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 Feb 9 comment Changing the Density of a liquid while its surface tension is constant Density and surface tension are tied together. If you think about what causes surface tension in liquids it's really the forces between molecules. Density is also related to the molecular spacing which relates to forces between molecules. If you want to change a liquid's density you need to change the molecular spacing, which in turn is going to change the surface tension. Anyways, you might look for a molecular theory to tell you if changing pressure and temperature can keep surface tension the same while changing density. Maybe, but maybe not. Feb 9 comment How to restict air flow without reducing pressure You want flow but you don't want to change pressure? Sounds like a contradiction.... Feb 8 comment Some Clarifications about Electricity No. Nothing outside of the system. It means there is a voltage difference between point "a" and point "b" which is due to the charge built up across the capacitor. Oct 25 awarded Yearling Oct 25 awarded Yearling Sep 22 answered How fast would a bullet travel through liquid air? Sep 22 answered Propellers and the Speed of sound Apr 12 comment If the MH370 black box did sink to 15000 ft, how long would it have taken? hyperphysics.phy-astr.gsu.edu/hbase/lindrg.html Apr 9 comment Minimum amount of fluid to experience turbulence? Not sure what you mean by "fluid volume." For a start, volume is not a dimensionless quantity and whether turbulence could occur within a volume of fluid is not a function of the physical volume of fluid but dimensionless quantities such as the Reynolds number. The Kolmogorov scale represents the smallest scale in a turbulent flow, which depends on the viscosity and what is driving the flow. Mar 7 comment Why will two bubbles floating on water surface attract each other? I'm not convinced they "attract" each other. When they bump into each other they tend to join and form a state of lower energy. Mar 6 comment What makes an abstract physical system describable by a “fluid” equations of motion? @SeanD Yes, there is a generalized notion of shear stress for each of those systems which is independent of deformation history and that is part of why you can approximate them as fluid continuum. If you want to understand shear stress on a molecular level you should look into the kinetic theory of gases. In that case shear stress is the result of momentum transport from molecular diffusion. I can imagine other sources of shear stress for other systems, but as long as it is independent of deformation history then the system is a fluid. Mar 6 comment What makes an abstract physical system describable by a “fluid” equations of motion? @SeanD Your question is now even more confusing. So what is it about a system obey a fluid model? The answer is quite simple and I know you don't find it enlightening, but it really is as simple as if it "doesn't support shear stress." There is no other magical quality that makes a system a "fluid." Mar 5 comment What makes an abstract physical system describable by a “fluid” equations of motion? Your answer is partial and indirect. For a start, that is not a valid definition for a fluid. A fluid is simply something that deforms continuously under shear stress. What you have given is an explanation as to when a material might be treated as a continuum. The author is indirectly asking about this as well, but you should correct your definition for a fluid. Mar 5 comment What makes an abstract physical system describable by a “fluid” equations of motion? A fluid is nothing more than a material that "flows" (deforms continuously under shear stress). I think what you are really trying to ask is under what circumstances can a material be modeled as continuum. There are many systems that lend themselves to continuum approximation. Some of those systems deform continuously under shear stress and are therefore analogous to fluids. Regardless, the equations of motion remain the same, but the equation of state of a fluid will always be independent of deformation history. Mar 5 answered Pressure in fluid mechanics of incompressible liquid Mar 4 comment Viscous fluid flowing around obstacle: would it deflect earlier? @Supernormal No, the solutions are not the same. The author describes them as "superficially similar" in that they share somewhat similar forms, but they are different. Feb 28 answered Viscous fluid flowing around obstacle: would it deflect earlier? Feb 28 comment Viscous fluid flowing around obstacle: would it deflect earlier? For stokes-flow ($Re << 1$, $M << 1$) there is an analytic solution for flow over a sphere (see "Viscous Fluid Flow" by White). It turns out that the solution is entirely independent of the fluid's viscosity. I think you should rephrase your question to indicate fixed, moderate, Mach numbers and moderate Reynolds numbers. That is probably the range where what you are asking can occur. My intuition as to the how the deflection varies with Reynolds number isn't quite able to answer your question though. Feb 28 comment Viscous fluid flowing around obstacle: would it deflect earlier? The dominant cause behind the fluid deflecting before reaching an obstacle is sound wave propagation. One might guess then that there is little, if any, change with the viscosity of the fluid. In fact, for high Reynolds numbers that is indeed the case. For low Reynolds numbers I can't say that I know what will happen. The question really needs to be asked with regard to both Mach number and Reynolds number. Feb 28 comment Branching lemon drop “smoke rings” "but I would assume that the 5 smaller rings would turn into 5 + X amount more, etc. At what point would they stop breaking up into smaller rings?" Basically it will depend on the Reynolds number and perhaps some other dimensionless quantity related to surface tension.