# Questions tagged [fluid-statics]

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### Will air push the water or pass through as bubbles?

This doubt is about a conundrum which is common to multiple scenarios. One such scenario is this: The valve is opened and the high pressure air is pushed towards the water in the channel with the ...
177 views

### Why do these experiments contradict the accepted theory about puddles? (+ my work)

A few months ago, I tried to tackle the problem of finding the shape, the height and other properties of a puddle (whenever a few mL of some liquid are dropped in a "uniform way" so that the puddle ...
3k views

### Inverted U manometer

I have some doubts on inverted U manometer. In the picture there is one U inverted manometer, the space above the liquid in the manometer is filled with air which can be admitted or expelled through ...
468 views

### Does the contact angle of a drop depend theoretically on the size of the drop?

According to Wikipedia, contact angle depends on the hydrophobicity of the surface and on thermodynamic equilibrium. But how does it in theory depend on the drop size? In other words, how would a ...
293 views

### The highest density of water

If temperature is decreased, volume decreases. so density increases. But in case of water, its density increases up to 277 K, then decreases. Why does this happen? Why 277 K is water's highest density?...
101 views

### Weird and unintuitive results from thermodynamics/hydrostatics for an isothermal atmosphere and possible explanation(?)

It is known from hydrostatics that for a fluid in equilibrium in a gravitational field, $$\frac{dP}{dz} = -ρg$$ Let us from now on suppose the atmosphere is isothermal and has temperature $T$. We ...
630 views

### How does the vertical distance beneath the surface of a fluid create pressure? (thought problem)

Specifically, I am trying to wrap my head around this scenario but can't do so solely by thinking about the weight of the fluid. Imagine a perfectly still lake. If you dive into the lake, the ...
73 views

### Could a hydrophobic surface increase a liquid's resistance to compression/displacement?

Imagine a quantity of an aqueous (yet slightly viscous) solution is resting on a hydrophobic surface with a contact angle around 100°. A downward force is then applied as a (repellant) surface is ...
132 views

### Is it possible to have a line rather than a point where the three states of a substance can exist?

Most of us are familiar with state diagrams that define which of the three states a substance will take given the pressure and temperature. And that some substances, such as water for example, exhibit ...
509 views

### Explaining the shape of a raindrop and a drop of mercury

I saw on wikipedia that the shape of a raindrop is explained by using Laplace pressure. But why? Since the drop is in motion, we shouldn't be able to use an hydrostatic law, am I wrong? The ...
27 views

### Why does fluid under pressure not flow into closed-off tube?

While doing an experiment, I saw an interesting situation. The set-up is shown: I plugged the end of the tube with my finger, and slowly started pouring in the liquid. However, as I poured, no liquid ...
70 views

### Knudsen Number and pressure

When computing the Knudsen number to know if the continuum hypothesis can be applied as $\frac{k_B T}{p \sqrt{2} \pi d^2 L}$, do we use the static or total pressure of the free stream? My object is ...
78 views

### Does tilting a liquid develop shear stress on its surfaces?

Please help me these confusions I have. If I tilt a cuboidal container containing an ideal fluid (non viscous liquid), does a shear stress develop on the liquid due to some shearing force acting ...
73 views

### Integrating to find net pressure and force on a sphere by a fluid

How would I go about solving for the net force on a sphere exerted by air pressure as a result of pressure being dependent only on the vertical position of the ball? When the sphere it is placed in a ...
183 views

### Can someone explain why there is water in this vacuum chamber?

I work at a water treatment plant that uses a vacuum system to pull the water into the filters and backwash the filters. In the middle of the vacuum system there is the vacuum chamber. What I don't ...
105 views

### If I replace water molecules with microscopic solid spheres, will they still reach the same height in communicating vessels?

I'm having an hard time understanding why liquids take the shape of the container or why they reach the same height in communicating vessels. I'm wondering if I try to simulate their behaviour in ...
94 views

### What is the physics behind lifting a submerged bucket out of water?

When you submerge a cylindrical bucket into the water and let all the air out, trying to lift the bucket back out is particularly difficult if we keep it vertical to the surface. In order for the ...
250 views

### What is the maximum gap that water won't pass through for hydrophobic surfaces?

If I had 2 hydrophobic surfaces (in this particular case, 2 sheets of coated glass), placed edge-to-edge, what is the maximum gap between the sheets where water would not drip between the sheets? Is ...
447 views

### Trouble deriving hydrostatic pressure equation

I'm having a difficulty deriving hydrostatic pressure. To be specific, I don't understand this image from Wikipedia: How does the fluid below act on the below surface? Is it the normal force? Or ...
2k views

### Under what conditions does Jurin's law work? (and what are its physical basis?)

I searched answer in wikipedia (and other sites) but I found disjointed and contradictory informations. In Jurin's law article they say that the law works if the radius of the tube is smaller than ...
147 views

### Given a wall on planet earth with a constant height is there a point at which a physicist needs to worry about the volume of water behind the wall?

I have a quick question, on the media outlet "Sploid" a post recently came out about this interesting flood wall in Austria that keeps water from destroying a community. The wall seems to contain a ...
106 views

### Stat mech explanation for separation of one liquid from another in gravity?

If one mixes two distinct ideal gases above the Earth's surface, one with a higher molecular mass than the other, then at equilibrium, their number density gradients will be such that at low heights, ...
91 views

### Reversibility of compressed air sinking in water

Consider an apparatus in which air is trapped in a small insulated container with an open bottom, and weighted with weights so that the air is slightly above neutral buoyancy. This apparatus is ...
106 views

### Shape of water on top of a thin sheet of stretched plastic

Consider a thin sheet of plastic (a square sheet for simplicity) that is stretched taught in a plane parallel to the ground. If a volume of water is then placed on top of the thin plastic sheet, then ...
29 views

### Pressure at depth $h$ of a spinning bucket at top of swing

This was a question posed to me some time ago and it has haunted me ever since. Hints and suggestions would be awesome. Say someone is spinning a bucket of water in a vertical circle of radius $r$ ...
46 views

### derive the density of water from its molecular structure?

Is it possible to derive the density of water by looking at its molecular structure? Is there a theoretical formula that describe the dependence with temperature? Thanks for the help!
19 views

### Liquid surface tension and molecular motion inside

In some of the descriptions I have found about surface tension, while they explain the phenomenon for liquid in an open container they use the following arguments: 1) A molecule at the surface feels ...
33 views

### Is it possible to find the exact value of the dynamic viscosity of a fluid by applying Stokes' law?

I am a high school student who is investigating the dynamic viscosity of a fluid. Typically, high school physics model the dropping of a spherical object vertically into a viscous fluid with the ...
41 views

### Resistance between steel wires in water delayed response

I am trying to make a wire wave gauge by measuring the resistance between two stainless steel wires. Theoretically, the output voltage reading should change the deeper the wires are in the water. If ...
58 views

### Will mass flow rate be the same in these tank-pipe-ideal fluid systems?

The equation of continuity for pipes $$\frac{\Delta m_{1}}{\Delta t} =\frac{\Delta m_{2}}{\Delta t}$$ states that mass flow rate inside a pipe doesn't depend on pipe diameter. I'm confused on ...
34 views

### Temperature relation to Archimedes principle

I have a probably very simple problem which I cant solve. I am weighing animals in seawater by placing them on a small PVC arm that is fixed on a balance and reaches into a beaker of seawater. On the ...
306 views

### Easy derivation of Pascal's Law

I would like to know a simple derivation of Pascal's Law, i.e. why the variation of pressure at a given point of an incompressible fluid also happens to all others.
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### What is the minimum amount of matter do I need to call it “bulk” matter?

I had been reading about bulk properties, but was confused what should the minimum amount of matter be for these properties to hold. For eg: Intensive thermodynamic variables will not remain ...
68 views

### Buoyancy question

Suppose we have a ball of volume V and a block of the same volume V whith same density. We submerge the ball into some kind of liquid so that $\frac{1}{2}V$ is submerged into the liquid. We do the ...
99 views

### Fluid statics - buoyancy force problem

The pictures above show the question and soution. The part I don't understand is why the air pressure is acting upwards against the plate instead of downwards on the water. Could someone explain this ...
47 views

### Derive the virial theorem from hydrostatic equikibrium

Consider the equation of hydrostatic equilibirium as follows as applied to a stellar object: $$\frac{dP}{dr } = \frac{-GM\rho}{r^2}$$ I know it is intimately related to the virial theorem but I ...
33 views

### How do fluids behave in space with regards to bouyancy and density?

Will a fluid loose it's bouyancy in zero-gravity. What would happen if an object is immersed in a liquid in space (if the liquid is still very much together and not spilling as big bubbles or droplets)...
37 views

### Effect of temperature on specific weight

How much of an impact has a temperature difference when the specific weight of an object in water is measured, e.g. the weight of a mussel in seawater. Would the two scenarios yield the same specific ...
58 views

### Which of the two events will occur first

Consider a bottle partially filled with water and it is sealed from everywhere so that no air can enter or exit from bottle.Now make a small hole at the bottom of the bottle and hang it vertically so ...
20 views

### Will hydraulics lever ever stop if we exert some force to it?

Suppose there is a simple hydraulics lever (like the one often shown when discussing about Pascal's Principle). If we exert some force into one of the piston without having any weight on the other ...
40 views

### How much energy is required to heat up a hot air balloon to make it neutrally buoyant?

To figure this out, I've tried to solve for $\Delta T$ for $PV=n_{cold}RT_{cold}$ and $PV=n_{warm}RT_{warm}$ which assumes that $PV$ is equal. This gave me the equation: \Delta T = \frac{T_{cold}^2 ...
32 views

### What is the difference of pressure of a gas in a closed container at a given distance from the Earth?

If I have a closed container of a given height $(h)$, located at a given distance from the centre of the Earth $(r_o)$, what would the pressure difference be at the top of the container vs the bottom? ...
142 views

### Movement of a helium filled vs lower-density-gas filled balloon inside an accelerated car

It's well known that a helium balloon inside of a car moves forward when the car accelerates, and backward when it slows down. What would happen, though, if a lower density gas was used instead of ...
127 views

### When exactly can the concept of stagnation pressure be applied?

Although I've attached a homework problem here, I'm only using it for reference for a conceptual question on stagnation pressure. From my understanding, in the photo attached above, the pressure at ...
26 views

### How to calculate the buoyance's work when releasing bar above water

The exercise is to calculate the buoyance's work when releasing a bar of mass $m$, lenght $L$ and cross section area $S$ just above water. My problem is that I am assuming my referencial on the water ...
560 views

### How do speed and density change in a turbulent flow?

Suppose a fluid passes from having laminar flow, to having a turbulent flow (like when passing after an object). How do fluid speed and fluid density change after that?
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### What's measured by weighting a fluid?

Suppose you have a glass of water at room temperature. Now suppose you put the glass of water on a weighting scale. What the quantity actually measures ? I have read that in fluids are basically lots ...
96 views

### Where does the buoyant force come from?

If I place a cube in water, the force at the top of the cube, F1 will be Ahρwg. Where, A = cross-sectional area h = height at the top ρw = density of water g = acceleration due to gravity All this ...