Physics

We are taught that calculus is all about "instantaneous changes" between 2 points, x and delta x in respect to time with x = 0 and delta x =1 For example, when we set x = 0, "are we "manipulating" time , ie stopping at a certain location " going a certain distance" as to derive the instantaneous change at that point " location" in time? If that's the case for our x = 0 "reference point" then what's the point of calculus and its "undefined location?" Another example, if x = 0 and delta x = 1, " what's the point of coordinate vectors?" example x = 3 y = 9?? Another good example, if Cartesian Space references 0 in the coordinate system -2, -3, -1, 0, 1 , …

I’m curious about the experience of a family member. They were with a female family member when she died. They made eye contact as she breathed her last breath, and saw a blue flash of light glow in her eyes and then leave her body. I was wondering if anyone has experienced a similar thing, or if this blue light can be explained by physics?

So for a geek convention coming up in a few months I was... considering dressing up as a cartoon animal. (Been going a little too mundane with my prior cosplay and wanted to make up for lost time.) But I'm worried about being too warm in it, so I want some sort of coolant between my outer costume and inner clothing. Icewater, I presume, would be a little too drastic and probably result in frostbite, if not hypothermia. (Even if I found a material watertight enough to make it work.) However, is there any other coolant whose equilibrium temperature is a little more comfortable; not too hot, not too cold?

Good day. Cut a strip of thin iron sheet (a coffee can after surgery by scissors) Say 25mm x 100mm x 0.5mm. Cover sharp edges with some vinyl-electrical tape. Wind maaany turns of fine wire coil from left towards right and keep winding back towards the starting point. Both coil ends are at the same side. That is sort of a Rogowski coil, with a flexible metal core. -I think- Now wrap the bendable strip coil thing onto an AC current carrying wire as the lower sketch. Will the winding present some voltage proportional to the big wire current ? -Ignore the decorative print at lower right-

So I was recently watching this video on YouTube and it shows a bunch of large balloons being popped in slow motion. (First example a few seconds shy of a minute in.) I can't tell whether that is dust or mist or something else at the outer edge of what used to be the balloon immediately after it was popped. Could the act of popping a balloon cause enough adiabatic expansion to bring the air in the immediate vicinity of the balloon to the dewpoint? Would the remnants of the balloon be aerosolized to the point of initially being visible as dust in the aftermath of the popping? Or is it a "little from column A, little from column B" solution with the aerosolized balloon bits…

I have noticed that this term gets thrown around quite often when people debate what's "scientific" and what is not. What I have NOT found is a definition or explanation of "falsifiability" that is unambiguous, coherent and logical! Apparently it's an alleged "property" of a theory or hypothesis, which for some mysterious reason makes it "scientific" (whatever that might mean - I'll get back to that later) So we need a "test" which could tell us whether a theory/hypothesis is "falsifiable" or not. In the context of science any such a "test" should be rigorously and unambiguously defined. The problem is, the only things I found are ambiguous and nonse…

As a chemist, I am struggling with this. In, say, a helium nucleus, the rest mass is less than the rest mass of the "free" neutrons and protons from which it is made - the so-called mass defect. That makes perfect sense to me because, to separate the nucleus into its components, you have to do work against the strong nuclear interaction that holds the nucleons together, i.e. an energy input is required, which of course is then reflected in a greater rest mass of the separated nucleons. And hence the converse occurs during fusion, leading to a net output of energy when the nucleons combine and become bound. But when it comes to the quarks that form a proton, say,…

Was trying to find hardness figures for nylon, delrin, ABS, teflon, poliethylenes ... and got lost in too many scales Shore, Brinnell flavors, Vickers, Rockwell, and metric and not metric, Is so much variety needed ? No consensus ? Very nasty to see so many and looking for the proper conversion. 😱

Good day. Let me know if I learned it right : The color of the lens is the color of a laser the goggle does NOT protect. The color of the lens is the one the laser beam color can pass trough. The protection from a laser is by reflecting, absorbing or blocking the beam and which of the three cannot be told by the color of the lens. Goggles can protect against several laser wavelengths except the one of the lenses.

Recently I saw comparison between an image taken by Spitzer and an image taken by James Webb space telescope (here, for example: https://www.planetary.org/space-images/spitzer-vs-jwst-in-infrared ). Why there are those rays of light visible around bright starts on the James Webb image (but not on the Spitzer image)? BTW, Is there a specific English term for this effect? Also, does anybody know what could be the exposition time needed to make these pictures (both, Spitzer and James Webb)?

Hello. Blue plus red emitters are used as grow light. The resulting hue is the sum of red+blue. Or is it the difference ? If the sum; what color is the difference of their wavelengths ? And what is the intended meaning for 'full spectrum' ? (Image borrowed from the web)

Am I to assume the tradeoffs depend on the scale on which they're being used? For photovoltaics, I'm not sure industrial use would necessarily be any more efficient than household use, on a per-panel level, but for thermal!solar, it sounds almost like the reasons for higher efficiency follow from thermodynamics itself... larger array of mirrors mean more sun rays converged means more sun rays on the same area means higher maximum temperature meaning higher difference between warm and cold reservoirs of the heat engine, which from what I recall from thermodynamics is more efficient, all else held constant. How do the initial costs and maintenance costs of each …

I have found droplets (of water, I assume) inside my chocolate wrappers. The wrappers hadn't been opened and didn't have holes in them, but they had been stored temporarily in a humid environment by mistake for maybe 30 minutes (they were at room temperature prior). Are the droplets likely to be condensation?

Hello all. Never had it fully clear, voids in knowledge... If a solution has -say all- the elements dissolved in water, and a direct electric current is passed in it; what elements will deposit on the electrodes if the voltage is 0.1V ; if it is 0.2V; if it is 0.5V; ... ... ... if it is 1.0V ? Can you cite examples for each voltage please, and which table tells it ? Electronegativity ? Electrochemical series ? Standard electrode potentials ?

I'm not sure whether this belongs in the physics of movies thread or not, but I figure I should have a spearate thread for video games given the interactive nature of the medium. In one part of Kaizo Mario 3, Mario is temporarily in projectile motion, until his trajectory crosses paths with a waterfall. At this point, the viscosity of his surroundings is no longer negligible, and therefore, he can push his feet downward against the water to propel himself upward. I assume it's exaggerated, and that one couldn't possible propel oneself to THAT extent in real life. But at the same time, this leaves me wondering whether it's po…

Originally I was going to make this a "relative motion" thread, but I think the notion of analyzing movie physics is more interesting overall. In "Rat Race," a mechanic startled by the passing supersonic landspeeder fires a bullet parallel to its path. (At about a minute and a half into the clip.) To the drivers, however, the bullet appears to be suspended in mid-air, as it is moving at approximately the same velocity as the landspeeder. 1. Would the bullet's path be kept horizontal for any non-negligible amount of time by air resistance, or would the vertical component of its motion immediately assume downward acceleration like ev…

My kitchen lacks one of those gradually-adjustable lights. Sometimes when I get up in the middle of the night I want a light that's just bright enough to help me see what I'm doing when I eat, but not as bright as the overhead lights. Sometimes, therefore, I use the refrigerator's partly-opened door. I'm fully aware that the refrigerator doesn't just produce light, but also heat, from the fact that relocating heat from the interior of the refrigerator to the exterior thereof converts electrical energy to heat energy in the process. However... since this is the same thing my electrical heaters in my apartment do anyway... does that mean that, so long as I adjus…

According to Einstein, if you go faster than the speed of light, you go back in time. Obviously you can't go back in time in the literal sense of the word, but you can go back in time in the sense that you can enter a parallel dimension where t=-1 compared to the present universe where t=0. Is that true?

So recent events got me thinking about the politics that led us here, including the 2016 election and Martin O'Malley's so-called "rain tax". It taxed impermeable surfaces that cause water to pick up pollutants as it flows, and further damaged his already not-so-hopeful career. While Vox is defending this tax (or at least criticizing flawed criticisms of it); and I get realize that there is often more to these issues than some buzzword will let on; what I am wondering is why there wasn't infrastructure in place; whether on a local or federal level; to actually collect rainwater so it could be transported to where it could be used. Seems a waste to just let some mere lawn …

A: In anticipation of future power outages, (last windstorm didn't do it but the next one could) I've been stuffing the top part of my freezer with tupperwaves full of ice. I'm figuring that because hot air rises and cold air sinks, that absorption of heat by melting ice would both preserve my frozen food for longer and double as an indication of how safe the food is by how much of the ice has melted. Would everything below this ice-water mixture be kept at 0 degrees centigrade due to sinking cold air, or is there something I'm missing? B: A little trickier is the question of how to keep items in refrigerator part well enough below room temperature for food no…

Does relativistic mass possess gravitational (passive) mass? Do you know any link that describes an experiment at CERN which proves or disproves the increase of weight in particle at near C?

Good day. Clearing the snow off my panels on he roof as it will be sunny the next week but still under 0oC and I wanted them generating, got curious... Sun does warm up solar panels, no doubt. Ambient too. But is panels heating only product of the solar radiation + environment or the panels themselves add warmth by the current produced ? In other words; a solar panel that has reached its temperature plateau under the sun while disconnected (no current flowing); it will increase its temperature when connected to a load and current flows, right ?

Sorry, if I am a bit off topic, but just a side question out of curiosity - can bouncing light in a fixed box have any wavelength, or only some discrete values?

What got stuck in its development, what is the bottleneck ? Have heard nothing in almost a decade 😟

Just prepared: https://arxiv.org/pdf/2112.12557 While textbook explanation of p-n junction ( https://en.wikipedia.org/wiki/P–n_junction ) looks quite heuristic, this is just using statistical mechanics - no "holes", only electron dynamics. Lattice 60x30 atoms below, dopants of different potentials are presented as red/green dots, grayness shows calculated electron densities, arrows show local currents. The model is: - use 3 types of potentials: of individual atoms + from external voltage + mean-field self interaction (from found charge density), - apply entropy maximizing diffusion ( https://en.wikipedia.org/wiki/Maximal_entropy_random_walk ), getting e.…