# Tag Info

159

I think that mere touching does not bring the surfaces close enough. The surface of a metal is not perfect usually. Maybe it has an oxide layer that resists any kind of reaction. If the metal is extremely pure and if you bring two pieces of it extremely close together, then they will join together. It's also called cold welding. For more information: What ...

113

They do, as Feynman said. If you have two copper pieces perfectly polished and you put them in contact, they will weld automatically (the copper atoms won't know what piece they belonged to). But in real life, oils, oxides and other impurities don't allow this process. Found it! Read Feynman's own words: If we try to get absolutely pure copper, if ...

27

When you look at a surface like sand, bricks, etc, the light you are seeing is reflected by diffuse reflection. With a flat surface like a mirror, light falling on the surface is reflected back at the same angle it hit the surface (specular reflection) and you see a mirror image of the light falling on the surface. However a material like sand is basically ...

24

Instead of a circular hole, let's think of a square hole. You can get a square hole two ways, you can cut it out of a complete sheet, or you can get one by cutting a sheet into 9 little squares and throwing away the center one. Since the 8 outer squares all get bigger when heat it, the inner square (the hole) also has to get bigger: Same thing happens ...

24

The Apollo 11 flag was included almost as an afterthought. It was just a month or so before liftoff, and someone at NASA slapped themselves on the head and said, "we need an American flag to plant at the landing site!" Someone rushed out to a local store (Sears?) and bought a standard nylon flag, which went to the Moon. Besides being bleached out by solar ...

22

Good question! Assuming the disc is uniform and isotropic (the same in different directions), the hole will expand in the same ratio as the metal. You can see this because the thermal expansion equation $$\mathrm{d} L = L\alpha\mathrm{d}T$$ applies to all lengths associated with the metal, including the circumference of the hole, since the edge of the hole ...

22

David Zaslavski's answer is correct and complete. But I want to propose a different way to look at the problem. Think of the disc that was cut out, and imagine that you heat it too, exactly as you heat the plate. After heating, the disc will fit in exactly to the hole, just as if it was first heated and then cut out. Therefore, the hole will expand.

21

This is a problem in the theory of cracks, but let me try to give an intuitive discussion on the level of linear elasticity. Imagine a rectangular sheet of material with two opposite ends (which I'll call East and West) being pulled apart slowly. The stress in the material due to the tension from the boundary conditions will be symmetric across the North ...

20

Your two questions are connected. There is a huge amount of empty space in aerographene (and other aerogels). However this space is filled with air, and precisely because it is filled with air it doesn't float. This is because the density reported is the density the material would have if the air was sucked out (i.e. in vacuum), and it is so low because ...

19

I believe this is essentially what happens in gilding, owing to the special properties of gold (malleability and lack of corrosion). Extremely flat surfaces can get stuck together due to Van der Waals forces as well as air pressure. I once accidentially stuck two quartz optical windows together, and had a hell of a time separating them.

17

There is a great paper from the group of Howard Stone on this subject: Wetting of flexible fibre arrays (freely available here, but for some reason I am not allowed to link to it normally: http://211.144.68.84:9998/91keshi/Public/File/34/482-7386/pdf/nature10779.pdf) They specifically study when 2 closely positioned parallel fibers (i.e. hairs) clump ...

16

I remember that the question in your title was busted in Mythbusters episode 72. A simple google search also gives many other examples. As for single- vs alternate-direction folding, I'm guessing that the latter would allow for more folds. It is the thickness vs length along a fold that basically tells you if a fold is possible, since there is always going ...

16

The thing is that paper fibers are really transparent (unless the paper has been painted some color, of course). The only reason paper blocks light is that its fibers are all “immersed” in air. Try to imagine what you would see with a very potent microscope: various clear tubes going in all directions. What happens to a ray of light entering this maze of ...

16

Two reasons: Oxides The roughness of the surface If the surface is rough, then the majority of the surface is touching the air gap between the two, not the opposite surface. A bond may form at the touching "peaks", but it will be weak compared to the rest of the metal because a very small fraction of the surface has actually bonded. In addition, metal ...

15

In addition to the question of bend radius - there is also an effect of surface scratches. Most materials are very strong - they fail because a surface flaw allows a stress concentration - ie a crack to form. glass fibre has a very smooth surface because of the way it is made and can be put under high stress without cracking. You can show this with a thick ...

14

Although I don't know anything about this, using some rough estimates I think I can get the right order of magnitude: Volume of graphite in a pencil: $10 cm$ cylinder of $1 mm$ thick = $0.314 mm^3$ (error: ~factor 2) Maximum surface a pencil can write: $50 km$ $\times$ $1$ mm = $10 m^2$ (error: ~factor 5) Thickness of the graphite layer: Volume / Surf. ...

12

No. Or at least, not without special caveats. Edit After comments by Mark Beadles, David Zaslavsky, and Ron Maimon, I should clarify that you cannot have a plain window that lets all light through in one direction but reflects all light coming the other direction. Thus, you cannot have a window that doesn't absorb radiation at all and also has the ...

11

Yes, it is possible to predict the color of a substance, but it is, in some cases, very complex. The color of a substance is decided at various levels. The most "trivial" level is the molecule in gas phase. You have a molecule, all by itself, and when you send some white light on it you provide all the colors of the spectrum. The electronic configuration ...

10

As far as I remember, there is some truth behind this statement: glass is inherently extremely strong, but it is fragile in practice because of microcracks on its surface. In water, glass dissolves to some extent. As a result, microcracks partly disappear, and glass becomes much stronger (at least for a while). I remember reading that A. F. Ioffe (I guess ...

9

See http://en.wikipedia.org/wiki/Thermal_conductivity In metals, I think it generally has to do with the higher valence band electron mobility, but it gets more interesting elsewhere. In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law, as freely moving valence electrons transfer not only ...

9

When you say "anti-gravity material", the closest thing I can think of is the hypothetical concept of negative mass: In theoretical physics, negative mass is a hypothetical concept of matter whose mass is of opposite sign to the mass of the normal matter. Such matter would violate one or more energy conditions and show some strange properties such ...

9

I'm afraid that outside the cartoon world vibrating won't allow a solid object to pass through another solid object. Solids won't pass through each other because the electrons in one solid interact with electrons in the other solid. There's a question about this already, How can I stand on the ground? EM or/and Pauli?, and there are lots of answers to it ...

9

What follows is very much the same as mgphys' answer, but I'm going to pedantic about what I mean. So imagine that I manufacture a substantial body of areographene, and then carefully slice out of it a rectangular prism that is $h$ by $l$ by $w$ in size. This gives a volume for the material of $V = h l w$. A put a analytic balance in a vacuum chamber and ...

9

Send an ultrasonic pulse through the gold bar and analyse the returning wave. This technique is actually used to detect impurities in gold bars. To quote this article: Where the wave encounters a region of material with different physical properties – particularly the density and elastic constants – to the rest of the metal, the beam is affected in a ...

9

I can't comment since I don't have the reputation for it, but I do have some relevant knowledge from my research in materials science. To add to what DumpsterDoofus said, it is very easy for two pieces of glass or polymer to bond if you clean them extremely well and ionize the surface. Look up plasma polymerisation. Moreover, you'd be surprised how much ...

8

A from a theoretical physics point of view not completely off the mark approach to anti-gravity effects comes from certain versions of supergravity described in wikipedia, which is a unified supersymmetric point-particle quantum field theory. Some particular versions of this theory not only contain the "usual" atractive graviton, a spin-2 particle, but in ...

8

Forever_a_Newcomer is on the right lines, but it's not like water dissolving salt. Paper is mostly made from cellulose fibres (depending on the type there may also be filers and glazes like clay). Cellulose molecules bristle with hydroxyl (OH) groups, and these form hydrogen bonds with each other. It's these hydrogen bonds that make the individual fibres ...

7

Typically, a laser will damage an optical surface in one of two ways. The first is just what you would expect: the laser heats the material up until something bad happens. The second is also pretty simple, but less common because (AFAIK) it is really only a problem with very short pulses (on the order of femtoseconds). In this case a small but rapidly ...

7

You're looking for something called a "light diode" or "optical isolator". The simplest is two polarizing sheets 45 degrees apart, with a 45 degree Faraday rotator in between. Here's why it doesn't violate the second law of thermodynamics. One of the components--the Faraday rotator--is thick and bulky and usually covers only a small area. So I guess it ...

7

These are "fake monopoles", in the sense that the north and south poles are not actually separated. They are the ends of thin tubes which behave like Dirac strings - like long thin twisted magnets. The tubes are formed due to geometrical frustration, which forces the magnetic field to be orientated either toward the outside or toward the inside of the ...

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