Wikipedia:Reference desk/Archives/Science/2018 July 29

= July 29 =

Force in rocketry, possibly for quarks inside proton, anywhere else?
Hi if t is time, p(t) is momentum of a pointlike object with mass, then classical physics says that p(t)=m(t)v(t) where m and v are mass and velocity of the opject, and that the net force F(t) is p’(t) = m(t)v’(t) + m’(t)v(t). In a rocket if the mass loss due to fuel consumption can make m’(t) a large enough negative number, can m’(t)v(t) become almost as great in magnitude though of opposite sign, to m(t)v’(t)? Is it possible to gain great increases in speed with small amounts of force this way? My second question is if quarks inside a proton could experience relativistic increases or decreases in mass so rapid that the contribution to net force of m’(t)v(t) is comparable or even much greater than that of mv’(t)? My 3rd question is that since the charges on quarks would complicate even the classical picture (what with electromagnetic radiation), are there noncharged particles that experience th strong force in the same way quarks do, so that a similar regime of m’v dominating could exist in a simpler way? Thank you Rich (talk) 03:38, 29 July 2018 (UTC)


 * Well, according to Newton's law F=m*a. So, in order to accelerate a ship of mass m, one needs a force F, there is no way around that except for gravitational slingshots (using planets/moons). If m diminishes, yes, the acceleration would be higher, but there is a limit to it (i.e. when all fuel got burned). With present-day technology it isn't possible to separate the quarks inside of a proton (anyway, not upon a spaceship). Tgeorgescu (talk) 04:13, 29 July 2018 (UTC)
 * No, F=ma when m’(t)=0.Rich (talk) 05:37, 29 July 2018 (UTC)
 * if one has to look at the the rocket case as a closed constant mass system that includes the exhaust gases, as i just learned a few seconds ago, how does one deal with a case where mass isnt being lost by or gained by something like fuel consumption, but instead by relativistic mass increase?Rich (talk) 05:50, 29 July 2018 (UTC)
 * In Newtonian mechanics the change in mass is not part of the force equation. If the mass is changing over time/space that will be included in calculations of impulse and work, but that's it. In the relativistic case, you simply use a corrected force equation, like p = γm0v, F = dp/dt. Someguy1221 (talk) 07:56, 29 July 2018 (UTC)
 * In "to accelerate a ship of mass m, one needs a force F, there is no way around that except for gravitational slingshots" there is no 'except'. No force == no acceleration. In a gravitational slingshot gravity is the force accelerating the spaceship. Doroletho (talk) 15:01, 29 July 2018 (UTC)

The chain rule derivative you do looks right. As said above, the first term is F = ma. The other term makes it F = ma + m'v. This extra term distinguishes Newtonian momentum from relativistic momentum. The classic case where this matters is that you can take a particle, put it in an accelerator and keep applying force to it, and after a while v scarcely changes - it's moving near the speed of light. Instead the relativistic mass keeps going up and up. To be sure, relativistic mass, while sensible as ever, is presently out of vogue, and you would be encouraged to do relativistic problems using a different formalism and a different definition of "mass"; hence instead of saying p = mv you say p = &gamma;m0v. But F is still dp/dt, and &gamma;m0 is still relativistic mass. Our article on momentum points at variable-mass system. But you have to watch the assumptions in those problems because mass never just "varies" (apart from the relativistic effects); it always comes or leaves taking its own momentum with it. Wnt (talk) 15:00, 29 July 2018 (UTC)

Sonorescence
The old Chambers Dictionary from 1908 defines sonorescence as the property of emitting sound under intermittent radiant heat or light. I can find this mentioned in some older books, e.g. Rubber World vol. 68 (1923): "In this application hard rubber is particularly valuable owing to the accuracy with which it can be machined and also to its remarkable sonorescent quality." Yet Wikipedia has no entry for sonorescence and I've never heard of this property before. Is it real, or was it an old erroneous belief? Does it have a different name today? Equinox ◑ 16:34, 29 July 2018 (UTC)


 * According to the latest OED (draft third edition of June 2017) the term is now obsolete, but the definition was: "The production of audible sounds by a substance when it absorbs periodically modulated light (or other electromagnetic radiation); the ability of a substance to do this." The OED also comments: "Now sometimes called the optoacoustic or photoacoustic effect."   Dbfirs  16:39, 29 July 2018 (UTC)


 * Thanks; just made a redirect. Equinox ◑ 16:42, 29 July 2018 (UTC)


 * You beat me to it!  Db<i style="color: #4fc;">f</i><i style="color: #6f6;">i</i><i style="color: #4e4;">r</i><i style="color: #4a4">s</i>  16:43, 29 July 2018 (UTC)

Different expression levels from same reporter inserted into the same place in the genome
I used homology directed repair to get a cassette expressing a fluorescent protein and (eukaryotic) antibiotic resistance gene in a specific place in the genome of a cell line. The cells were cultured under selective medium but for some reason they all show different levels of expression of the fluorescent protein. Why might that happen? It would be interesting to culture them in a live cell imaging microscope to see if high-fluorescence cells divide into high-fluorescence daughter cells but I'm not going to have the opportunity. The cells in this image are fully confluent because I was about to extract the DNA from them but the variation in expression exists at lower confluence also.

Maybe it's an oversimplification to think they're all the same cells and they actually have developed their own epigenetic deviations which have resulting in the differing expression? They were all resistant to that antibiotic though and the antibiotic was expressed from the same transcript as the fluorescent protein via a 2A sequence. --129.215.47.59 (talk) 18:33, 29 July 2018 (UTC)


 * This is the expected result for most things. I mean, not necessarily that level of variance between clonal cells, but variance in general. "Stochastic gene expression" is virtually an entire field of study at this point. I would recommend going to your favorite research search engine and search for recent reviews on that topic. The stochasticity can arise from transcription factor abundance/activity/binding, epigenetic marks, transcript stability, or translation rates. Or even protein stability. It's a whole field! Some genes are more prone to this than others, and variance can also be influenced by environmental factors and genetic background. If the cells were recently transfected and not subject to clonal expansion yet, whether the transgene is likely to be identical in each would depend on the method by which you got it in there. Someguy1221 (talk) 02:24, 30 July 2018 (UTC)


 * This [maybe not -- see below] likely has to do with heterochromatin formation or other epigenetic aspects of the genome.  See position-effect variegation as one example, though to be sure I don't know whether you have yours next to heterochromatin.  Note though that putting multiple copies of something into the genome will make heterochromatin; it's the body's despamming mechanism.  Steve Henikoff put out a lot of the key papers in the area.  There are some neat examples - the Pigmented Extraepidermal Tissues mutant reportedly formed five different sublineages with different (internal) pigmentation patterns from a single originating mutant  though four were lost during an air conditioner outage.  I should also comment that "unstable transgene expression" is typically downward and hence disappointing - it was not that uncommon for lines of transgenic mice to just stop producing the transcript as a new heterochromatin region became established, though modern methods allow for better control over both the position and the number of copies.


 * Actually looking at your picture though, I realize this might be simpler than my first thought. It is possible that some variable aspect of the plating is influencing the expression -- for example, the cells might be at different points of the cell cycle, or some might be experiencing contact inhibition while others aren't, or many other things.  The effects I described above should be somewhat clonal in nature, but it looks like you grew up those cells together, and there's no obvious correlation between neighboring cells.  There's a lot of variation, but not variegation.  If I saw patches of lower expression it would be more likely to be a DNA-based effect.  See if you can synchronize these cells in the cell cycle and look at them at known times and/or with cell cycle markers as controls. Wnt (talk) 14:29, 30 July 2018 (UTC)


 * There is of course the far more straightforward explanation that your cultures aren't pure. There could be differential uptake/expression of your contruct in different cells. What cell line is it supposed to be, and have you checked that lately? Cell line contamination is a major issue (and one of the many reasons I only work with primary cells). Fgf10 (talk) 07:06, 31 July 2018 (UTC)