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Why does air get trapped below water pouring into a glass?

You're seeing a vortex. It's a clump of bubbles in the water flow, just as a smoke ring is a cluster of smoke particles in air flow. The circulation is a trap for its contents.
Whit3rd's user avatar
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2 votes
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Interesting Aerofoil Logical Fallacy

There is no possibility for perpetual motion, but you have to be careful about how you treat the drag. In your diagrams, the air particles all flow with uniform velocity, which suggests you are ...
Roger Yang's user avatar
3 votes

Are there any substances that exist as plasmas or supercritical fluids at STP?

Plasma: To maintain a substance in the plasma phase the energy associated with the temperature, $k_\text{B}T$, has to be near to, or greater than, the ionization energy. At room temperature, $k_\text{...
Jos Bergervoet's user avatar
17 votes

Open or closed windows in a tornado?

Why did people open their windows before a tornado hit their home? There is a significant pressure drop inside a tornado - It can be about 10% lower than ambient. So, when a tornado passes over and ...
Ritzthephysibeast's user avatar
0 votes

Why don't disturbances/wind move at the speed of sound in air?

Consider two large pressure vessels connected by a pipe that has a valve. When we open the valve, the air flow velocity (wind speed) in the pipe is a function of the pressure difference between the ...
KDP's user avatar
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0 votes

Why don't disturbances/wind move at the speed of sound in air?

Sound moves faster because it consists of energy that's skipping through the air molecules. The air molecules themselves don't move except a short little bit of vibration. When the wind blows, mass as ...
Miss Understands's user avatar
0 votes

Poking the water droplet developed at the spout of a leaky tap

When you poke, the surface of the water is no longer spherical, so the water begins to flow continuously. And when it drips, the water can return to a spherical shape, so it drips again in the form of ...
enbin's user avatar
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0 votes

Why does the pet's water bowl overflow?

The reason may be simple, because your container not only has angular movement, initially T-2 tilts forward, then T-1 tilts backward, but also horizontal movement from right to left during this ...
enbin's user avatar
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-2 votes

Why does the pet's water bowl overflow?

If you check the tide chart for your local area you will find that the bowl overflows only during the incoming tide and is directly proportionate to what stage the tide is at when you set the bowl ...
StillWill's user avatar
1 vote

Can siphons improve the efficiency of hydroelectric dams?

No, you cannot gain any efficiency. Remember the bernoulli equation: $P_1 + \frac12\rho v_1^2 + \rho gh_1 = P_2 + \frac12\rho v_2^2 + \rho gh_2$ If you have water at rest at $P_{atm}$ on the top, and ...
Pato Galmarini's user avatar
1 vote

Why will low pressure pulling higher pressure air towards it make "thrust"?

Low pressure air means a lack of air molecules. Normal pressure air means a normal amount of air molecules. Low pressure means that the combination of the number of molecules and the speed of the ...
BowlOfRed's user avatar
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12 votes

Why does the pet's water bowl overflow?

The two horizontal sloshing modes could have slightly different frequencies (e.g. if the rotational symmetry is not perfect). If you then start with the combination of both, but out of phase, which is ...
Jos Bergervoet's user avatar
18 votes

Why does the pet's water bowl overflow?

If the mat is thick and soft enough to be compliant, then it will couple with the moving mass of the dish and water and allow the dish to tip back and forth, causing the spillage. This can be directly ...
niels nielsen's user avatar
40 votes
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Why does the pet's water bowl overflow?

A guess. Perhaps there are multiple modes present. It takes a while for them to have an interference maximum at the edge of the bowl.
mmesser314's user avatar
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1 vote

Why there is a delay before canned fruits float to the top?

Just a guess. If berries contain air, the air expands as the berry gets hot. This increases the volume of the berry, but not its weight. The berry floats when the weight of the berry is less than the ...
mmesser314's user avatar
  • 41.1k
1 vote

Why there is a delay before canned fruits float to the top?

All fruits have hair, and a water resistant wax finish which lets rain drops roll off. It also provides a surface film of air bubbles if the fruit is immersed in water. Most easily observable with ...
Roland F's user avatar
1 vote

Incompressible flow condition intuition

As you have said it is derived from the continuity equation (which is one of Euler fluid equations) $$\frac{\partial \rho}{\partial t}+\nabla\cdot(\rho \mathbf u)=0$$ if the density of the fluid $\rho$...
Mauricio's user avatar
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3 votes
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Incompressible flow condition intuition

An intuitive explanation comes from the meaning of the word divergence itself. If the divergence of the velocity is zero, it means that there is no (net) inflow or outflow of fluid from a given volume....
CompassBearer's user avatar
-1 votes

Incompressible flow condition intuition

You ask"How is there a relation between the density of the fluid and its speed"? There is none $\nabla u=0$ says only that the velocity does not change for example in x direction .
trula's user avatar
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1 vote

Can one make a jet of air bounce without spreading?

The water in the video exists as droplets that are held together by their surface tension. After a droplet hits the hydrophobic surface and is flattened out by the impact, it's the surface tension, ...
Solomon Slow's user avatar
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1 vote

Is there a way to achieve a long jet of air?

You'd have to find a way to maintain a low Reynolds number ($\text{Re}$) as the jet progresses. $$\text{Re} = \frac{\rho u L} {\mu}$$ $\rho$ being density, $u$ being fluid speed, $L$ is the ...
mike1994's user avatar
  • 978
-1 votes
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Is there any phenomenon where opposite reaction (Newton's 3rd Law) is not fulfilled?

While Newton's Third Law of Motion is a fundamental principle in physics, there are some situations where it may appear to be violated or where the reaction force is not immediately apparent. However, ...
Ansh Tandon's user avatar
0 votes

Is there any phenomenon where opposite reaction (Newton's 3rd Law) is not fulfilled?

All forces occur in pairs. There are no isolated forces. See; http://hyperphysics.phy-astr.gsu.edu/hbase/Newt.html As stated in the link, there are no known exceptions. Hope this helps
Bob D's user avatar
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0 votes

Is the pressure at the top of the wings on Mars also lower than the local atmospheric pressure on Mars?

It doesn't matter whether a wing is operating on Earth or Mars, or what the atmospheric pressure happens to be. The lift on a wing depends on the average pressure on its upper surface being lower than ...
D. Halsey's user avatar
  • 2,170
1 vote

Is the pressure at the top of the wings on Mars also lower than the local atmospheric pressure on Mars?

I'm going to go out on a limb and saying "Yes". There is only one plane on Mars, and it is rotary wing, so you can read about that (https://en.wikipedia.org/wiki/Ingenuity_(helicopter)). ...
JEB's user avatar
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1 vote
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Is Steady Flow necessarily Streamlined Flow? or Vice Versa?

Here are some definitions: A streamline is a path (not necessarily constant in time) such that the tangent at each point of the streamline denotes the velocity of the particle at that point. A ...
CompassBearer's user avatar
1 vote

Is Steady Flow necessarily Streamlined Flow? or Vice Versa?

Is steady flow necessarily streamlined flow ? Yes. Steady flow is defined as flow that is not a function of time. In a steady flow, the streamlines (curves that are tangent to the velocity vector ...
gandalf61's user avatar
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1 vote

How exactly does a seashell make the humming sound?

The question to the answer is: The seashell does not make noise. The answer to the question: What is the source of what you hear if you close your one ear with a cup and close the other ear competely ...
Roland F's user avatar
0 votes

Why doesn't a hydraulic lever violate conservation of energy?

But like using the kinematics formula we get equal displacement. Like we know that mass of the lever increases proportionately with Area so F1/m1 = F2/m2 Now, S = ut + 1/2at^2 S = 1/2at^2 S1 = 1/2F1/...
Deeptajit Nag Chowdhury VIII I's user avatar
1 vote

Why the density have anything to do with initiation of subduction?

First velocities are very low and masses are very large. Momentum ($mv$) is large, but kinetic energy ($mv^2$) is small. So when two plates crash into each other, it is a fender bender. The kinetic ...
mmesser314's user avatar
  • 41.1k
1 vote

How exactly does a seashell make the humming sound?

Neither of the three explanations are true. Actually, the seashell amplifies and reflects the sound of your blood flowing through the blood vessels in your ear! That is what you hear as humming.
user402514's user avatar
0 votes

Why rain drops are spherical while water droplets on a glass surface are flat?

To add: If the surface upon which the droplet of water lands is tilted, then gravity will try to pull the droplet in the down direction. The leading contact angle of the sliding droplet will be large ...
niels nielsen's user avatar
1 vote

Which is the cause and which one is the effect between the shear stress and velocity gradient in a fluid flow?

The primary reason is the relative motion of the walls and fluid viscosity. If you work with a viscous fluid, both no-slip condition at solid wall is satisfied and viscosity introduces shear stresses ...
basics's user avatar
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0 votes
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Energy of a flowing fluid

Shouldn't the net force acting on the fluid element by the surrounding fluid, as it is pushed through a distance $dx$, be equal to the changes in the kinetic and potential energies of the fluid ...
Ján Lalinský's user avatar
0 votes

Energy of a flowing fluid

I hope you can find the answer to all your questions in the balance of kinetic energy. Differential form. The differential form of this equation is derived from the scalar product of the momentum ...
basics's user avatar
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4 votes
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Why rain drops are spherical while water droplets on a glass surface are flat?

The answer is surface tension. When a droplet falls through the air, you only have one type of boundary that you need to account for (water-air $\sigma_{WA}$). In contrast, for a droplet on a flat ...
Adam's user avatar
  • 187
5 votes

How exactly does a seashell make the humming sound?

The shell with a small opening forms a Helmholtz resonator that is excited by noise from outside. Image from Formulas of Acoustics, F.P. Mechel pp72. The Wikipedia article has relevant equations for a ...
Spehro Pefhany's user avatar
12 votes

How exactly does a seashell make the humming sound?

While inside your house, place a shell or a cup or a drinking glass or even a cupped hand over your ear and you will observe the same phenomena. Assuming there is no significant wind inside your house ...
KDP's user avatar
  • 6,142
8 votes

How exactly does a seashell make the humming sound?

Your third option is the closest to the case. The seashell forms a cavity resonator, which will have some characteristic frequency response determined by things like the shape of the shell etc. ...
Riley Scott Jacob's user avatar
0 votes

External force in the Navier-Stokes momentum equation

Although the question is old, the answer may still be useful to someone else. In my opinion, the pressure imposed by the piston should be included in the equation, not as boundary conditions, rather ...
Carla's user avatar
  • 1
1 vote

Where does the pressure go when the fluid leaved the pipe?

If the pressure at $3$ and $1$ are both atmospheric, the fluid is not going to move. If you want the fluid to move the pressure at $1$ will have to be larger than atmospheric.
mike stone's user avatar
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4 votes
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Why do most fish wiggle their tails side to side, and not up-down?

As suggested in the question, symmetry tells us that the caudal (tail) fin of a balanced, neutrally buoyant, underwater animal or vehicle in an isotropic unbounded aquatic environment should work ...
David Bailey's user avatar
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0 votes

Will the divergent section of the tube generate pressure below atmospheric pressure?

it will not result in a pressure lower than atmospheric pressure Yes, it can ($\sim$ cit.) Example I suspect you've never dealt with an old motor-bike or with a carburetor, that device needed to add ...
basics's user avatar
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