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27-495: A ladder is a vertical or inclined set of rungs or steps commonly used for climbing or descending. There are two types: rigid ladders that are self-supporting or that may be leaned against a vertical surface such as a wall , and rollable ladders, such as those made of rope or aluminium , that may be hung from the top. The vertical members of a rigid ladder are called stringers or rails (US) or stiles (UK). Rigid ladders are usually portable, but some types are permanently fixed to

54-404: A carabiner . If a leaning ladder is placed at the wrong angle, the risk of a fall is greatly increased. The safest angle for a ladder is 75.5°; if it is too shallow, the bottom of the ladder is at risk of sliding, and if it is too steep, the ladder may fall backwards. This angle is achieved by following the 4 to 1 rule for a ladder placed on a vertical wall: for every four feet of vertical height,

81-441: A gallows , while others attribute it to ancient Egyptian traditions involving pyramids and triangles representing the trinity of the gods, and passing through the triangular shape made by a ladder against a wall was seen as desecration. Ladders have also been linked to the crucifixion of Christ , with author and scientist Charles Panati noting that many believe a ladder rested against the cross that Christ hung from, making it

108-416: A fellow worker is not available for "footing" the ladder. Footing occurs when another worker stands on the lowest rung and so provides much greater stability to the ladder when being used. However footing a ladder should be seen as a last resort for a safe placement. The anchor point is usually a ring cemented into a slot in the brick wall to which the rungs of a ladder can be attached using rope for example, or

135-649: A horizontal table. Although the word horizontal is commonly used in daily life and language (see below), it is subject to many misconceptions. In general or in practice, something that is horizontal can be drawn from left to right (or right to left), such as the x-axis in the Cartesian coordinate system . The concept of a horizontal plane is thus anything but simple, although, in practice, most of these effects and variations are rather small: they are measurable and can be predicted with great accuracy, but they may not greatly affect our daily life. This dichotomy between

162-506: A larger scale the gravitational field of the Earth, which is at least approximately radial near the Earth, is not radial when it is affected by the Moon at higher altitudes. Neglecting the curvature of the earth, horizontal and vertical motions of a projectile moving under gravity are independent of each other. Vertical displacement of a projectile is not affected by the horizontal component of

189-437: A point P and designate a direction through P as vertical. A plane which contains P and is normal to the designated direction is the horizontal plane at P. Any plane going through P, normal to the horizontal plane is a vertical plane at P. Through any point P, there is one and only one horizontal plane but a multiplicity of vertical planes. This is a new feature that emerges in three dimensions. The symmetry that exists in

216-556: A structure, building, or equipment. They are commonly made of metal, wood, or fiberglass , but they have been known to be made of tough plastic. Ladders are ancient tools and technology. A ladder is featured in a Mesolithic rock painting that is at least 10,000 years old, depicted in the Spider Caves in Valencia, Spain . The painting depicts two humans using a ladder to reach a wild honeybee nest to harvest honey . The ladder

243-434: A symbol of wickedness, betrayal and death. In comedic children's media, the image of a character walking under a ladder being the cause or result of bad luck has become a common trope. Vertical direction Conversely, a direction or plane is said to be horizontal (or leveled ) if it is perpendicular to the vertical direction. In general, something that is vertical can be drawn from up to down (or down to up), such as

270-466: Is colour-coded to indicate the amount of weight the ladder is designed to hold, the certification class and its use. The color of the safety label specifies the class and use. A common superstition in English-speaking countries is that walking under a ladder is seen as bad luck . Some sources claim that this stems from the image of a ladder being propped up against a wall looking similar to

297-434: Is depicted as long and flexible, possibly made out of some sort of grass. Rigid ladders are available in many forms, such as: Rigid ladders were originally made of wood , but in the 20th century aluminium became more common because of its lighter weight. Ladders with fiberglass stiles are used for working on or near overhead electrical wires , because fiberglass is an electrical insulator . Henry Quackenbush patented

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324-512: The x -axis, in which case the y -axis is then automatically determined. There is no special reason to choose the horizontal over the vertical as the initial designation: the two directions are on par in this respect. The following hold in the two-dimensional case: Not all of these elementary geometric facts are true in the 3-D context. In the three-dimensional case, the situation is more complicated as now one has horizontal and vertical planes in addition to horizontal and vertical lines. Consider

351-533: The North Pole and as such has claim to be a horizontal plane. But it is. at the same time, a vertical plane for points on the equator. In this sense, a plane can, arguably, be both horizontal and vertical, horizontal at one place , and vertical at another . For a spinning earth, the plumb line deviates from the radial direction as a function of latitude. Only on the equator and at the North and South Poles does

378-519: The apparent simplicity of a concept and an actual complexity of defining (and measuring) it in scientific terms arises from the fact that the typical linear scales and dimensions of relevance in daily life are 3 orders of magnitude (or more) smaller than the size of the Earth. Hence, the world appears to be flat locally, and horizontal planes in nearby locations appear to be parallel. Such statements are nevertheless approximations; whether they are acceptable in any particular context or application depends on

405-415: The applicable requirements, in particular in terms of accuracy. In graphical contexts, such as drawing and drafting and Co-ordinate geometry on rectangular paper, it is very common to associate one of the dimensions of the paper with a horizontal, even though the entire sheet of paper is standing on a flat horizontal (or slanted) table. In this case, the horizontal direction is typically from the left side of

432-422: The context of a 1-dimensional orthogonal Cartesian coordinate system on a Euclidean plane, to say that a line is horizontal or vertical, an initial designation has to be made. One can start off by designating the vertical direction, usually labelled the Y direction. The horizontal direction, usually labelled the X direction, is then automatically determined. Or, one can do it the other way around, i.e., nominate

459-403: The extension ladder in 1867. The most common injury made by ladder climbers is bruising from falling off a ladder, but bone fractures are common and head injuries are also likely, depending on the nature of the accident. Ladders can cause injury if they slip on the ground and fall. To avoid this, they tend to have plastic feet or base pads which increase friction with the ground. However, if

486-655: The ladder foot should move one foot from the wall. Both scenarios can cause significant injury, and are especially important in industries like construction, which require heavy use of ladders. The European Union and the United Kingdom established a ladder certification system – ladder classes – for any ladders manufactured or sold in Europe. The certification classes apply solely to ladders that are portable such as stepladders and extension ladders and are broken down into three types of certification. Each ladder certification

513-450: The launch velocity, and, conversely, the horizontal displacement is unaffected by the vertical component. The notion dates at least as far back as Galileo. When the curvature of the Earth is taken into account, the independence of the two motion does not hold. For example, even a projectile fired in a horizontal direction (i.e., with a zero vertical component) may leave the surface of the spherical Earth and indeed escape altogether. In

540-409: The plastic is badly worn, the aluminium may contact the ground increasing the chance of an accident. Ladder stabilizers are also available to increase the ladder's grip on the ground. One of the first ladder stabilizers or ladder feet was offered in 1936 and today they are standard equipment on most large ladders. A ladder standoff, or stay, is a device fitted to the top of a ladder to hold it away from

567-415: The plumb line align with the local radius. The situation is actually even more complicated because Earth is not a homogeneous smooth sphere. It is a non homogeneous, non spherical, knobby planet in motion, and the vertical not only need not lie along a radial, it may even be curved and be varying with time. On a smaller scale, a mountain to one side may deflect the plumb bob away from the true zenith . On

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594-487: The same fundamental principle. When the curvature of the Earth is taken into account, the concepts of vertical and horizontal take on yet another meaning. On the surface of a smoothly spherical, homogenous, non-rotating planet, the plumb bob picks out as vertical the radial direction. Strictly speaking, it is now no longer possible for vertical walls to be parallel: all verticals intersect. This fact has real practical applications in construction and civil engineering, e.g.,

621-475: The tops of the towers of a suspension bridge are further apart than at the bottom. Also, horizontal planes can intersect when they are tangent planes to separated points on the surface of the Earth. In particular, a plane tangent to a point on the equator intersects the plane tangent to the North Pole at a right angle . (See diagram). Furthermore, the equatorial plane is parallel to the tangent plane at

648-406: The two-dimensional case no longer holds. In the 2-dimension case, as mentioned already, the usual designation of the vertical coincides with the y-axis in co-ordinate geometry. This convention can cause confusion in the classroom. For the teacher, writing perhaps on a white board, the y -axis really is vertical in the sense of the plumbline verticality but for the student the axis may well lie on

675-557: The vertical to be perpendicular to the horizon in his 1636 book Perspective . In physics, engineering and construction, the direction designated as vertical is usually that along which a plumb-bob hangs. Alternatively, a spirit level that exploits the buoyancy of an air bubble and its tendency to go vertically upwards may be used to test for horizontality. A water level device may also be used to establish horizontality. Modern rotary laser levels that can level themselves automatically are robust sophisticated instruments and work on

702-458: The wall. This enables the ladder to clear overhanging obstacles, such as the eaves of a roof , and increases the safe working height for a given length of ladder because of the increased separation distance of the two contact points at the top of the ladder. It has become increasingly common to provide anchor points on buildings to which the top rung of an extension ladder can be attached, especially for activities like window cleaning, especially if

729-531: The y-axis in the Cartesian coordinate system . The word horizontal is derived from the Latin horizon , which derives from the Greek ὁρῐ́ζων , meaning 'separating' or 'marking a boundary'. The word vertical is derived from the late Latin verticalis , which is from the same root as vertex , meaning 'highest point' or more literally the 'turning point' such as in a whirlpool. Girard Desargues defined

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