Wikipedia:Reference desk/Archives/Science/2023 October 29

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October 29

Red-bodied swallowtail

What is the size range of Pachliopta polydorus? (Pictures probably OK as long as they're not too big -- the article didn't mention anything about it having vertical stripes like the barred swordtails!) 2601:646:9882:46E0:804:549:949E:C896 (talk) 07:52, 29 October 2023 (UTC)

I have added size data with a reference. Feel free to add any other details which you think are pertinent; see Help:Editing for details or ask at the nice people at Wikipedia:Help desk if you get stuck. Alansplodge (talk) 11:20, 29 October 2023 (UTC)

To answer your question directly here (I appreciate why you may be reluctant to look at the article): "Average wingspan size is 72 mm for males and 76 mm for females." The illustrated specimens have no vertical stripes at all, just some fine horizontal ones on the forewings. {The poster formerly known as 87.81.230.195} 46.65.231.103 (talk) 12:36, 29 October 2023 (UTC)

Thanks! So, about 3 inches -- which is a very reasonable size for a swallowtail (most of them are about 3-4 inches, i.e. monarch-size in my parlance, although some species can be as small as 2 inches, and a few -- fortunately only a few -- can be 6 inches or more)! And yes, I've already taken a look at the article (both with the pictures off, and later with the pictures on, although both times were before User:Alansplodge added the size data) -- and this is one beautiful critter!  :-) 2601:646:9882:46E0:80DA:EA0B:DFEC:D821 (talk) 03:52, 30 October 2023 (UTC)

Oh? Why might someone be reluctant to look at an article about butterflies? —Tamfang (talk) 23:19, 3 November 2023 (UTC)

Well, because some people may be afraid of either butterflies in general, or of certain kinds of butterflies -- and the latter is the case with me, but only of Papilio glaucus and other species which look similar to P. glaucus, so Pachliopta polydorus is completely OK! 2601:646:9882:46E0:E8CD:5F6:7849:92ED (talk) 06:30, 4 November 2023 (UTC)

Kinetic energy, joule and time

An object having a stable kinetic energy outside of any interaction, this energy is given in joules and therefore its value does not vary with time here. Apart from the physical dimension of the joule is ML²T^-2, therefore dependent on time. So, how should we take kinetic energy, potential energy? How to explain this? Malypaet (talk) 22:02, 29 October 2023 (UTC)

Clarify "dependant on time". The height of a tree depends on time because its value depends on when one chooses to measure it. A speed depends on time in the sense of equalling a distance differentiated with respect to an instant of time^{see note}. The dimension of speed is L T^-1. Energy can be measured in SI units as kg⋅m²⋅s⁻² having, as you correctly say, dimensions M L² T⁻². Potential energy (dynamite) is just kinetic energy (the boom) waiting to be released. ^{Note: It is more practical to evaluate average speed during a finite time by dividing the finite distance travelled by the finite time.} Philvoids (talk) 22:47, 29 October 2023 (UTC)

For work I understand SI units kg m² s^-2, as there is a force, energy depends on time, in 1s you get n Joule, in 2s you get 2n Joule, m s^-2 is an acceleration. Thus, for work, the energy delivered depends on the delivery time. But when the force that gave an object the kinetic energy disappears, the kinetic energy doesn't change over time, it just becomes potential energy/work like dinamite. What I'm trying to understand or explain is how to differentiate between kinetic energy and work, because the first does not change over time while the second does. Malypaet (talk) 09:24, 30 October 2023 (UTC)

A physical quantity may or may not vary as a function of time. This is not related to whether the dimension of the quantity involves a non-zero power of ${\mathsf {T}}.$ Philvoids's example, the height of a tree, is a time-dependent quantity whose dimension is just ${\mathsf {L}}.$ The speed of light does not depend on time, but its dimension is ${\mathsf {LT}}^{{-}1}.$ Whether the magnitude of a physical property of some physical object or physical system "depends on time" is not so much determined by the property as by the object or system of which it is a property.  --Lambiam 10:49, 30 October 2023 (UTC)

Sorry, but a light pulse, a radar pulse or a laser pulse obeys the same speed and times law than any object (except they are mass less) , like a plane or a bullet, in the same frame of reference. These pulses travel at maximum the speed of light and otherwise according to the resistance of the medium crossed. They therefore depend on time. And the energy they carry travels with them, as power. That's applied physics. Malypaet (talk) 12:54, 30 October 2023 (UTC)

Let that energy be trapped within an object with v=0. Overall, the power vanishes, but the mass-energy within remains, it is constant and can still do work. Modocc (talk) 13:49, 30 October 2023 (UTC)

Right, if you write about light, absorbed by an object with v=0, the power become thermic energy, that become again radiant energy. This is the planck's law. Or it become electric power, or it become chemical reaction storing the energy flow of the power... But the subject is about kinetic energy and joule vs time vs work. Malypaet (talk) 20:47, 30 October 2023 (UTC)

My point is that you are right that the magnitude of KE need not depend on time. This can be understood in the sense that energy, qualitatively, can do work and the amount of work which is well-defined. Modocc (talk) 22:26, 30 October 2023 (UTC)

The OP's polite apology is appreciated but needless for correctly citing applied physics. The travel of Electromagnetic radiation of energy through a vacuum in forms such as light, radar or laser that are mentioned cannot be adequately explained the same way as the flights of planes or bullets. The hypothetical massless Photon particle somehow carries momentum and electromagnetic radiant energy but our Classical physics concepts of speed and time prove unable to fully describe its behavior. We are forced to confront the Wave–particle duality of the elusive photon, whose understanding comes from recognizing its quantum mechanical nature: light as a wave interferes and light as a particle follows a trajectory. The OP would like to identify the photon energy as potential or kinetic but its duality blurs the distinction: where the photon acts as a particle we may call its energy kinetic (in motion it has only a small relativistic mass and energy directly related to its emission wavelength by $E={\frac {\,h\,c\,}{\lambda }}$ where ${\textstyle h}$ is the Planck constant.) As long as the photon can be regarded as a wave its energy is indefinitely preserved before an eventual interaction so we can call its energy potential. Whether kinetic or potential, the energy is the same given by the equation. One may pursue its dimensionality and confirm that ${\textstyle h}$ has dimension M L² T^-1. Philvoids (talk) 22:43, 30 October 2023 (UTC)

Reference needed at salt marsh

A new editor removed a statement in the salt marsh article because it didn't have a reference. Is there a reference for the statement that was removed? Bubba73 ^{You talkin' to me?} 23:13, 29 October 2023 (UTC)

Resolved

Bubba73 ^{You talkin' to me?} 06:22, 30 October 2023 (UTC)