# Tag Info

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

Convection in meteorology is often assumed to be more or less adiabatic, and convection is what creates cumulus clouds. You can imagine clouds as bubbles rising inside a water tank, they also have a ...
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### Do densities of the same object differ in different fluids?

First note that since oil and water have different densities, they will exert different buoyant forces on the cube, since this force is given by $$F_b=\rho Vg$$ where $V$ is volume and $g$ is the ...
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### Do densities of the same object differ in different fluids?

Your mistake is trying to calculate two separate densities and then add them together. The correct approach is Calculate the mass of water displace day by the cube. Calculate the mass of oil ...
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### Why do clouds have well-defined boundaries?

Cumulus clouds are opaque (optically thick). The interior of a large cloud is very dark (as you've experienced if you've flown through one in an airliner in the daytime during ascent or descent). When ...
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Accepted

### Are bacterial flagella the most efficient propulsion system at the microscopic scale?

The most important difference between bacteria and everyday propulsion is size. Bacteria are microscopic. There are two sources of resistance when moving through water. First, flowing water generates ...
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Accepted

### Why do superfluids even climb walls?

Superfluids are described by a zero viscosity $\eta = 0$. Microscopically, this means that the intermolecular forces between molecules are so small that the individual atoms move together instead of ...
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### It is possible to have a drag force which is non-Lipschitz?

It may be mathematically possible to have a model with an exact finite stopping time by choosing a non-Lipschitz function in the differential equation. But it is nothing more than a mathematical game, ...
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### Is the pressure-gradient force that causes hydrostatic equilibrium entropic, contact, or both?

Two thoughts occur to me. First, we can model an ideal gas by allowing that there are short-range repulsive forces between molecules (the 'hard spheres' model), and the molecules themselves take up a ...
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1 vote

### Why is this happening to the milk?

It probably is a surface tension effect with the milk not wetting the plastic surface. Creating surface requires the input of energy and the reverse is also true. The milk starts off with a very large ...
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### Shear force oscillating plate

Yes. Simply because no other force can push things in the horizontal direction in this setup.
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1 vote
Accepted

### A question on Hydraulics from a 19th century book

Take a strong glass tube about three feet in length, and tie over one end a piece of well-soaked bladder. When thoroughly dry, . . . . The dry bladder provides a stiff and air-tight seal at one end of ...
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1 vote

### Why does hot water shoot out of my reusable coffee cup when I invert it during cleaning?

This is a weaker form of the thermos-squirting effect. Sloshing the hot water (even gently) increases its surface area and therefore increases the evaporation rate into the air inside the cup. This ...
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1 vote

### Clapping-like sound from large planar object hitting a surface

The basic idea is simply that you need to expel air of volume $\pi h R^2$ through the periphery of area $2 \pi h R$, implying that for a given $\dot{h}$ there's an outflow proportional to $R$ and a ...
1 vote

### What about negative (absolute) pressure in Bernoulli Equation

The Bernoulli equation is derived assuming the fluid is incompressible, which is an idealization even for liquids, and even more for gases. This idealization is valid provided density $\rho$ in the ...
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1 vote

### Does a tower bell ringing prevent thunderstorms?

The energy scale of thunderstorm is much larger than that of a bell (most other devices built by humans), so it is unlikely that such devices can affect thunderstorms in a controllable way. A single ...
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Accepted

### The relationship between height of water and the rotational speed inside a cup

Consider a cup filled with a liquid whose density is $\rho$ which is rotating about its axis of symmetry with a constant angular velodity $\omega$. Let the height to the lowest point of the liquid ...
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1 vote

### How to survive a direct hit of a Tsunami?

The problem with surviving a tsunami is not the water. It's the stuff in it. Once it has made contact with land, your average tsunami wave will contain cars, tree trunks, shipping containers, houses ...
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### How to survive a direct hit of a Tsunami?

No. Multiple reasons: The "other side" is far away. Tsunami waves are extremely broad. You would need to swim several km, or wait underwater several minutes. The currents are too strong. ...
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### Tsunami effects of a nuclear explosion

Yes, destructive tsunamis can be created by current warheads. This source provides in Chapter VII an equation to obtain the amolitude of the biggest wave in a nuclear tsunami, which I have turned into ...
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### Empty Sealed Coke Cans

My first guess would be that corrosion has created pinholes. A second guess would be that pinholes are the result of manufacturing defects. Either way, pinholes allow water to evaporate over the ...
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1 vote

### Can we derive Stokes' drag law from Navier-Stokes' equation?

Yes you can find a derivation in Landau and Lifshitz's book on fluid mechanics in the chapter about viscous fluids. The calculation is rather involved. Note that this is true only in the limit of a ...
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Accepted

### Why does rain feel like a chain reaction?

I believe in the phenomenon you are describing, rain starts from the cloud tops where the temperatures are cooler and fall through the cloud. As the cooler raindrops fall through the cloud, they take ...
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### Some questions about density perturbations in the early universe

Question 1 The pressure appears because General Relativity generalizes Newtonian gravity to include relativity. Instead of mass sourcing a gravitational field, in GR the source of gravity is the ...
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### Am I understanding Pascal's law the right way?

If we compare a liquid and a solid in a similar pressure situation, the result can be also the same. Suppose a spherical thick steel container, with a hole for entrance where a piston can be installed....
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Accepted

### Divergence theorem in index notation

Maybe it helps you visualize if we expand things out. Let $A_{il} = \epsilon_{ijk}r_{j}\sigma_{kl}$, such that the first integral is: $$\tau_{i} = \displaystyle{\int A_{il}n_{l} dA}$$ Notice that we ...
1 vote

### It is possible to have a drag force which is non-Lipschitz?

Let's look at what you are after from a half physical and half mathematical point of view. Assume that you have a drag force which satisfies the following sensible conditions: 1- It is local in time, ...
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