Questions tagged [fluid-statics]

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109
votes
8answers
44k views

Why does a helium filled balloon move forward in a car when the car is accelerating?

I noticed that when I had a helium filled, latex balloon inside of my car, it moved forward in the cabin as I accelerated forward. The faster I accelerated forward, the faster the balloon went from ...
67
votes
3answers
6k views

Why is dry soil hydrophobic? Bad gardener paradox

When I forget to water my plants, and their soil becomes very dry, during the next watering I can see that the soil becomes hydrophobic. I can even see pockets of air between the repelled blob of ...
58
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8answers
12k views

Given that ice is less dense than water, why doesn't it sit completely atop water (rather than slightly submerged)?

E.g. If we had a jar of marbles or something else of different densities and shook it, the most dense ones would go to the bottom and the less dense ones to the top. (Image Source) If I put a cube of ...
56
votes
7answers
8k views

Is the weight of the aircraft flying in the sky transferred to the ground?

Is the weight of the aircraft flying in the sky transferred to the ground? Is the weight of people swimming in the pool also transferred to the ground? How can we prove it?
53
votes
4answers
7k views

Why does a cup with 100 g water float when placed on another cup with 50 g of water?

Imagine we have cup A with 50 g of water and cup B (smaller in width than A) with 100 g of water. Now put cup B into cup A. If the width of both cups are of comparable size then the cup with ...
47
votes
3answers
11k views

Why doesn't this perpetual motion machine using the buoyant force work?

I realize this isn't possible, but I can't see why not, especially if you change the model a little bit so that the balls simply travel through a tube of water on the way up, rather than exactly this ...
43
votes
4answers
21k views

Does the sea level increase if an iceberg melts?

It was claimed that if an iceberg melts in the ocean, the sea level won't change as the ice displaces as much water as there will be melted water. The other claim was that the sea level should rise ...
41
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3answers
7k views

Can ants walk on liquid mercury? [closed]

I'm not a physicist, but I was wondering: Can ants walk on dense liquid like mercury and why?
38
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8answers
6k views

Can a battleship float in a tiny amount of water? [duplicate]

Given a battleship, suppose we construct a tub with exactly the same shape as the hull of the battleship, but 3 cm larger. We fill the tub with just enough water to equal the volume of space between ...
35
votes
4answers
6k views

What is the energy source if a tall water tank is used to transfer floating objects upwards instead of cables with motors?

Suppose I want to transport some logs from the ground to the roof of a tower. Originally I can use a lift, or some cables, or even move the logs upwards manually; then the energy is converted to the ...
32
votes
3answers
3k views

What really is a fluid?

My question may be pretty basic, but I feel it is important to ask this as I've gone through several texts and none offer me the clarity I seek. The question is: What is a fluid? What is flow? If we ...
29
votes
5answers
4k views

Can a submerged object in a free falling fluid sink faster than falling?

I've been wondering about how sunken objects would behave if you could instantaneously flip their container with water. Like if you had a bucket filled with normal tap water and you dropped a ball in ...
26
votes
4answers
7k views

Why does a cork float to the side of a glass?

What is this physical phenomenon and when it's happen ? Please take a look at this picture: Notice: This is a cork ball attracted to sides of a glass of water
24
votes
9answers
23k views

Will a bathtub full of water weigh more if I add something that floats in the water?

Let's say I have a bucket or something bigger, like a bathtub full of water. I weigh that bath and get some value, $x$. Then I add a small boat made of wood to that bathtub that doesn't touch the ...
24
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13answers
16k views

Would a gas “weigh” less than a liquid if they have the same mass?

Thought experiment: I acquired two boxes of the same dimensions and same weight. One box contains $1\ \mathrm{kg}$ of water at room temperature while the other box has $1\ \mathrm{kg}$ of water, but ...
24
votes
4answers
13k views

The Density of Clouds

Clouds are made up of tiny water or ice droplets, depending on temperature. This implies that cloud density is greater than that of dry air. Why don't clouds sink through their surrounding atmosphere ...
20
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4answers
1k views

Which weighs more in atmosphere, $1\,{\rm kg}$ of steel or $1\,{\rm kg}$ of feathers?

I'm having a discussion at the moment regarding the mass of $1\,{\rm kg}$ of feathers and $1\,{\rm kg}$ of steel. The person I'm arguing with states that $1\,{\rm kg}$ of feathers will be lighter ...
17
votes
9answers
4k views

Why does atmospheric pressure act on us?

I have a bit of misconception about weight which I want to clarify. The air pressure is explained as the weight of the air column above our head acting per unit area. But since air is not continuous ...
17
votes
5answers
44k views

How far can water rise above the edge of a glass?

When you fill a glass with water, water forms a concave meniscus with constant contact angle $\theta$ (typically $\theta=20^\circ$ for tap water): Once you reach the top of the glass, the water-air ...
16
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4answers
3k views

Why do we need to apply more force to push a submerged object deeper?

If the buoyant force acting on a body submerged in a liquid,say water, does not depend on depth, why does it become increasingly difficult to push an object deeper and deeper. I know that the buoyant ...
15
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9answers
5k views

How to emulate 40ft (12 m) of water? [closed]

I am part of an underwater robotics team, and our next year's challenge requires us to compete at a depth of 40ft. We don't have any pools nearby to test in, so I want to emulate this 40ft of pressure ...
15
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5answers
18k views

If force is a vector, then why is pressure a scalar? [duplicate]

By definition pressure is the perpendicular force applied to a unit area. So it has a direction which is perpendicular to the area. So it should be a vector. But I did sone googling and found out that ...
15
votes
1answer
323 views

What makes a bottle full of water soo bouncy?

Lets take a small coke bottle(plastic one). Now fill the bottle entirely with water not even letting a small air bubble, however we cannot stop forming a tiny bubble. Now this water bottle when ...
14
votes
7answers
6k views

How to keep a helium balloon between 1 to 5 meters above ground? (without it being tied)

I understand that helium balloons rise because their density is less than air, so they can rise up to a point where the air surrounding it has the same weight as the balloon. I was thinking to fill it ...
13
votes
8answers
2k views

Can a ship float in a (big) bathtub?

I am confused. Some sources say it is possible at least theoretically ( http://www.wiskit.com/marilyn/battleship.jpeg ) and some say it is not true ( http://blog.knowinghumans.net/2012/09/a-...
13
votes
2answers
25k views

Is there a way to fill Tank 2 from Tank 1 through Gravity alone?

I am a newbie in water system design but I am currently faced with the exact situation below on my land, and I need to know whether gravity alone is sufficient in order to fill Tank 2 from Tank 1, as ...
13
votes
4answers
316 views

How quickly should a fluid come to hydrostatic equilibrium?

Let's say I'm holding a one-liter water bottle, full of water, which I then drop. Before dropping the water bottle, the equilibrium is for there to be a pressure gradient in the water canceling the ...
12
votes
10answers
5k views

Why does a helium balloon rise? [duplicate]

This may be silly question, but why does a helium ballon rise? I know it rises because helium is less dense than air. But what about the material of the ballon. It is made up of rubber/latex which is ...
12
votes
4answers
2k views

A water pipe from sea level to beyond the atmosphere

If a pipe extended from just above the ocean floor to outside the atmosphere, would water be sucked up it by the vacuum beyond the atmosphere? If a hole was made in the pipe, above sea level, how ...
12
votes
3answers
22k views

Hydrostatic pressure - doesn't density vary with depth?

Our class is learning about hydrostatic water pressure and we have been told that we can calculate the force of the liquid on an object at any depth using "the density x 9.8 x the depth". However, as ...
11
votes
3answers
932 views

Fluid Pressure: The Collision Model - Approximation or Truth?

Microscopically, the pressure exerted by a fluid on a surface in contact with it is caused by collisions of molecules of fluid with the surface. As a result of a collision, the component of a molecule'...
11
votes
2answers
5k views

How is the water meniscus at the edge of a capillary tube?

Suppose we have a capillary tube in which water can rise to a height of x cm. If we dip the tube such that the height above the surface is less than x, then how will the water meniscus be at the edge ...
10
votes
3answers
6k views

What is the meniscus shape?

What is the shape of a meniscus? I suppose that the problem is very complex, but is the solution known at least for a liquid that wets the wall in a big vessel? (exponential, maybe?)
10
votes
2answers
503 views

Name of device with air capsule inside water

Sorry for the title being so vague. I'm looking for the name of something in physics to do with pressure. It is a bottle of water with a small capsule with air inside it. When the bottle is squeezed, ...
10
votes
4answers
40k views

Why does the water level equalize in a series of tubes?

Say I have a series of tubes (not the internet) looking like this, where w represents water: ...
10
votes
1answer
20k views

Question on the hydrostatic paradox

The hydrostatic paradox, states that the height of the water in any container is independent of the shape of the container. This does make sense since it is only the height of the fluid ...
9
votes
4answers
3k views

Why does pressure increase with water depth?

Let's say I took a tiny metal sphere that when put under water has the surface area to allow, at any point in time, to be surrounded by up to 1000 water molecules. Now lets say we put this sphere ...
9
votes
2answers
1k views

Why do bubbles initially stick to the side or the bottom of a glass while boiling?

When boiling water, I've noticed that bubbles will appear to grow at the bottom of the glass for a period of time and then rise. I've read that bubbles form from nucleation centers but I don't know ...
9
votes
2answers
835 views

Does capillary rise violate hydrostatic paradox?

If $p$ is a pressure and $p_A = p_{\text{atm}} + hdg,\,$ $p_B = p_{\text{atm}}$, is hydrostatic paradox violated, shouldn't $p_A=p_B$?
9
votes
4answers
684 views

When is a water level no level?

We are all familiar with the ancient but much venerated and still in use today water level. It's simple, right? You fill a tube with water, make sure that all of the bubbles are excluded, and then ...
9
votes
2answers
1k views

At the molecular level, how is the pressure at the bottom of a lake higher than at the top?

Surely the temperature of the molecules is the same throughout the water. Using $p = \rho g h$ seems to assume a constant density as well. But then how is it that the force per unit area on an object ...
8
votes
4answers
6k views

What stops water from a moon pool from filling the inside of a submarine? [duplicate]

In some submarines, divers can exit and enter through a moon pool, an opening in the bottom of the submarine. We can clearly see ocean's water flowing in the moon pool, but it won't come inside. Why ...
8
votes
2answers
743 views

How is it possible for tall trees to pull water to heights more than 10m?

Which force actually drives water so high up, since pure atmospheric pressure will only get you up to about 10 meters if you're using suction and a long straw and yet tallest trees are over 100 meters ...
8
votes
3answers
884 views

Meniscus attached to an inclined plate

To be more specific, suppose a hydrophilic infinite plate is stuck into a semi-infinite region of water, above the water is a semi-infinite region of air, when the plate is stuck into the water ...
7
votes
9answers
45k views

What property of objects allow them to float?

I used to think that the shape of an object determines its ability to float (boat-shaped objects are more likely to float, and spheres tend to sink). But my friend, who is fond of making me look ...
7
votes
3answers
56k views

Can pockets of air exist underwater?

If yes, why don't they fill up with water, and can you breathe the air there? Like, it's not exactly atmosphere there, but an underwater cave with higher ceiling. P.S. Possible that it has a ...
7
votes
4answers
1k views

Why doesn't hydrostatic pressure glue objects to the seafloor?

Consider a cement block of the same density as sea-water resting on the (perfectly polished) seafloor. I also assume the side-faces of the cement block are highly polished, so that no vertical ...
7
votes
3answers
2k views

Validity of the Navier Stokes equations for turbulent flows

The derivation of the Navier-Stokes equation presupposes that the pressure, $p$, and velocity, $u_i$, are infinitely differentiable, so that the forces in each face of the fluid element can be ...
7
votes
2answers
11k views

How do simple bird waterers work?

I made a simple bird waterer made from two 1.25L PET bottles. Let's call these two bottles the reservoir and the tray. Setup The tray is a bottle with the top cut off to leave only the bottom third. ...
7
votes
1answer
657 views

Shape of a water drop

Some years ago (1987 time frame), a mechanical engineering professor asked a question in a graduate level heat transfer class that I have never been able to solve. The questions is: "Given the ...

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