# Tag Info

28

Assuming you start with a full bottle of water, when you tip the bottle upside down, a 'partial vacuum' (ie below atmospheric pressure) is created at top of the bottle as the water pours out the bottom. Atmospheric air then 'bubbles through' the mouth of the bottle to compensate. This slows down the flow of water through the mouth of the bottle. Each time ...

22

When water leaves the bottle, the pressure above it drops. This reduces the net force pushing the water out of the opening, until it stops and a bubble can rise up. When the bubble has left the mouth of the bottle, the water can start flowing again. The stop-start of the water, and the reduced pressure inside the bottle, contribute to the lower flow rate in ...

6

A friend of mine once overheard a conversation between a father and his child on a public transit bus on a windy day. Child: "Why does the wind blow?". Father: "Some places are cold and some places are warm. That is not fair. Thus, the wind takes the cold air away and moves it to the warm place so that everybody's happy." - IRO-bot, from Earth Science ...

5

I know that 'Blowing air is called Wind' but what I don't know is, how is wind formed? And I don't want the answer from Google Search. I want to know more about wind in atomic or molecular level It is not out of a quirk of physicists that even though we have an enormous knowledge of how the microscopic framework of atoms and molecules works, we still ...

4

I think you've understood it all, air gets into the bottle faster. Without the vortex, the air is able to pull on the liquid, preventing it from escaping. This is why you can pour orange juice faster if the opening is at the top, rather than the bottom. It also stops it splashing.

4

The rotation of the Earth causes a force called the Coriolis force. This does have an effect on ocean currents, but the effect is only significant on length scales of hundreds of miles. Over the diameter of a shell, even a big one, the Coriolis force is completely swamped by other effects like tides, local currents, random thermal fluctuations or whether a ...

4

Both viscosity and surface tension are connected theoretically to inter-molecular forces, but they are still very different concepts. Viscosity force is a force that acts only when the fluid is moving and acts to decrease the gradient of velocity in it. Viscosity is a characterization of the fluid itself. Roughly speaking, it says how fast momentum of ...

3

This is essentially Pascal's law. I would like to add that it depends on the fluid being in equilibrium, as well as incompressible. Otherwise local pressure differences can build up and the fluid can behave rather differently. I like to look at this problem from a thermodynamics point of view. One way to think of this is that the pressure of a fluid is a ...

3

As is given in Jamie's answer I'll assume the surface is a revolution about $r=0$, that the mean curvature is proportional to the pressure difference, and that the radius of the cup is much larger than the inverse of this mean curvature. In this case the mean curvature can be specified as $$K_m = \frac{r''}{2(1+r'^2)^{\frac32}}$$ As in Jamie's answer the ...

3

The fluid in the tube is not water as some might think but an organic solvent called Dichloromethane. The reason the bubbles form is due to the fact that the fluid is heated at the base of the tube to it's boiling point which is a low 103.3 F degrees. You can almost get it boiling by holding it in your hand. The bubble is actually the vapor form of the ...

3

The downstream side of the nozzle is much more important to maintaining the efficiency of the nozzle by controlling expansion wave. It also influences the uniformity of the flow exiting the nozzle. Symmetry would be perfectly fine, but you'd end up making the converging section bigger than it needs to be. Here's a bit more about the design of the nozzle ...

2

Because the bag deflates as milk leaves it, the volume of the bag decreases and the pressure remains constant so the milk pours smoothly. When pouring from a can, which does not deform like the bag, the pressure inside the can decreases as liquid leaves the can. The pressure differential creates a potential that pulls air into the can, interrupting the flow ...

2

The question is therefore : why doesn't a fluid flow out a bottle smoothly ? The "bottleneck" phenomenon is caused by the lack of pressure in the can/bottle. As the liquid flows out, the pressure inside decreases because the volume of the container is fixed. When the pressure inside the bottle reach a given threshold, the outside air tends to flow in the ...

2

Every time, when you deal with differential equations, the first step is to put it into dimensionless form. There are more reasons for that. First, "small" and "large" has no meaning in dimensional forms, since you can always change the system of units. Second, nature knows no units. Now, when there is no exact solution (this is often the case), you can ...

1

The specific gravities would be the same if the levels of the two side were the same after liquid-II was added. When I try this, my logic seems to be flawed too. I don't get 1.12. The level on side II has not changed. The level on side I has risen 2 cm. So 2 cm of liquid-II were added. Consider the horizontal plane 2 cm below the top of side II. Below ...

1

Consider the following image, we have some fluid volume, $V$, having density $\rho$ and traveling at a velocity $v$ along a pipe with some cross-sectional area $A$. The rate at which the water flows through the pipe is called the volumetric flow rate. This is given by, $$\frac{dV}{dt}\equiv Q=\mathbf v\cdot\mathbf A$$ where $V$ is the volume of the ...

1

As @JánLalinský nicely explains, surface tension is measured between two fluids, while viscosity is measured within one. Say that you have a droplet of some liquid this means that if you change the surrounding medium the liquid-surrounding surface tension changes, while the viscosity of the droplet will not. That said, if you keep the surrounding medium ...

1

This is d'Alembert's principle. The basic, very general idea is to take Newton's second law applied to an accelerating mass, and write it as $F-ma=0$. That is, we take the $ma$ term and pretend it's another force balancing the $F$ term. This allows us to think about the dynamic, accelerating mass as if it's a static system. The $ma$ term is what's referred ...

1

The sound velocity depends on the sound frequency (dispersion). The flow must be locally faster than the frequency of the downstream disturbances. If the latter are such that their sound velocity is small, the local flow velocity may be chosen small too. Note, that the sound velocity depends also on the void fraction. If there are bubbles (even locally - ...

1

As you rightly suspected, in general, if $\vec{v}_1$ and $\vec{v}_2$ are two solution of the Euler equation then $\vec{v}_1 + \vec{v}_2$ is not a solution because of the nonlinear term. However, in many cases, one (or both) of the velocities will make the nonlinear term zero allowing you to add them. For example, in the case of a steady flow in an ...

1

The system you describe, a water tower with pumps and turbines, is a version of a pumped hydro storage system. The amount of energy stored in a pumped hydro storage system is defined by the elevation, i.e. the height that the water is lifted; multiplied by the volume of water that is lifted (assuming that the height the water will fall, is the same as the ...

1

There was a recent publication addressing exactly this question. From the abstract: Here, we show that the overall foaming-over process can be divided into three stages where different physical phenomena take place in different time scales: namely, the bubble-collapse (or cavitation) stage, the diffusion-driven stage, and the buoyancy-driven stage.

1

Tribology (not the study of tribes!) is the study of what happens when things 'rub'. This involves friction and wear when solids rub against other solids (such as in mechanical bearings) and the effect of liquids (such as 'lubricants') and other fluids. Friction at a solid-liquid interface is still called friction. It is a 'damping' or 'dissipative' force, ...

1

Well, in this case the origin presents an unstable equilibrium point, in the sense that if you put a test particle exactly at the origin, and there is no noise/fluctuation, it will stay there, but if there is even a tiniest fluctuation it will drift off origin. The reason is that $u=(0,0)$ at $(x,y)=(0,0)$. Put it another way, the equations of motion for a ...

1

Dust sticking to things is a complex process but can be broken down into several stages and analyzed. First though lets define our dust. Dust Size The aerodynamics of dust are most easily approximated by pretending all of the particles are spheres with a density equal to water ($1000 \frac{kg}{m^3}$). Each particle is assigned an aerodynamic diameter that ...

1

The friction between a solid and liquid is a function of viscosity. The best way to answer this is with a model setup called Couette flow where a fluid sandwiched between two plates is sped up by the movement of the top plate: Image source: University of Virginia, Physics 152 taught by Michael Fowler The friction force $F$ that the fluid exerts on ...

1

Basicaly, atmospheric wind are created from pressure differences from one area respective to another, so air molecule are pushed toward the lower pressure zone. This air molecules movement is called "wind". One application of this simple principle can be described by the so called Venturi effect derived from Berbouilli's equation  P*_\text {1} + ...

1

When a region heats, and another region somewhere cools it creates a difference in pressure. Hot air rises, and it then goes towards the region of low pressure to equal the pressure at both regions. Now, why does the air rise? I mean, why hot air rises. A simple explanation is: Hot air is less dense and experiences a buoyant force, just like a bubble of ...

1

Pneumatic circuit components are close to what you are looking for. You can buy pneumatic components that allow you to do a wide variety of operations online from places like McMaster-Carr, or directly from the manufacturer, e.g., from Clippard. I'm not so sure there are too many off-the-shelf components that are direct analogues to a particular electrical ...

1

You are not missing anything. $P = \frac{F}{A}$ so then your second equation becomes: $P = \mu \frac{u}{y}$ For the actual pressure: $P_a = \mu_a \frac{u_a}{y_a}$ Since $u_a = u_0$ because you require it to and $y_a = y_0$ because the boundary separation doesn't change, when you divide $P_a$ by $P_0$: $\frac{P_a}{P_0} = \frac{\mu_a}{\mu_0}$ and as ...

Only top voted, non community-wiki answers of a minimum length are eligible