# Tag Info

Accepted

### If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?

Adding another perspective to the existing answers: In your usual diving scenario, water is not confined to the points in space it occupied before, while a slab of ground is – on account of water ...
• 4,995
Accepted

### Why do clouds have well-defined boundaries?

Clouds are fuzzier than they look. Clouds get their white colour from Mie scattering of light from water droplets of size comparable to the wavelength of light. But for smaller droplets Rayleigh ...
• 27.9k
Accepted

### Why doesn't oil produce sound when poured?

The noise is generated by turbulent flow. Turbulence in the flow generates turbulence in the air at the interface between the air and the liquid surface, and that turbulence in the air is what we hear ...
• 334k

### If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?

In simple terms, water (or any fluid) will move out of the way; concrete won’t (unless it is hit very hard). The important properties are viscosity and elasticity rather than compressibility.
• 35.5k
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### When I drink with a straw, is the atmospheric pressure pushing the water or me sucking it?

Those are just different ways to name the same thing. Ultimately it is atmospheric pressure that pushes the liquid up the straw but normally the atmosphere wouldn't do that: the reason the water moves ...

### Material which becomes less dense as pressure is applied

Becoming less dense with pressure is an energy-producing quality; mass being conserved, volume must increase as pressure rises for density (mass/volume) to have that response. So, I'd say that ...
• 9,015
Accepted

### What is cold wind?

Technically, it all is the same motion. The difference is magnitude and direction and how you separate out the superposition of them. Temperature is a result of the components of motion (vectors) of ...
• 7,720

### Difficulty for a fireman to hold a hose which ejects large amounts of water at a high velocity

You are right, somewhat. For a straight hose, the naive argument is that since the hose pushes the water forwards, the water must push back on the hose by Newton’s third law, resulting in a backwards ...
• 812

### Why doesn't oil produce sound when poured?

Actually, two questions: The existence of the sound: The most common source of the flowing liquid sound is air bubbles popping. Surface waves (both over the open surface and inside the bubbles ...
• 6,113

### Why do clouds have well-defined boundaries?

I just want to expand on Anders’s otherwise great answer, because I think based on the text of the question this paragraph is more what you're asking for: There are doubtless other forces keeping ...
• 35.1k
Accepted

### Why do all fighter jets and aerobatic airplanes have flat wings?

The "equal-time fallacy" is alive and kicking, as shown in your question and the other answers. Look, here is the best explanation I've seen about how wings work and airplanes fly: https://...
• 16.6k
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### How does a river freeze when the water keeps moving?

You know that ice is less dense than water. Then, water that freezes will stay at the surface. Also, take in mind that water will only freeze on the surface. Then as you said, any ice that forms will ...

### Why doesn't a hot air balloon burst even if we keep heating the air in balloon for a long time , shouldn't the air pressure become so high to resist?

Since the balloon isn't sealed and the bottom has an opening, air will escape through this opening thus preventing the point where the pressure inside the balloon is so high that it would explode.
• 25.6k

### How does a river freeze when the water keeps moving?

The river does not freeze in its moving parts. It begins to freeze from the banks in places where the water is motionless or nearly so. Then the frozen area gradually grows towards the center of the ...
• 4,364
Accepted

### How can a Kestrel hover in the wind?

A free-body diagram for a fixed-wing airfoil takes into account four interactions: weight, thrust, lift, and drag. For an unpowered airfoil, the thrust is zero. [source] These are approximately ...
• 74k

### Direction of water through a pipe

You could probably use ultrasound, as is done for measuring the speed of blood in arteries. I say 'probably' because there mustn't be too large a discrepancy between acoustic impedances of the pipe ...
• 28.3k

### If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?

So why isn't the impact of diving into water equivalent to that of diving on hard concrete? Water is rather incompressible, but it is not very hard. It deforms rapidly under shear stress, unlike ...
• 68.7k

### Direction of water through a pipe

A standard technique (https://en.wikipedia.org/wiki/Ultrasonic_flow_meter): Place two ultrasound transducers along the pipe (a few centimetres is OK) and measure the propagation time of signals in ...
• 3,658

### Why do all fighter jets and aerobatic airplanes have flat wings?

Is there any particular reason(s)? There are actually two reasons. Fighter jets are designed to fly at supersonic speed. Airfoil camber (that is the proper name you were looking for) helps at ...
• 2,396

### Material which becomes less dense as pressure is applied

It is impossible to have a homogeneous thermodynamically stable substance in which pressure increases along with the volume. If this happens, then the material can lower its energy by shrinking one ...
• 2,477

### In zero $g$ would someone inside a water bubble be unable to swim and drown?

The physics in that film isn't very realistic in parts. She could swim out of the bubble, although the couldn't just float to the 'top'. One major flaw in that scene is that the bubble would form at ...
• 13.4k

### When I drink with a straw, is the atmospheric pressure pushing the water or me sucking it?

It is the atmospheric pressure pushing it. In a vacuum, if you tried sucking on a straw nothing would happen (you wouldn't even be able to suck). Or if you sealed a rigid container so the only opening ...
• 7,720

### Why doesn't a hot air balloon burst even if we keep heating the air in balloon for a long time , shouldn't the air pressure become so high to resist?

Hot air balloons take advantage of the density of hot air, like you correctly mention in your post. Pressure won't cause the balloon to suddenly burst given that the hot air balloon has an exit ...
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Temperature is related to motion, yes. And wind is motion. But numbers matter. A particle with temperature $T$ (in absolute units, like Kelvin) will typically have a kinetic energy of approximately $... • 74k 14 votes ### Does it make sense to model the kinetic energy as$T=\frac{1}{2}mv\,|v|$instead of$T=\frac{1}{2}mv^2$? No, because kinetic energy is not a vector, so we cannot have it be proportional to a vector. Kinetic energy is a scalar quantity, and so$T=\frac12m\mathbf v\cdot|v|$cannot be used. The drag force ... • 53.8k 13 votes ### Am I making a fatal error with this simplification? That's not right, unfortunately. The principle governing this situation is the continuity equation, which says that the total flow rate past any given point is constant. Since the flow rate is given ... • 53.7k 13 votes Accepted ### Momentum of a gas expanding into a vacuum chamber in outer space At any moment there will be particles moving in each direction, bouncing on the walls and changing direction. The pressure on the walls of the chamber is due to this bouncing. Those who go through the ... • 3,656 13 votes ### Does a tower bell ringing prevent thunderstorms? There is enormous energy in a storm, which will be randomly oriented with respect to a bell tower. The energy in the bell's sound will be radially falling with$1/r^2\$ and is not directional. The ...
• 223k

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