# Tag Info

89

You have created a rather poor pinhole camera (camera obscura). You can see an "image" of the sky, a green space (trees) and even a reddish brown blur that is your driveway. This is not diffraction or refraction - it's geometrical (classical) optics. Because the hole is pretty big, you see a very blurry image. But basically, the light from the sky falling ...

66

The process you describe is called granular convection$^1$. It happens because under random motion it's easier for a small particle to fall under a big one than vice versa. Let's assume that all the particles are made of the same material so there are no density differences in play. If you agitate the particles then temporary voids will open between ...

27

The sort of dishcloths generically known as J Cloths are made from a material called Viscose rayon: This material is derived from cellulose and like cellulose it interacts with water. Water breaks hydrogen bonds formed within the fibres. This makes the fibres softer, and the exposed hydroxyl groups make the surface more hydrophilic. It's the latter ...

13

I'm going to respond to (v1) of the question, which asks why the night sky is dark (black and unlit) compared to the day sky even though there are many light sources at night. The updated question references Olber's paradox, which has been answered many times before. Like most things we see in everyday life, there are a number of reasons contributing to ...

10

I googled it and found the term Brazil-nut Effect but couldn't found any proper explanation. What is the physical explanation of this effect? I answered another question on granular convection some time ago, this is the physical explanation: When you shake a mixture you create some gaps between the nuts. Because of gravity, small nuts will fall ...

8

Steam is caused when water vapor condenses. This is caused by the air having too much water vapor for it to hold. When you have a lot of heat under the pan, the air above the pan is quite hot and can hold a lot of water. The water evaporating from the pan disperses into the atmosphere and doesn't condense. When you turn off the heat, the pan and food ...

7

Who is interested can find detailed information at wiki, or here The problem is known (as you added in your edit) as Olbers' paradox, and was posed already in the mid 1500's, by Johannes Kepler in 1610 and even later by Edmond Halley in the eighteen century, and curiously, even the novelist an poet Edgar Allen Poe anticipated possible explanations as to why ...

6

Paper is opaque because there are mineral fillers in between the cellulose fibers, which are in a crystalline form. When surrounded by air, they appear opaque due to the higher difference in the index of refraction between the two. When wet, the paper appears translucent because there is a lower difference in the index of refraction between the crystals and ...

5

Don't even think about this!! REALLY. The risk is probably low, but the outcome of a broken rope is pretty severe in climbing. Too severe for the sake of a tug of war game. This is an advanced materials engineering question. 9kN is presumably the rope's rating, right? 50-60 grownups will easily impart this force onto the rope (30 at each end, each pulling ...

5

The front of the train compresses air which can blow you away, while at the back of the train air rushes back in after the train has displaced it. This backdraft is especially troublesome in closed areas such as subways, where a train exits a small tunnel near a platform and the displaced air rushes back into the vacated tunnel. Next time you see a big truck ...

5

From Wikipedia: A spoiler is an automotive aerodynamic device whose intended design function is to 'spoil' unfavorable air movement across a body of a vehicle in motion, usually described as turbulence or drag. A rear spoiler is designed to change the flow of air over the rear of the vehicle. This change in air flow increases downforce on the rear of ...

4

Three things to consider: The temperatur $T$ of the glass, the thermal conductivity $\kappa$ of the glass, and the fact that you are actually comparing conduction from skin-to-glass with convection from skin-to-air, which might not be a fair comparison. To no. 1., remember that your hand is much warmer than the glass. The glass temperature is not ...

4

The following passage is extracted from Stephen Hawking's book "A Brief History of Time": In fact, various contemporaries of Newton had raised the problem, and the Olbers article was not even the first to contain plausible arguments against it. It was, however, the first to be widely noted. The difficulty is that in an infinite static universe ...

3

The problem is that while mass is the same everywhere on earth, weight is not - it can vary as much as 0.7% from the North Pole (heavy) to the mountains of Peru (light). This is in part caused by the rotation of the earth, and in part by the fact that it is not (quite) a sphere. When you are interested in "how much" of something there is - say, a bag of ...

3

The Cheerios effect is mostly due to surface tension and is most prominent among objects comparable in size to or smaller than the meniscus of the fluid. It's a type of capillary action.

3

Assuming your question is essentially: "Why do the flakes actually 'clump' together ?", the answer is one that involves probability. Suppose you have only two wet corn-flakes on the surface of milk isolated from each other. Say somehow they start moving toward each other. As they come together, they stick. Now, once stuck, the flakes will not seperate ...

3

Firstly, I suspect that oxidation will throw a spanner into any such plan. For metals like aluminum, which have a very high affinity for oxygen, a "virgin" surface will begin to tarnish almost immediately. The second problem is that metals have microstructure. Neighboring crystals in a polycrystalline aggregate such as a metal piece have to satisfy certain ...

3

what kind of wings would a person need to fly? long and thin. Human powered flapping-wing aircraft (ornithopter) It seems this couldn't take off under human power alone but, once airborne, could produce enough thrust to overcome drag for up to 145 meters in 20 seconds of flight. to achieve flight on the limited power of the human engine, the ...

2

What would be the benefits of rear spoilers in cars, like this one? On cars like that one, there are no significant objective benefits. There are some disadvantages increased weight, so higher fuel consumption increased drag, so higher fuel consumption On racing cars on racing tracks at high speeds, there can be increased downforce on the rear wheels ...

2

Why can't we reach the ends of rainbow? The WIkipedia article you linked to contains the explanation The rainbow is not located at a specific distance, but comes from an optical illusion caused by any water droplets viewed from a certain angle relative to a light source. Thus, a rainbow is not an object and cannot be physically approached. Indeed, ...

2

Like any object moving through a fluid, a high-speed train distorts the air as it moves through it. Broadly speaking, there are three main regions of flow structure around a high-speed train: the upstream distortion, boundary layer and wake. These can be collectively referred to as the slipstream. The effects of the slipstream on a static observer (e.g. a ...

2

I suspect it's a surface tension question. When a dish cloth is slightly damp, then water is already between the fibers. Putting a drop of water in touch with that fiber (and water), the water will be drawn from the drop into the space between the fibers. By contrast, if the cloth is really dry, then when the drop is touching the fiber, it first needs to ...

1

If all you did was drink distilled water, it would not be terribly healthy for you. But most people eat as well - and that combination probably includes "healthy" minerals. Unless you binge-drink distilled water without any food, it's unlikely to cause serious health effects. Having said that - ions are pretty small - certainly smaller than 15 nm. But their ...

1

The spoiler spoil the air flow at the rear side. When the car is cruising, from the frame of reference of the car, air flows around it from front to back. If the air flow is smooth and if there is a large velocity at the rear side then there will be pressure drop. Thus, there is a low pressure at the rear side and higher pressure on the front side, creating ...

1

A full 3D model might be too complex for this case, because then you need to know everything about the wall, and the tree, and the interaction between the two in great detail. I think a simplified approach might be more suitable. I'm not sure if this is oversimplified, but let me take a swing at it: If we assume the tree is supported by both the ground and ...

1

To start the ball rolling, I found a thread at http://www.reddit.com/r/askscience/comments/1rhpc1/how_does_clingfilm_saranwrap_stick_to_itself_does/ which contained some interesting information: the formulation of Saran wrap was changed a few years ago when Dow sold the brand to SC Johnson the new material is made with polyethylene which doesn't stick very ...

1

I'm fairly sure that the ridges exist to hold an inventory of liquid. A simple way to argue this is to note that the ridged loop has a much larger total surface area. However, I'm sure there are more complicated physics with the ridged surfaces. Since it helps brace the liquid within a ridge, it's sure to hang on to more water than what a flat surface of ...

1

You can reach the end of a rainbow, if you make one using a hose pipe on a sunny day. It is possible to locate the ends to within a few cm

1

Caller ID is sent after the first ring. Ironically, even though phone companies sequenced things that way to avoid delaying the first ring, some phones want to avoid bothering people until after the caller ID information is received or would be expected (if the caller hangs up or someone answers the phone before the caller ID information is sent, there ...

1

Actually, the surface of the ice is frozen, until you step on it, then it melts. When pressure on the ice is increased, it can melt if it is not too cold. This is why ice skates work so well on the ice. They create a lot of pressure; creating a water layer between the ice and blade. This water layer decreases the friction so the blade slides easily. From ...

Only top voted, non community-wiki answers of a minimum length are eligible