52 votes

Could there theoretically exist a material so light, that it can levitate in the air just due to the in height decreasing air pressure?

A hot air balloon, or a helium-filled balloon floats in air, so either might meet your criteria. If you're looking for a solid material, perhaps a sphere of very sparse aerogel, with its outside ...
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  • 24k
24 votes

Why does fluid pressure decrease as fluid velocity increases according to Bernoulli's principle?

I just can't wrap my head around why pressure decreases as velocity increases This is a classic misunderstanding of Bernoulli's equation. What Bernoulli's equation actually says is that the velocity ...
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  • 53.4k
22 votes
Accepted

What force accelerates a liquid moving in a narrowing pipe?

You are right. From continuity of the incompressible fluid you have $$A_1 v_1 = A_2 v_2.$$ So obviously the velocity is changing. Thus the fluid is accelerated, and therefore there must be a force ...
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21 votes

What is the mechanism that transforms pressure into velocity?

Q: What is the mechanism that transforms pressure into velocity? A: Pushing. Seriously, that's the answer. Bernoulli's equation is usually stated as “pressure drops when velocity increases”, but I ...
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20 votes
Accepted

Water Bottle Rocket Thrust - two calculation methods not matching

The first method is correct. In the second you have assumed that the pressure at the nozzle is still $P$ despite the water exiting with some velocity. i.e. You have neglected the dynamic pressure. ...
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  • 111k
16 votes

Why can we assume that Velocity at the top of large tank is zero?

By conservation of volume, you must have the same flow rate at the surface as the flow rate through the tube/exit. If the tank is at least 10 times the diameter then it has at least 100 times the area....
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  • 4,382
14 votes
Accepted

What is Pressure Energy?

The technical answer First I need to explain entropy. Suppose you have any system which you can only see at a certain granularity: we say that you see its "macrostate" but this could be any set of "...
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  • 35k
14 votes

What is the cause and what is the effect in the Bernoulli effect?

The answer is that the pressure change is the cause and the flow velocity is the effect. Note that the Bernoulli theorem applies to the changes of $P$, $\rho$, $u$, $\phi$ and $v$ along a streamline. ...
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12 votes

Why does Bernoulli's principle work?

Seems to me that the "meat" of your question is not about Bernoulli's principle, but rather a related concept called "conservation of mass" . I'll address that first, and get to Bernoulli's principle ...
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12 votes
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Intuition behind Bernoulli's equation

Usually, Bernoulli's principle is derived from the conservation law of energy by realising that pressure $P$ is not only a force per area, $F/A$, but also an energy per volume, $\frac{E}{V}=\frac{F\...
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  • 7,746
11 votes

Intuition behind Bernoulli's equation

The fluid has to speed up as it enters the narrower region. That means that the bit of fluid just entering the region has to be being pushed from behind. So the pressure behind it must be larger ...
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  • 41.8k
11 votes

Why does fluid pressure decrease as fluid velocity increases according to Bernoulli's principle?

i just can't wrap my head around why pressure decreases as velocity increases.. When velocity increases then you obviously have acceleration. Now what is causing this acceleration? As always (...
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11 votes
Accepted

What is the mechanism between whirlpools repelling and attracting each other?

For liquids that are moving Bernoulli's principle states that $$P+h\rho g+\frac{1}{2}\rho v^2$$ is constant. This means that the pressure exerted by a faster flowing liquid is reduced (for the same ...
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  • 13.4k
10 votes

What is Pressure Energy?

When a fluid is squeezed, as in a cylinder by a piston, work is done on the fluid. This work 1) elevates the pressure (pressure energy), and 2) the temperature (heat energy). (If the cylinder is ...
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9 votes
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Fuel tank draining

Hint: If you divide by $\sqrt {h(t)}$ the equation separates.
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9 votes

Why does moving air have low pressure?

It's all about conservation of momentum, $F=ma$. Fluid can only change velocity by experiencing a force, and the only force it can feel is a pressure difference. So if there's a velocity difference, ...
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9 votes
Accepted

Does continuity equation hold if the flow is accelerated?

In the form you've stated it ($A_1 v_1 = A_2 v_2$), the continuity equation is only holds for incompressible fluids. So what you've found is that the type of accelerated flow you're describing cannot ...
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9 votes

Flow rate of an open tap

The average flow rate must stay the same all the way down, unless water is piling up somewhere. A possible exception is if the fall is so far that some of the water evaporates on the way down. But ...
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  • 71.8k
8 votes
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What exactly is the 'lift' of a sailboat as explained by Bernoulli principle

Bernoulli's Principle is one very small piece of a large mathematical theory that explains lift. Unfortunately, most attempts to explain lift using Bernoulli's principle without the overall ...
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8 votes

What is the mechanism that transforms pressure into velocity?

Normally (excluding nuclear reactions etc.), it is the electromagnetic ("em" from here on) repulsion at the root of all. A push is nothing but electromagnetic repulsion between what is ...
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  • 4,443
8 votes

Water Bottle Rocket Thrust - two calculation methods not matching

Often these rockets are launched off a vertical section of pipe or rod that extends up inside the bottle, acting as a piston until the rocket has moved far enough to clear the end of the pipe. During ...
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  • 8,972
8 votes

What force accelerates a liquid moving in a narrowing pipe?

There is more mass per area behind than ahead of the constriction so since ppressure is force divided by area there develops a pressure difference
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  • 3,525
7 votes
Accepted

Problem understanding basic sail mechanics

Newtonian Aspect So the force generated by the sail will perpendicular to the sail. This is because only the momentum of the air particles perpendicular to the sail changes the component changes the ...
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  • 1,820
7 votes

How much effect does the Bernoulli effect have on lift?

Sometimes you will see statements like 'some of the lift is caused by Bernoulli's principle and some of it is caused by Newton's laws", but this is the wrong way to think about it. The fact is that ...
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7 votes
Accepted

Is Magnus effect a corollary of Bernoulli principle?

This is a most excellent and astute question. Ultimately it comes down to experiment: the model below works pretty well for many fluids. What this must mean therefore is that the loss is small enough ...
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7 votes

Are eductors working on the Bernoulli principle?

What is the reason behind suction? The main reason behind suction in an eductor is due to a special case of Bernoulli principle called the Venturi Effect. When fluid flows through a pipe, it has a ...
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  • 1,235
7 votes

Where does $W_{net} = \Delta K_e + \Delta P_e$ come from?

You need to add surnames to just "work." W-E theorem is $$W_\text{total}=\Delta K_e$$ For conservative forces $$W_\text{cons}=-\Delta P_e $$ Consequently, if you combine both formulae, since ...
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  • 8,019
6 votes
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Pressure $p$ in the Bernoulli equation

Bernouilli is all about conservation of energy. The drop in pressure is necessary so that work can be done on the incompressible fluid - because when it flows faster, it has more kinetic energy and ...
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  • 116k
6 votes

Is a causal relationship implied by Newton's 2nd Law?

Is there any causality implied by Newton's 2nd Law? It depends on what you mean by "causality" and "Newton's 2nd law". I would rather not go into various meanings. Newton's original formulation of ...
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6 votes

Why decrease in velocity will increase pressure?

Pressure is momentum transfer due to molecular collisions once you have subtracted out their average motion. So decrease in pressure due to increase in average speed may be construed as transfer of ...
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