Mike Dunlavey
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 Mar 16 comment Why aren't clouds multi-colored? Clouds are multi-colored. Mix together all the colors, and what you see is white. In fact, I'm typing this on a screen that looks white. A magnifying glass shows that it is not white. It is red, green, and blue. Mar 15 comment Free falling and bouncing back Look up discontinuity. Here, the velocity is negative, then positive. You can't really find a time when it is in between, not unless you want to model at the atomic level. Functions that are discontinuous cannot be differentiated, so calculus doesn't work across discontinuities. So how do you model it? In sections, that's how. Mar 10 comment How is pressure reduced on the upper part of an aircrafts wing? Ethan, maybe I've mentioned this to you before, but I would encourage you to read John Denker's on-line book about aviation. He just makes things so incredibly clear. Mar 9 comment How is pressure reduced on the upper part of an aircrafts wing? Air cannot be pulled (or pushed) by anything other than a pressure difference. Mar 7 comment What Are Logarithms? @MatthewMorrissette: Well, I was a professor once, so I don't mind answering elementary questions, but that makes me an outlier. There are people on this site who can answer questions at the cutting edge of physics. The more physics you learn, the more you will see how great this website is. Mar 7 comment Why does pressure decrease as velocity increases? It's very simple. The only way the velocity of a parcel of gas can increase is by being pulled by lower pressure (or pushed by higher). So there's a direct relationship between velocity and pressure. (A violation of this rule is if something else accelerates the gas, as in the case of a conductive plasma in a magnetic field.) Feb 25 comment Why does wind sound the way it does? @Carl: That's why I said "white" :) Feb 25 comment Why does wind sound the way it does? It depends what it's blowing through - trees, whatever. Anyway, it's "white" noise. Feb 21 comment Would a Tesla Valve create a pressure differential? @ChesterMiller: Right, I think that's what Joe was asking, and I think the electrical analog is the corresponding situation. If in Joe's boxes a pressure differential would develop, then you could have it drive a little motor in a tube going from one box to the other. Perpetual motion machine! Feb 17 comment Gravity and elliptical orbits @JeanValsean; Not if the satellite is traveling fast. In general, newtonian orbits are conic sections, like a hyperbola. In fact, you've heard of "gravity assist" to accelerate spacecraft? That's when a fast-moving object loops past a planet and picks up speed from it. In the planet's frame, the orbit is a hyperbola. Feb 15 comment Did/Will gravitational wave displaced earth? Thanks. Dumb questions: 1) what is spiral time? 2) how do they know it is 1.3b light years distant? Feb 14 comment Can a submarine be powered by a wind turbine? @Hohmannfan: The point is, just have the sail travel in a circle. Then the sail is not traveling directly into the wind. It is traveling at an angle to the wind, but the vehicle is traveling directly into the wind (or wherever it is pointed). Feb 13 comment Why didn't 0.2A at 2V kill me? Because your skin has a lot of resistance. Feb 9 comment Why does a propeller suck in air from the front? @jameslarge: Yes of course there's a pressure difference. The wrong explanation is the one that says the air parcels reunite at the trailing edge - the "equal transit time" fallacy. Feb 5 comment 2 airplanes same size and shap different mass Not steeper, just faster. Both lift and drag are proportional to velocity squared, so their ratio is roughly constant, L/D. The tangent of the descent angle is just D/L. So if Weight is greater, Lift must be greater, so Velocity increases as the square root of Lift, and Drag increases linearly with Lift. So - same angle. For a small plane, the L/D ratio is about 9. For a jet airliner it is more like 25. For a sailplane, 30. Feb 4 comment Adding cinnamon to my coffee makes it stop whirling much faster: Why is thats? @HolgerFiedler; I'm with you. I suspect the coffee below is still spinning. Feb 4 comment What is the difference between vortexing and centrifuging? +1 The difference is in the angular velocity as a function of radius. In a vortex the angular velocity decreases as $1/r$, while in a centrifuge it is constant, so in a centrifuge the centripetal acceleration $\omega^2r$ increases with radius, so things more rapidly float/sink to their preferred depth. Feb 2 comment Why is an airplane propeller so different from a boat propeller in shape? I'm not sure I believe that other answer. This is just a comment because I don't really know. I can only point out that they are both rotary wings, and the lift/drag ratio of a wing is better if it has higher aspect ratio - long and narrow. Another factor is cavitation - water propellers cannot have too large of a pressure difference, to minimize steam bubbles, so they might have to make up for it with area. But I don't really know. Feb 2 comment If the Earth is curved, why don't planes need to adjust attitude to stay parallel to the ground? @Dargscisyhp: So you should know what I mean, having had your hands on the controls yourself. Aircraft power is not on/off, if it more/less. Aircraft are always gliding. Whether they glide down or glide up is just a matter of whether the thrust is less than the drag or more. If you want the plane to maintain constant height above the earth, that's one power setting. If you want it to follow a "straight line" and ascend, that takes a higher power setting. If you want to descend to the airport, you decrease the power, to idle if necessary. Feb 2 comment If the Earth is curved, why don't planes need to adjust attitude to stay parallel to the ground? @DavidHammen: Yeah, sure. Good planes, properly loaded, damp that out. (I'm talking about common aircraft. Aerobatics and fighters are a different story.)