)
 * That's where I guessed the need to not let your spin get out of hand came from. If you can hold rigid maybe you can find a really balanced spread eagle that accelerates you rotationally very slowly. If the parachute jerk is unconformable or dangerous standing up and/or at 200mph (~4 times air resistance forces of spread eagle) then they'd probably slow back down and change to falling on their belly which seems a more advanced dive than the usual move to the belly by the time you're only a few tens of mph and adjust to rising resistance. I've seen intentional cool rolling into a spinning ball but film cut soon after the plane, maybe there's a sticking limbs out on the downspin side to kill rpm part they didn't show. Sagittarian Milky Way (talk) 18:42, 2 April 2022 (UTC)
 * I think in addition to the exact pose, the rigidity of the body would have a very significant influence on the behaviour of the falling body. That is, if you lock your body in any particular pose, your fall will be more controlled and less random than if the body is all loose. --Stephan Schulz (talk) 11:26, 2 April 2022 (UTC)
 * See also the Red Bull Stratos high altitude skydiving project with Austrian skydiver Felix Baumgartner, who jumped from a capsule 38 km up, but "An uncontrolled spin started within the first minute of the jump which could have been fatal, but it ended at 01:23 when Baumgartner regained stability". 46.102.221.177 (talk) 15:41, 2 April 2022 (UTC)
 * Lucky guy. Sagittarian Milky Way (talk) 18:45, 2 April 2022 (UTC)
 * So it could be chaotic like the pendulum attached to pendulum thing? Sagittarian Milky Way (talk) 18:53, 2 April 2022 (UTC)
 * Dzhanibekov effect.ogv]]
 * I don't know, because the spinning T-handle in zero-gravity behaves in a regular pattern. But that's probably completely overwhelmed by air resistance effects, except for Baumgartner who was above most of the air at the time. Card Zero  (talk) 20:13, 2 April 2022 (UTC)
 * Doddy Hay (a test 'pilot' of the Martin-Baker ejector seat) writes of two American test 'pilots' doing similar tests from an altitude of 50,000 feet. Lt. Henry Neilsen found himself flat on his back and spinning so fast he was unable to perform the 'orthodox remedial action of rolling himself up into a ball in order to reduce the area of body-surface being presented to the air.' Fortunately, he eventually landed safely. His colleague, Capt. Ed Sperry, repeated the jump, and warned by his colleague's report began at the very start of free-fall 'to thresh around wildly with his arms and legs, kicking and punching in every direction. ... His random gymnastics served to break up the airflow' and he did not spin. Possibly a static body will naturally tend to rotate in a specific direction, and if the position does not change, the rotation will only be reinforced; whereas a dynamic body will be pushed to turn in many different directions and have much less tendency to spin? It also suggests that at least one 'natural freefall pose' may be "spreadeagled and spinning".--Verbarson talkedits 22:52, 2 April 2022 (UTC)
 * The top speed of mach 1.25 was reached at 42 seconds, so similar force levels to regular skydiving by then. Sagittarian Milky Way (talk) 23:00, 2 April 2022 (UTC)

Cordyline fruticosa
I inherited a Ti plant (Cordyline fruticosa) that I’ve been taking care of for about six years now in a large container. I estimate its age at about eight or nine. I have two choices going forward. One, I can continue keeping it in a container and making do, or two, attempt to plant it outside in a spacious area. There are benefits and drawbacks to both choices. My question has more to do with the botany of container raised plants. Since it has spent the majority of its life in a container (and never flowered so far, as I’m told it takes many years for that to happen), is it even worth considering moving it to open soil in a natural habitat conducive to its future free from the container? Furthermore, I am concerned that it has other problems. It has survived a spate of mites (which were recently destroyed by peppermint spray), but it may be currently suffering from a case of Phytophthora, in which case the literature recommends either destroying it or letting it live out its life. I would appreciate any insight into this dilemma. My understanding is that there is no known treatment for Phytophthora. Also, how do I, a layman, go about confirming and verifying Phytophthora is the correct diagnosis? Is there a simple test I can perform at home?Viriditas (talk) 09:27, 2 April 2022 (UTC)
 * Phytophthora nicotianae: Leaves near the soil are water-soaked and have brown, irregular, zonate dead areas.
 * Phytophthora parasitica: Lesions form mainly on lower leaves close to the potting medium. They are initially water-soaked, brown, zonate areas with irregular margins.
 * These symptoms are also generically reported for Phytophthora. Here is something about controlling Phytophthora root rot, but it does not look very promising. Replanting outside may spread the pathogen to the soil and other plants. --Lambiam 10:53, 2 April 2022 (UTC)
 * Sorry if I wasn’t clear. I’m just not convinced that Phytopthora is the correct diagnosis.  The plant looks really healthy right now, except for the lower leaves which seem to fit with the aforementioned diagnosis.  Is there anything experimental I could try?  I was thinking of changing out the soil in the container for starters. It feels like a shame to kill off a plant that is almost a decade old and shows no sign of dying soon. Based on my limited knowledge of plants, I don’t think the Phytophthora diagnosis is correct.  To my untrained eye, it looks like a simple nutrient deficiency. How do I determine which nutrient it is lacking? In the hopes that I’m right, I just fed it a very small amount of 24(N), 8(P205), and 16(K20).  Is this kind of plant food safe for a Ti plant, and how long will it take to notice any changes? Viriditas (talk) 20:56, 2 April 2022 (UTC)
 * A preliminary diagnosis of Phytopthora is made based on the symptoms. For a more definitive confirmation, a lab test is needed. Apparently, there are DIY test kits.
 * Thanks. The good news is that 48 hours after adding the 24-8-16 nutrient solution, the plant is now thriving and producing the red streaks along the Ti leaf edges.  I’ve never seen it this healthy.  So it was a nutrient deficiency. I’ve also been told it’s important to use filtered water with Ti plants, so I’m going to try that next. Viriditas (talk) 21:31, 3 April 2022 (UTC)