Surface Go 2

Originally concerned about the surface in relation to transport and sports and more commonly used on linear features, this key is now increasingly used with certain areas of type natural=*. Note, however, that the values of natural=* and surface=* must not be confused, e.g. natural=grassland vs surface=grass and natural=glacier vs surface=ice. For broader descriptions of surfaces see Landcover.

For roads for motor vehicles there there is typically an assumption that the surface is surface=paved unless otherwise stated. Paved in OpenStreetMap is non-specific and may cover sealed, tarmac, asphalt, bitumen, even sett or true cobblestone. surface=unpaved is treated as the opposite of paved. More specific tags can be used for surfaces which are normally classified into paved or unpaved for routing purposes. Navigation software should assume that roads-that-are-not-paved will have slower driving speed (and therefore longer driving time) and may be impassable in some weather conditions.

Surface Go 2

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Rendering software convention varies, but generally roads-that-are-not-paved are shown in a different colour but same width as their paved cousins or use the same colour but are dashed. Rendering of road surface in standard OSM style is a complicated subject.

In some cases when other tags are not sufficient to describe the road conditions good enough the use of smoothness=* and maxspeed:practical=* can be considered. maxspeed:practical=* can be useful in situations where other tags are not sufficient to describe what kind of traveling speed could be reasonably expected. If there is one good surface=unpaved road, where practical speed is 60 km/h, and a second bad road with concrete:lanes surface allowing a practical speed 10 km/h, users or routing software could be misguided to choose the second road for routing, because by default paved roads are assumed to allow much higher speeds than unpaved roads.

There are no default values for surface, it is generally considered as OK and desirable to tag it explicitly for all roads. In case of missing surface=* data consumers may sometimes successfully guess values based on location of object and values of highway=*, lit=*, tracktype=*, tiger:reviewed=no and other tags[1][2][3]. Only in some cases, like on highway=motorway, it can be assumed that all of them will be paved.

The primary use is navigation. A router for bicycles could avoid an otherwise normal road that is tagged as having surface=sand. For vehicles, the router might tie-break by preferring a slightly longer asphalt route over a earth/compacted/gravel/cobblestone route. Pedestrians probably also prefer any paved value over a potentially muddy footpath. The tag can furthermore be used to use the right texture when rendering, for example to indicate which ways are unpaved.

Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to float on a water surface without becoming even partly submerged.

Because of the relatively high attraction of water molecules to each other through a web of hydrogen bonds, water has a higher surface tension (72.8 millinewtons (mN) per meter at 20 C) than most other liquids. Surface tension is an important factor in the phenomenon of capillarity.

Surface tension has the dimension of force per unit length, or of energy per unit area.[4] The two are equivalent, but when referring to energy per unit of area, it is common to use the term surface energy, which is a more general term in the sense that it applies also to solids.

Due to the cohesive forces, a molecule located away from the surface is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have the same molecules on all sides of them and therefore are pulled inward. This creates some internal pressure and forces liquid surfaces to contract to the minimum area.[2]

Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the imbalance in cohesive forces of the surface layer. In the absence of other forces, drops of virtually all liquids would be approximately spherical. The spherical shape minimizes the necessary "wall tension" of the surface layer according to Laplace's law.

Another way to view surface tension is in terms of energy. A molecule in contact with a neighbor is in a lower state of energy than if it were alone. The interior molecules have as many neighbors as they can possibly have, but the boundary molecules are missing neighbors (compared to interior molecules) and therefore have higher energy. For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized. The minimized number of boundary molecules results in a minimal surface area.[5]As a result of surface area minimization, a surface will assume a smooth shape.

Surface tension of a liquid is the force per unit length. In the illustration on the right, the rectangular frame, composed of three unmovable sides (black) that form a "U" shape, and a fourth movable side (blue) that can slide to the right. Surface tension will pull the blue bar to the left; the force F required to hold the movable side is proportional to the length L of the immobile side. Thus the ratio .mw-parser-output .sfrac{white-space:nowrap}.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tion{display:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center}.mw-parser-output .sfrac .num,.mw-parser-output .sfrac .den{display:block;line-height:1em;margin:0 0.1em}.mw-parser-output .sfrac .den{border-top:1px solid}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}F/L depends only on the intrinsic properties of the liquid (composition, temperature, etc.), not on its geometry. For example, if the frame had a more complicated shape, the ratio F/L, with L the length of the movable side and F the force required to stop it from sliding, is found to be the same for all shapes. We therefore define the surface tension as

= F 2 L . {\displaystyle \gamma ={\frac {F}{2L}}.} The reason for the 1/2 is that the film has two sides (two surfaces), each of which contributes equally to the force; so the force contributed by a single side is L = F/2.

Surface tension of a liquid is the ratio of the change in the energy of the liquid to the change in the surface area of the liquid (that led to the change in energy). This can be easily related to the previous definition in terms of force:[7] if F is the force required to stop the side from starting to slide, then this is also the force that would keep the side in the state of sliding at a constant speed (by Newton's Second Law). But if the side is moving to the right (in the direction the force is applied), then the surface area of the stretched liquid is increasing while the applied force is doing work on the liquid. This means that increasing the surface area increases the energy of the film. The work done by the force F in moving the side by distance x is W = Fx; at the same time the total area of the film increases by A = 2Lx (the factor of 2 is here because the liquid has two sides, two surfaces). Thus, multiplying both the numerator and the denominator of tag_hash_124 = 1/2F/L by x, we get = F 2 L = F x 2 L x = W A . {\displaystyle \gamma ={\frac {F}{2L}}={\frac {F\Delta x}{2L\Delta x}}={\frac {W}{\Delta A}}.} This work W is, by the usual arguments, interpreted as being stored as potential energy. Consequently, surface tension can be also measured in SI system as joules per square meter and in the cgs system as ergs per cm2. Since mechanical systems try to find a state of minimum potential energy, a free droplet of liquid naturally assumes a spherical shape, which has the minimum surface area for a given volume.The equivalence of measurement of energy per unit area to force per unit length can be proven by dimensional analysis.[8]

The quantity in parentheses on the right hand side is in fact (twice) the mean curvature of the surface (depending on normalisation).Solutions to this equation determine the shape of water drops, puddles, menisci, soap bubbles, and all other shapes determined by surface tension (such as the shape of the impressions that a water strider's feet make on the surface of a pond).The table below shows how the internal pressure of a water droplet increases with decreasing radius. For not very small drops the effect is subtle, but the pressure difference becomes enormous when the drop sizes approach the molecular size. (In the limit of a single molecule the concept becomes meaningless.)

When an object is placed on a liquid, its weight Fw depresses the surface, and if surface tension and downward force become equal then it is balanced by the surface tension forces on either side Fs, which are each parallel to the water's surface at the points where it contacts the object. Notice that small movement in the body may cause the object to sink. As the angle of contact decreases, surface tension decreases. The horizontal components of the two Fs arrows point in opposite directions, so they cancel each other, but the vertical components point in the same direction and therefore add up[5] to balance Fw. The object's surface must not be wettable for this to happen, and its weight must be low enough for the surface tension to support it. If m denotes the mass of the needle and g acceleration due to gravity, we have

To find the shape of the minimal surface bounded by some arbitrary shaped frame using strictly mathematical means can be a daunting task. Yet by fashioning the frame out of wire and dipping it in soap-solution, a locally minimal surface will appear in the resulting soap-film within seconds.[8][13] 9af72c28ce