Wikipedia:Reference desk/Archives/Science/2021 January 25

= January 25 =

Why don't we consider air resistance in calculating gravity?
I mean If we drop feather on earth, it takes more time to Earth. Rizosome (talk) 17:20, 25 January 2021 (UTC)
 * We do. It's called Terminal velocity. ←Baseball Bugs What's up, Doc? carrots→ 17:24, 25 January 2021 (UTC)

I am talking about classical mechanics not fluid dynamics. Rizosome (talk) 17:35, 25 January 2021 (UTC)
 * In a vacuum, a feather and a lead weight will fall at the same rate. ←Baseball Bugs What's up, Doc? carrots→ 17:36, 25 January 2021 (UTC)
 * I'm not sure what you are asking. Air resistance is a factor in the speed of fall, but it is irrelevant to the strength of gravitational attraction. --Khajidha (talk) 17:50, 25 January 2021 (UTC)
 * The answer was given already by Baseball Bugs: we do. Yes, in classical mechanics. BTW, will you, please, start indenting what you write properly? Without that I can't reply to your not-the-most-recent comment without breaking the whole talk history. --CiaPan (talk) 18:22, 25 January 2021 (UTC)

Cotton piece falls to Earth slower than brass sphere, so why it is irrelevant for gravitational attraction? Rizosome (talk) 17:53, 25 January 2021 (UTC)
 * Because the force of gravity is the same on both, but the force of air resistance is different. It is the combination of the two forces that determines rate of fall. --Khajidha (talk) 17:55, 25 January 2021 (UTC)
 * (the acceleration due to gravity is the same for both; but the force will be different) but otherwise Khajidha is right. --Floquenbeam (talk) 17:59, 25 January 2021 (UTC)
 * Yeah, sorry about that. Biologist trying to explain physics. And trying to do so across an apparent language barrier with Rizosome. --Khajidha (talk) 18:03, 25 January 2021 (UTC)
 * Sometimes people take buoyancy into account. When talking about the weight of an airship, people might mean the net weight, id est, the weight minus the buoyancy. This net weight is easier to measure than the actual weight. But drag? That would complicate matters. It's a function of velocity. PiusImpavidus (talk) 18:35, 25 January 2021 (UTC)
 * By the Newton's law the acceleration is proportional to the force, so if you say the acceleration due to gravity is the same for both pieces, then the gravitational force is same for them, too. That, of course, under a silent assumption that items have equal masses. Then the net force will be different, e.g. due to different aerostatic buoyant force, caused by different sizes which in turn result from different density of materials; and more important, as Khajidha said, due to different aerodynamic resistance force, which results from different sizes (and possibly different shapes), which finally leads to different terminal velocity. --CiaPan (talk) 09:48, 27 January 2021 (UTC)
 * {{re:CiaPan} any particular reason we’re assuming the brass sphere and cotton ball have the same mass? That’s not pedantry so much as it is confusion. —Floquenbeam (talk) 13:04, 27 January 2021 (UTC)
 * speaking of confusion, re-pinging because I can’t type. . —-Floquenbeam (talk) 13:07, 27 January 2021 (UTC)
 * it’s hard when your mind deteriorates so rapidly in full public view. . —Floquenbeam (talk) 13:09, 27 January 2021 (UTC)
 * It wasn't specified, whether one is heavier or bigger - and which one - so it's safe to suppose they are meant to be of same weight or of the same size. Otherwise one could mischievously assume 1 milligram piece of brass foil versus 1 ton of tightly compressed cotton and argue about surprising, counter-intuitive result. Possibly I chose equal masses as a reminiscence of a puzzle I was told many years ago... (It was "What is heavier, one kilogram of iron or one kilogram of feathers?") --CiaPan (talk)15:11, 27 January 2021 (UTC)
 * Anyway, they were not assumed to be balls. --CiaPan (talk) 15:13, 27 January 2021 (UTC)


 * A brass sphere resting on the surface of Earth falls slower than a cotton piece released in the air. In fact, its (relative) velocity is zero. You can't get much lower than that. The force exerted on the sphere is the resultant of two forces: one due to the gravitational attraction of the Earth, and an opposite one, in this case the ground reaction force, which cancel each other. In general, the acceleration of an object results from the combination of all forces acting on the object. So for a feather or piece of cotton, we must consider both gravitation and air resistance, which is what physicists do. They would also do that for a brass sphere, except that when a sizable brass sphere is released from a moderate height, the effect of air resistance is so small that it is negligeable for most practical purposes. --Lambiam 12:50, 26 January 2021 (UTC)
 * If you want to know where something lands then you may also have to consider the Coriolis force. Objects don't fall vertically on Earth (except at the poles). And if you drop a really heavy object then Earth's attraction to the object will also accelerate Earth towards it so the collision happens faster – both because the place of the collision changes and because the attraction grows when the objects come closer. PrimeHunter (talk) 14:55, 26 January 2021 (UTC)
 * As to the latter effects, see also  --Lambiam 16:36, 26 January 2021 (UTC)
 * You may also take an aerostatic buoyant force into consideration, if you're on the surface of Earth. Actually, there is also a centrifugal force, resulting from the Earth's daily rotation. Additionally, let's not forget about a wind. For a brass sphere lying  on the ground it might be negligible (unless it's a hurricane at the moment), but for 'a feather or piece of cotton' it certainly should be considered. CiaPan (talk) 09:56, 27 January 2021 (UTC)
 * And also consider the hydrostatic buoyant force if the sphere came down in an ocean, or the effect of a slope if that is where the sphere landed, or dogs that try to pick up the ball. If it came down from high it will be sizzling hot, and then if it lands on ice there will be some interesting effects. The fall of a feather may be precipitated by precipitation. And so on. --Lambiam 10:42, 27 January 2021 (UTC)
 * Just to continue this tangent, one of the shortcomings of Newtonian mechanics is the use of forces becomes insanely complicated and irreducible when you start to bring all of these factors in to play. Newtonian mechanics is the most famous formulation of classical mechanics, but it is not the only one, and sometimes not even the most useful.  Lagrangian mechanics is more mathematically complex than the rather algebraic approach of Newton, but it also allows for much more rigorous predictions of chaotic systems where Newton's "force" methods fall short.  -- Jayron 32 14:12, 27 January 2021 (UTC)

Heat energy transfer to gases
As I sit beside a radiator, I can see the result of the hot air rising as it moves the curtain above, but what exactly is going on here? Specifically, how does the heat energy within the radiator (or any material that is not mixing with its surroundings through convection) get transferred to the air (or any adjacent gas)? Our article on radiators states that "most radiators transfer the bulk of their heat via convection instead of thermal radiation", so does that mean that a small amount of thermal radiation is exciting the atoms in the air, resulting in convection? Or are the atoms of the air hitting the radiator and then bouncing off with increased speed? And why does such heating result in the air (gas) expanding? PaleCloudedWhite (talk) 18:48, 25 January 2021 (UTC)


 * Of course some atoms of the air are "hitting the radiator and then bouncing off with increased speed", in other words, receiving hear by conduction. Why not? --142.112.149.107 (talk) 22:50, 25 January 2021 (UTC)


 * Is that the whole story? I'm looking for a clear explanation of how gases acquire energy from non-gaseous surroundings. When I read this teaching resource, I find statements like "with an increase in temperature, the particles [of a gas] gain kinetic energy and move faster" - but it doesn't explain how the gain in kinetic energy occurs. It also makes me wonder how gases lose energy, as the same resource states that "unlike collisions between macroscopic objects, collisions between particles [in a gas] are perfectly elastic with no loss of kinetic energy" - so, once atoms in a gas are moving faster, what makes them slow down? PaleCloudedWhite (talk) 23:20, 25 January 2021 (UTC)


 * They bounce off slower molecules, speeding those up, and slowing down themselves. Greglocock (talk) 03:01, 26 January 2021 (UTC)


 * Of course, once those air molecules have been 'sped-up', they will spread around the room by convection; additionally, there will be some direct thermal radiation that will heat up some further air molecules and all surfaces within line-of-sight, as you can easily observe by placing your hand or face next to (not above) the radiator a few inches away from it. {The poster formerly known as 87.81.230.195} 90.200.40.9 (talk) 04:46, 26 January 2021 (UTC)


 * Courtesy links: natural convection (fluid particles "bouncing" on the solid surface) and thermal radiation.
 * Air is mostly transparent to IR radiation at ambient temperature (otherwise, thermal cameras would not work). (Exception: water vapour and CO2 have big absorption bands in the near-IR spectrum, so a foggy bathroom might not be so transparent). Hence, air molecules do not significantly heat up by radiation.
 * Nonetheless, depending on the radiator design, it might send significant radiation to the solid surfaces of the room (which then heat up the room by convection on those). According to, for radiant panels (= a hot surface) you get a radiative exchange coefficient of about 5W/m2/K at slightly-above-ambient temperatures (top of page 3, cited to a ref I did not bother to check because the figure looks about right). This is pretty decent compared to natural convection values (the "90% radiation" claim only really applies to ceilings but it is >50% in most designs).
 * However, the more standard "radiator" design is a convector, where air goes goes in and out the body of the "radiator". The geometry is designed to increase the convection exchange coefficient, plus the air is heated up inside the "radiator" at a higher temperature than the outside surface temperature, so the proportion of convection vs. radiation is higher.
 * For either mode of heat transfer, the efficiency of transfer is limited by the maximum surface temperature. I would assume there are regulations in place to prevent people getting burned when they accidentally touch their radiator. The surface is usually around 80°C. Tigraan Click here to contact me 09:56, 26 January 2021 (UTC)
 * If air molecules do not heat up significantly by radiation, I'm wondering why is there such an apparent loss in radiant heat transfer across a room? i.e. if I place my hand just above a hot radiator (or just in front of an electric fire, which I'm assuming is emitting much more radiant heat), I can feel heat, but I can't feel the heat on the other side of the room (maybe I can with an electric fire, but the heat is much reduced). Is this because the heat energy has been converted to increased kinetic energy of atoms in the air? PaleCloudedWhite (talk) 10:46, 26 January 2021 (UTC)
 * The infrared radiation spreads out, reducing the flux. You see this by taking a large piece of tinfoil paper, holding your hand near a radiator and moving the paper between your hand and the radiator and removing it so that you clearly feel the difference in heat due to the radiation. If you do this near the middle of a large radiator, then moving your hand at a larger distance has far less effect than doing this at the top of the radiator. You can also move at some distance from the radiator and use the tinfoil as a reflector and see if you can detect the reflected radiation with your hands. If you manage to shape the tinfoil in a parabolic shape, you can amplify the flux. Count Iblis (talk) 12:29, 26 January 2021 (UTC)
 * Iblis provides a great answer here, but just to add a courtesy link, radiation decreases with distance because it obeys the inverse square law; basically a mathematical description of the fact that the same energy has to occupy larger and larger volumes of space as you move outwards from a source, decreasing its density. -- Jayron 32 13:38, 27 January 2021 (UTC)
 * In central heating systems, where heat is transferred from a central heater to radiators using hot water, 80°C is kind of a hard limit, as otherwise the water might boil. Further, for heating systems other than electric resistance heating (which is really inefficient), overall efficiency is maximised when the temperature is minimised. Heat loss through the chimney is minimised by cooling the exhaust in a counter-flow heat exchanger, but is limited by the temperature of the water returning from the radiators. Heat pumps are more efficient when their warm side is cooler. You can compensate for the low temperature by using a large surface area, which is why modern buildings tend to use floor heating. It only needs a heat source at about 35°C, which can be reached with an efficient heat pump or industrial waste heat. PiusImpavidus (talk) 09:54, 27 January 2021 (UTC)


 * The OP is asking good questions, but hard questions. They sound simple until you get down to some of the "fuzzy edges" around our definitions.  Firstly, there is the problem of "medium scale thermodynamics" to get into.  When we talk about things like temperature and convection and things like that, these are usually defined in terms of statistical thermodynamics, however we also want to understand what is going on at the atomic level, that is what happens when one molecule of a gas hits one atom of the radiator and the atom on the radiator transfers its energy to that molecule of gas.  It sounds like that should scale, but it kinda doesn't.  This is because you're trying to treat the atomic interactions under the rules of classical mechanics and they simply don't follow those rules.  We need to use quantum mechanics in order to describe these interactions, and at that level, things like atoms and molecules don't interact like well defined little objects.  In reality all heat transfer is radiation.  That's because the way in which two colliding molecules will exchange energy is by exchanging photons; the gauge boson (force carrier) of the interaction between two molecules is the electromagnetic force carrier, the photon.  What is radiation?  It's heat transfer via photon.  The difference between "conduction" and "radiation" is just the distance scales we (semi-arbitrarily) define for those particular categories. What we define as "conduction" "convection" and "radiation" at the macro scale is really just an artifact of "scaling up" from the quantum world to the classical world and using classical models that make life easier; those categories simply break down at the molecular level.  The categories are useful, they just don't scale well when trying to bridge the gap between the atomic and the macro worlds.  -- Jayron 32 13:32, 27 January 2021 (UTC)

Brazilian pepper?
I think I have a picture of Brazilian pepper berries, leaves and, stems but I'm not a botanist. Can someone tell me if it is Brazilian pepper or something else? If something else, will need rename. The berries and leaves look right, but the stems are red-tinged -- Deep fried okra ( talk ) 19:19, 25 January 2021 (UTC)
 * Courtesy link: Brazilian pepper. --Lambiam 20:53, 25 January 2021 (UTC)
 * Read that. Does not definitively answer my question. Perhaps if a botanist knows? -- Deep fried okra ( talk ) 21:16, 25 January 2021 (UTC)
 * From the picture I think you're correct, . The leaf is odd-pinnate compound, the right size, and they're slightly toothed. Matches my pictures of a Brazil pepper I once massacred, apart from the red rachises (stems) but I've seen plenty of other pictures on the web with that, it could just be the colour of new growth. If you have a picture of the main trunk, the bark pattern can be handy for ident. Honestly though I think you're on the money. I'm not a botanist but i am a gardener who spent a short time working in Southern California, whatever that counts for lol. Zindor (talk) 23:41, 25 January 2021 (UTC)