Mike Dunlavey
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 Feb 17 answered How can I understand $\mathrm ds^2 = -c\,\mathrm dt^2 + [\mathrm dx-v_s(t)f(r_s)\mathrm dt]^2 +\mathrm dy^2 +\mathrm dz^2$ in the simplest way? 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 14 answered Can a submarine be powered by a wind turbine? Feb 13 comment Why didn't 0.2A at 2V kill me? Because your skin has a lot of resistance. Feb 12 answered How can I calculate how high an arrow goes when all I know is its initial speed? Feb 10 revised Is it possible that a plane in turbulences has more downward acceleration than gravity's added 517 characters in body Feb 9 answered Why doesn't Torque of masses on a pulley depend on the friction between the rob and the pulley according to a question in the physics 1 book 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 8 revised How can a product of Bra and Ket be a scalar if they are matrices? added 67 characters in body Feb 8 answered How can a product of Bra and Ket be a scalar if they are matrices? Feb 8 revised Is it possible that a plane in turbulences has more downward acceleration than gravity's added 119 characters in body Feb 8 revised Is it possible that a plane in turbulences has more downward acceleration than gravity's added 83 characters in body Feb 8 revised Is it possible that a plane in turbulences has more downward acceleration than gravity's added 266 characters in body Feb 8 answered Is it possible that a plane in turbulences has more downward acceleration than gravity's Feb 6 answered How can I clarify the difference between free falling and Newton's law? 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.