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Jan
31
comment $\Delta T$, temperature units. How to use fahrenheit in this formula
Looks cool! Good luck.
Jan
31
comment $\Delta T$, temperature units. How to use fahrenheit in this formula
Multiply by 5/9
Jan
31
comment Why does a flat clover-like shape fall slower when it is rotating?
@DiomidisSpinellis: There's a video here (and there are many more like it) illustrating the concept of AoA and wing stall.
Jan
30
comment Why does a flat clover-like shape fall slower when it is rotating?
@DiomidisSpinellis: If you are a tiny person attached to the end of a blade, and if the motion downward is 1 m/s, and since the toy is spinning you also have a tangential motion forward of, say, 10 m/s, then the air is coming toward you at an angle of roughly 6 degrees below the horizon, at a speed of roughly 10 m/s. Even if the blade you are attached to has zero pitch (i.e. perfectly flat), that blade will be functioning as a wing, not a parachute, i.e. it will be flying, not stalled, therefore seeing appreciable lift. Stall AoA is between 10 & 20 degrees.
Jan
30
comment Why does a flat clover-like shape fall slower when it is rotating?
@Olin: Yeah, helicopters do not reverse rotation direction when they autorotate, they pitch the blades down. Regarding the "whole disk" acting like a parachute, I'm sure you've seen this. As a sometime pilot myself, I'm very aware of the essential importance of angle of attack. A parachute (not a paraglider) is essentially stalled (90 degree AoA). Rotor blades are flying, not stalled (AoA < 20 degrees). The airflow follows both upper and lower surfaces. So calling the spinning toy basically a circular parachute doesn't hold water, I think.
Jan
29
comment Why does a flat clover-like shape fall slower when it is rotating?
"Approximate a disk for the purpose of air resistance"? "rotor end up spinning opposite of how the engine was driving it"? Sure you don't want to reconsider those?
Jan
29
comment Pressure in fluid mechanics of incompressible liquid
Keep in mind that the Bernoulli relation is simply energy conservation. Ignoring gravity, the only way the fluid can change speed is if there is a pressure gradient for it to fall through.
Jan
29
comment Physics and Linear Differential Equations
@RoopamSinha: I would be careful of the term "linear", if what you mean is that only first powers of state elements, not multiplied by other state elements, appear on the right hand sides. In my experience in pharma, nonlinear differential equations appear all the time.
Jan
24
comment How physical objects (e.g. Earth and Apple) are aware and do computation about each other?
@Yasser: Another way is to simply accept not knowing! We don't know now, and maybe we will in the future, maybe not. Does a blank piece of paper need an explanation? Look, hundreds of years ago, there was no conflict between science and theology, because scientists saw themselves as trying to understand more about creation. Theology was not seen as a substitute for science, until some hard-nosed people insisted it was, and we still see that trouble today. Personally, I keep those issues entirely separate and private, and I wish more people would do the same :)
Jan
24
comment How physical objects (e.g. Earth and Apple) are aware and do computation about each other?
@KeithThompson: Totally agree. In one of Feynman's paperbacks, he tells the story of his father being asked, if there's a pumpkin in a wagon, and the wagon is pulled, why does the pumpkin roll to the back? His father said, basically, everybody knows it does, but nobody knows why.
Jan
24
comment How physical objects (e.g. Earth and Apple) are aware and do computation about each other?
@Yasser: Newton couldn't answer that question, so he assumed God did it. Einstein actually made a theory of it, which helps us understand it, regardless of any deity, but it does not not require the physical objects to know that theory.
Jan
24
comment How physical objects (e.g. Earth and Apple) are aware and do computation about each other?
If you put one block on top of another, do the blocks need to know math to know they are 2 blocks tall? No. Maybe we do.
Jan
24
comment Does rotation cause gravity?
Just to be light-hearted, some children were once asked what causes wind. The answer: the trees waving their leaves.
Jan
22
comment Who popularized $E=mc^2$?
6 August, 1945, is when the world became aware.
Jan
22
comment Why is the Fraunhoffer Pattern the Fourier transform of the slit?
There's a pretty good explanation here.
Jan
22
comment Sailing against the wind. Is this a fair model?
That model is good, and you can use the same model to show how to go downwind faster than the wind, because the "sail" can always be turned to an angle such that the balls hit it, and the normal force has a forward component.
Jan
20
comment Non-determinitric flow of water stream on vertical porcelain surface
There's probably a better answer, but the water streaming down the surface has surface tension, that apparently affects it chaotically, the same way rivers meander. Flow in a straight line is unstable, because the slightest curve tends to grow on the outside.
Jan
13
comment Mirrors into Infinity
+ Nice, and you get a similar effect if you have 1) a TV camera, connected to 2) a closed-circuit TV monitor, and 3) the camera is looking at the monitor. In the 70s this was an example of electronics "freaking out".
Jan
11
comment Homopolar motor and Lorentz force
+ I'd never seen that before. What a nifty demonstration! If you have a wire cutting through magnetic field lines a current is induced. Then the reverse has to be true also, and that's what you're seeing.
Jan
11
comment How does the pressurization in a house's water supply allow adjustable flowrates from a tap?
I'm sure you're learned in your Ph.D. studies in fluid dynamics that pipe networks and wire networks are analogous, and an adjustable tap is just like a variable resistor.