1.
Force
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In physics, a force is any interaction that, when unopposed, will change the motion of an object. In other words, a force can cause an object with mass to change its velocity, force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity and it is measured in the SI unit of newtons and represented by the symbol F. The original form of Newtons second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. In an extended body, each part usually applies forces on the adjacent parts, such internal mechanical stresses cause no accelation of that body as the forces balance one another. Pressure, the distribution of small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of materials, or flow in fluids. In part this was due to an understanding of the sometimes non-obvious force of friction. A fundamental error was the belief that a force is required to maintain motion, most of the previous misunderstandings about motion and force were eventually corrected by Galileo Galilei and Sir Isaac Newton. With his mathematical insight, Sir Isaac Newton formulated laws of motion that were not improved-on for nearly three hundred years, the Standard Model predicts that exchanged particles called gauge bosons are the fundamental means by which forces are emitted and absorbed. Only four main interactions are known, in order of decreasing strength, they are, strong, electromagnetic, weak, high-energy particle physics observations made during the 1970s and 1980s confirmed that the weak and electromagnetic forces are expressions of a more fundamental electroweak interaction. Since antiquity the concept of force has been recognized as integral to the functioning of each of the simple machines. The mechanical advantage given by a machine allowed for less force to be used in exchange for that force acting over a greater distance for the same amount of work. Analysis of the characteristics of forces ultimately culminated in the work of Archimedes who was famous for formulating a treatment of buoyant forces inherent in fluids. Aristotle provided a discussion of the concept of a force as an integral part of Aristotelian cosmology. In Aristotles view, the sphere contained four elements that come to rest at different natural places therein. Aristotle believed that objects on Earth, those composed mostly of the elements earth and water, to be in their natural place on the ground. He distinguished between the tendency of objects to find their natural place, which led to natural motion, and unnatural or forced motion
2.
Machine
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A machine is a tool containing one or more parts that uses energy to perform an intended action. Machines are usually powered by chemical, thermal, or electrical means, historically, a power tool also required moving parts to classify as a machine. However, the advent of electronics has led to the development of power tools without moving parts that are considered machines, a simple machine is a device that simply transforms the direction or magnitude of a force, but a large number of more complex machines exist. Examples include vehicles, electronic systems, molecular machines, computers, television, the word machine derives from the Latin word machina, which in turn derives from the Greek. The word mechanical comes from the same Greek roots, however, the Ancient Greeks probably have borrowed the word mekhane from the ancient Hebrews. The ancient Greeks were familiar with the Hebrew Scriptures and language, a wider meaning of fabric, structure is found in classical Latin, but not in Greek usage. This meaning is found in late medieval French, and is adopted from the French into English in the mid-16th century, in the 17th century, the word could also mean a scheme or plot, a meaning now expressed by the derived machination. The modern meaning develops out of specialized application of the term to stage engines used in theater and to siege engines. Simple Machines are commonly reckoned to be Six in Number, viz. the Ballance, Leaver, Pulley, Wheel, Wedge, compound Machines, or Engines, are innumerable. The word engine used as a synonym both by Harris and in later language derives ultimately from Latin ingenium ingenuity, an invention, perhaps the first example of a human made device designed to manage power is the hand axe, made by chipping flint to form a wedge. A wedge is a machine that transforms lateral force and movement of the tool into a transverse splitting force. The idea of a simple machine originated with the Greek philosopher Archimedes around the 3rd century BC, who studied the Archimedean simple machines, lever, pulley and he discovered the principle of mechanical advantage in the lever. Later Greek philosophers defined the five simple machines and were able to roughly calculate their mechanical advantage. Heron of Alexandria in his work Mechanics lists five mechanisms that can set a load in motion, lever, windlass, pulley, wedge, and screw, however the Greeks understanding was limited to statics and did not include dynamics or the concept of work. In 1586 Flemish engineer Simon Stevin derived the mechanical advantage of the inclined plane, the complete dynamic theory of simple machines was worked out by Italian scientist Galileo Galilei in 1600 in Le Meccaniche. He was the first to understand that simple machines do not create energy, the classic rules of sliding friction in machines were discovered by Leonardo da Vinci, but remained unpublished in his notebooks. They were rediscovered by Guillaume Amontons and were developed by Charles-Augustin de Coulomb. The word mechanical refers to the work that has produced by machines or the machinery
3.
Power (physics)
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In physics, power is the rate of doing work. It is the amount of energy consumed per unit time, having no direction, it is a scalar quantity. In the SI system, the unit of power is the joule per second, known as the watt in honour of James Watt, another common and traditional measure is horsepower. Being the rate of work, the equation for power can be written, because this integral depends on the trajectory of the point of application of the force and torque, this calculation of work is said to be path dependent. As a physical concept, power requires both a change in the universe and a specified time in which the change occurs. This is distinct from the concept of work, which is measured in terms of a net change in the state of the physical universe. The output power of a motor is the product of the torque that the motor generates. The power involved in moving a vehicle is the product of the force of the wheels. The dimension of power is divided by time. The SI unit of power is the watt, which is equal to one joule per second, other units of power include ergs per second, horsepower, metric horsepower, and foot-pounds per minute. One horsepower is equivalent to 33,000 foot-pounds per minute, or the required to lift 550 pounds by one foot in one second. Other units include dBm, a logarithmic measure with 1 milliwatt as reference, food calories per hour, Btu per hour. This shows how power is an amount of energy consumed per unit time. If ΔW is the amount of work performed during a period of time of duration Δt and it is the average amount of work done or energy converted per unit of time. The average power is simply called power when the context makes it clear. The instantaneous power is then the value of the average power as the time interval Δt approaches zero. P = lim Δ t →0 P a v g = lim Δ t →0 Δ W Δ t = d W d t. In the case of constant power P, the amount of work performed during a period of duration T is given by, W = P t
4.
Lever
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A lever is a machine consisting of a beam or rigid rod pivoted at a fixed hinge, or fulcrum. A lever is a body capable of rotating on a point on itself. On the basis of the location of fulcrum, load and effort and it is one of the six simple machines identified by Renaissance scientists. A lever amplifies an input force to provide an output force. The ratio of the force to the input force is the mechanical advantage of the lever. The word lever entered English about 1300 from Old French, in which the word was levier and this sprang from the stem of the verb lever, meaning to raise. The verb, in turn, goes back to the Latin levare, itself from the adjective levis, the words primary origin is the Proto-Indo-European stem legwh-, meaning light, easy or nimble, among other things. The PIE stem also gave rise to the English word light, the earliest remaining writings regarding levers date from the 3rd century BC and were provided by Archimedes. Give me a place to stand, and I shall move the Earth with it is a remark of Archimedes who formally stated the correct mathematical principle of levers. The distance required to do this might be exemplified in astronomical terms as the distance to the Circinus galaxy - about 9 million light years. It is assumed that in ancient Egypt, constructors used the lever to move, a lever is a beam connected to ground by a hinge, or pivot, called a fulcrum. The ideal lever does not dissipate or store energy, which there is no friction in the hinge or bending in the beam. This is known as the law of the lever, the mechanical advantage of a lever can be determined by considering the balance of moments or torque, T, about the fulcrum. T1 = F1 a, T2 = F2 b where F1 is the force to the lever. The distances a and b are the distances between the forces and the fulcrum. Since the moments of torque must be balanced, T1 = T2, the mechanical advantage of the lever is the ratio of output force to input force, M A = F2 F1 = a b. Levers are classified by the positions of the fulcrum, effort. It is common to call the force the effort and the output force the load or the resistance
5.
Mechanism (engineering)
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A mechanism is a device designed to transform input forces and movement into a desired set of output forces and the movement. In this context, his use of machine is generally interpreted to mean mechanism, the combination of force and movement defines power, and a mechanism is designed to manage power in order to achieve a desired set of forces and movement. A mechanism is usually a piece of a process or mechanical system. Sometimes an entire machine may be referred to as a mechanism, examples are the steering mechanism in a car, or the winding mechanism of a wristwatch. From the time of Archimedes through the Renaissance, mechanisms were considered to be constructed from simple machines, such as the lever, pulley, screw, wheel and axle, wedge and inclined plane. It was Reuleaux who focussed on bodies, called links, in order to use geometry to study the movement of a mechanism, its links are modeled as rigid bodies. This means distances between points in a link are assumed to be unchanged as the moves, that is the link does not flex. Thus, the movement between points in two connected links is considered to result from the kinematic pair that joins them. A mechanism is modeled as an assembly of rigid links and kinematic pairs, Reuleaux called the ideal connections between links kinematic pairs. He distinguished between higher pairs which were said to have contact between the two links and lower pairs that have area contact between the links. J. Phillips shows that there are ways to construct pairs that do not fit this simple. Lower pair, A lower pair is a joint that has surface contact between the pair of elements. This imposes five constraints on the movement of the links. This imposes five constraints on the movement of the links. A cylindrical joint requires that a line in the body remain co-linear with a line in the fixed body. It is a combination of a joint and a sliding joint. This joint has two degrees of freedom, a spherical joint, or ball joint, requires that a point in the moving body maintain contact with a point in the fixed body. This joint has three degrees of freedom, a planar joint requires that a plane in the moving body maintain contact with a plane in fixed body
6.
Rigid bodies
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In physics, a rigid body is an idealization of a solid body in which deformation is neglected. In other words, the distance between any two points of a rigid body remains constant in time regardless of external forces exerted on it. Even though such an object cannot physically exist due to relativity, in classical mechanics a rigid body is usually considered as a continuous mass distribution, while in quantum mechanics a rigid body is usually thought of as a collection of point masses. For instance, in quantum mechanics molecules are often seen as rigid bodies, the position of a rigid body is the position of all the particles of which it is composed. To simplify the description of this position, we exploit the property that the body is rigid, if the body is rigid, it is sufficient to describe the position of at least three non-collinear particles. This makes it possible to reconstruct the position of all the other particles, however, typically a different, mathematically more convenient, but equivalent approach is used. Thus, the position of a body has two components, linear and angular, respectively. The same is true for other kinematic and kinetic quantities describing the motion of a body, such as linear and angular velocity, acceleration, momentum, impulse. This reference point may define the origin of a coordinate system fixed to the body, there are several ways to numerically describe the orientation of a rigid body, including a set of three Euler angles, a quaternion, or a direction cosine matrix. In general, when a body moves, both its position and orientation vary with time. In the kinematic sense, these changes are referred to as translation and rotation, indeed, the position of a rigid body can be viewed as a hypothetic translation and rotation of the body starting from a hypothetic reference position. Velocity and angular velocity are measured with respect to a frame of reference, the linear velocity of a rigid body is a vector quantity, equal to the time rate of change of its linear position. Thus, it is the velocity of a point fixed to the body. During purely translational motion, all points on a body move with the same velocity. However, when motion involves rotation, the velocity of any two points on the body will generally not be the same. Two points of a body will have the same instantaneous velocity only if they happen to lie on an axis parallel to the instantaneous axis of rotation. Angular velocity is a quantity that describes the angular speed at which the orientation of the rigid body is changing. All points on a rigid body experience the same velocity at all times
7.
Archimedes
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Archimedes of Syracuse was a Greek mathematician, physicist, engineer, inventor, and astronomer. Although few details of his life are known, he is regarded as one of the scientists in classical antiquity. He was also one of the first to apply mathematics to physical phenomena, founding hydrostatics and statics and he is credited with designing innovative machines, such as his screw pump, compound pulleys, and defensive war machines to protect his native Syracuse from invasion. Archimedes died during the Siege of Syracuse when he was killed by a Roman soldier despite orders that he should not be harmed. Cicero describes visiting the tomb of Archimedes, which was surmounted by a sphere and a cylinder, unlike his inventions, the mathematical writings of Archimedes were little known in antiquity. Archimedes was born c.287 BC in the city of Syracuse, Sicily, at that time a self-governing colony in Magna Graecia. The date of birth is based on a statement by the Byzantine Greek historian John Tzetzes that Archimedes lived for 75 years, in The Sand Reckoner, Archimedes gives his fathers name as Phidias, an astronomer about whom nothing is known. Plutarch wrote in his Parallel Lives that Archimedes was related to King Hiero II, a biography of Archimedes was written by his friend Heracleides but this work has been lost, leaving the details of his life obscure. It is unknown, for instance, whether he married or had children. During his youth, Archimedes may have studied in Alexandria, Egypt and he referred to Conon of Samos as his friend, while two of his works have introductions addressed to Eratosthenes. Archimedes died c.212 BC during the Second Punic War, according to the popular account given by Plutarch, Archimedes was contemplating a mathematical diagram when the city was captured. A Roman soldier commanded him to come and meet General Marcellus but he declined, the soldier was enraged by this, and killed Archimedes with his sword. Plutarch also gives an account of the death of Archimedes which suggests that he may have been killed while attempting to surrender to a Roman soldier. According to this story, Archimedes was carrying mathematical instruments, and was killed because the thought that they were valuable items. General Marcellus was reportedly angered by the death of Archimedes, as he considered him a valuable asset and had ordered that he not be harmed. Marcellus called Archimedes a geometrical Briareus, the last words attributed to Archimedes are Do not disturb my circles, a reference to the circles in the mathematical drawing that he was supposedly studying when disturbed by the Roman soldier. This quote is given in Latin as Noli turbare circulos meos. The phrase is given in Katharevousa Greek as μὴ μου τοὺς κύκλους τάραττε
8.
Virtual work
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Virtual work arises in the application of the principle of least action to the study of forces and movement of a mechanical system. The work of a force acting on a particle as it moves along a displacement will be different for different displacements, among all the possible displacements that a particle may follow, called virtual displacements, one will minimize the action. This displacement is therefore the displacement followed by the according to the principle of least action. The work of a force on a particle along a displacement is known as the virtual work. The principle of work had always been used in some form since antiquity in the study of statics. It was used by the Greeks, medieval Arabs and Latins, working with Leibnizian concepts, Johann Bernoulli systematized the virtual work principle and made explicit the concept of infinitesimal displacement. He was able to solve problems for both bodies as well as fluids. Bernoullis version of virtual work law appeared in his letter to Pierre Varignon in 1715 and this formulation of the principle is today known as the principle of virtual velocities and is commonly considered as the prototype of the contemporary virtual work principles. In 1743 DAlembert published his Traite de Dynamique where he applied the principle of work, based on the Bernoullis work. His idea was to convert a dynamical problem into static problem by introducing inertial force, consider a point particle that moves along a path which is described by a function r from point A, where r, to point B, where r. It is possible that the moves from A to B along a nearby path described by r + δr. The variation δr satisfies the requirement δr = δr =0, the components of the variation, δr1, δr2 and δr3, are called virtual displacements. This can be generalized to a mechanical system defined by the generalized coordinates qi. In which case, the variation of the qi is defined by the virtual displacements δqi. Virtual work is the work done by the applied forces. When considering forces applied to a body in equilibrium, the principle of least action requires the virtual work of these forces to be zero. Consider a particle P that moves from a point A to a point B along a trajectory r and it is important to notice that the value of the work W depends on the trajectory r. Suppose the force F is the same as F, the variation of the work δW associated with this nearby path, known as the virtual work, can be computed to be δ W = W ¯ − W = ∫ t 0 t 1 d t
9.
Gear train
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A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term refers simply to the gearbox that uses gears and gear trains to provide speed. In British English, the term refers to the whole drivetrain, including clutch, gearbox, prop shaft, differential. In American English, however, the term more specifically to the gearbox alone. The most common use is in vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a high rotational speed, which is inappropriate for starting, stopping. The transmission reduces the engine speed to the slower wheel speed. Transmissions are also used on bicycles, fixed machines. Often, a transmission has multiple gear ratios with the ability to switch between them as speed varies and this switching may be done manually or automatically. Directional control may also be provided, single-ratio transmissions also exist, which simply change the speed and torque of motor output. The output of the transmission is transmitted via the driveshaft to one or more differentials, while a differential may also provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds as it changes the direction of rotation. Conventional gear/belt transmissions are not the mechanism for speed/torque adaptation. Alternative mechanisms include torque converters and power transformation, automatic transmissions use a valve body to shift gears using fluid pressures in conjunction with an ecm. Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, in support of pumping, milling, most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft. This means that the shaft of a gearbox rotates at a slower rate than the input shaft. A gearbox can be set up to do the opposite and provide an increase in speed with a reduction of torque. Some of the simplest gearboxes merely change the rotational direction of power transmission. Many typical automobile transmissions include the ability to select one of several gear ratios, in this case, most of the gear ratios are used to slow down the output speed of the engine and increase torque
10.
Mechanical system
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A mechanical system manages power to accomplish a task that involves forces and movement. Mechanical is derived from the Latin word machina, which in turn derives from the Doric Greek μαχανά, Ionic Greek μηχανή contrivance, machine, engine, similarly Merriam-Webster Dictionary defines mechanical as relating to machinery or tools. This can be seen in Watts steam engine in which the power is provided by expanding to drive the piston. The walking beam, coupler and crank transform the movement of the piston into rotation of the output pulley. Finally, the pulley rotation drives the flyball governor which controls the valve for the input to the piston cylinder. Power flow through a system provides a way to understand the performance of devices ranging from levers and gear trains to automobiles. Human and animal effort were the power sources for early machines. Natural forces such as wind and water powered larger mechanical systems, modern water turbines use water flowing through a dam to drive an electric generator. Windmill, Early windmills captured wind power to generate rotary motion for milling operations, modern wind turbines also drives a generator. This electricity in turn is used to drive motors forming the actuators of mechanical systems, engine, The word engine derives from ingenuity and originally referred to contrivances that may or may not be physical devices. A steam engine uses heat to boil water contained in a pressure vessel and this principle can be seen in the aeolipile of Hero of Alexandria. This is called a combustion engine. An automobile engine is called a combustion engine because it burns fuel inside a cylinder. A jet engine uses a turbine to compress air which is burned with fuel so that it expands through a nozzle to provide thrust to an aircraft, and so is also an internal combustion engine. Power plant, The heat from coal and natural gas combustion in a boiler generates steam that drives a turbine to rotate an electric generator. A nuclear power plant uses heat from a reactor to generate steam. This power is distributed through a network of lines for industrial and individual use. Motors, Electric motors use either AC or DC electric current to generate rotational movement, Electric servomotors are the actuators for mechanical systems ranging from robotic systems to modern aircraft
11.
Robot
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A robot is a machine—especially one programmable by a computer—capable of carrying out a complex series of actions automatically. Robots can be guided by a control device or the control may be embedded within. Robots may be constructed to take on human form but most robots are designed to perform a task with no regard to how they look. By mimicking a lifelike appearance or automating movements, a robot may convey a sense of intelligence or thought of its own. These technologies deal with automated machines that can take the place of humans in dangerous environments or manufacturing processes, or resemble humans in appearance, many of todays robots are inspired by nature contributing to the field of bio-inspired robotics. These robots have also created a branch of robotics, soft robotics. From the time of ancient civilization there have many accounts of user-configurable automated devices and even automata resembling animals and humans. As mechanical techniques developed through the Industrial age, there appeared more practical applications such as automated machines, remote-control and wireless remote-control. The word robot was first used to denote a fictional humanoid in a 1920 play R. U. R. by the Czech writer, Karel Čapek but it was Karels brother Josef Čapek who was the words true inventor. Electronics evolved into the force of development with the advent of the first electronic autonomous robots created by William Grey Walter in Bristol. The first digital and programmable robot was invented by George Devol in 1954 and was named the Unimate, there are concerns about the increasing use of robots and their role in society. Robots are blamed for rising unemployment as they replace workers in increasing numbers of functions, the use of robots in military combat raises ethical concerns. The possibilities of robot autonomy and potential repercussions have been addressed in fiction, the word robot can refer to both physical robots and virtual software agents, but the latter are usually referred to as bots. Closely related to the concept of a robot is the field of Synthetic Biology, the idea of automata originates in the mythologies of many cultures around the world. Engineers and inventors from ancient civilizations, including Ancient China, Ancient Greece, since circa 400 BC, myths of Crete include Talos, a man of bronze who guarded the Cretan island of Europa from pirates. In ancient Greece, the Greek engineer Ctesibius applied a knowledge of pneumatics and hydraulics to produce the first organ, in the 4th century BC, the Greek mathematician Archytas of Tarentum postulated a mechanical steam-operated bird he called The Pigeon. Hero of Alexandria, a Greek mathematician and inventor, created numerous user-configurable automated devices, the 11th century Lokapannatti tells of how the Buddhas relics were protected by mechanical robots, from the kingdom of Roma visaya, until they were disarmed by King Ashoka. Yan Shi proudly presented the king with a life-size, human-shaped figure of his mechanical handiwork made of leather, wood, in 1066, the Chinese inventor Su Song built a water clock in the form of a tower which featured mechanical figurines which chimed the hours
12.
Linkage (mechanical)
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A mechanical linkage is an assembly of bodies connected to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid, the connections between links are modeled as providing ideal movement, pure rotation or sliding for example, and are called joints. A linkage modeled as a network of links and ideal joints is called a kinematic chain. Linkages may be constructed from open chains, closed chains, or a combination of open, each link in a chain is connected by a joint to one or more other links. Thus, a chain can be modeled as a graph in which the links are paths and the joints are vertices. The movement of a joint is generally associated with a subgroup of the group of Euclidean displacements. The number of parameters in the subgroup is called the degrees of freedom of the joint, mechanical linkages are usually designed to transform a given input force and movement into a desired output force and movement. The ratio of the force to the input force is known as the mechanical advantage of the linkage. The speed ratio and mechanical advantage are defined so they yield the number in an ideal linkage. A kinematic chain, in which one link is fixed or stationary, is called a mechanism, perhaps the simplest linkage is the lever, which is a link that pivots around a fulcrum attached to ground, or a fixed point. As a force rotates the lever, points far from the fulcrum have a greater velocity than points near the fulcrum. Because power into the lever equals the power out, a force applied at a point far from the fulcrum equals a larger force applied at a point near the fulcrum. The amount the force is amplified is called mechanical advantage and this is the law of the lever. Two levers connected by a rod so that a force applied to one is transmitted to the second is known as a four-bar linkage, the levers are called cranks, and the fulcrums are called pivots. The connecting rod is also called the coupler, the fourth bar in this assembly is the ground, or frame, on which the cranks are mounted. Linkages are important components of machines and tools, examples range from the four-bar linkage used to amplify force in a bolt cutter or to provide independent suspension in an automobile, to complex linkage systems in robotic arms and walking machines. The internal combustion engine uses a slider-crank four-bar linkage formed from its piston, connecting rod, relatively simple linkages are often used to perform complicated tasks. Interesting examples of linkages include the windshield wiper, the bicycle suspension, in these examples the components in the linkage move in parallel planes and are called planar linkages
13.
Toothed belt
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A timing belt, toothed belt, cogged belt or cog belt, or synchronous belt is a non-slipping mechanical drive belt. It is made as a belt with teeth moulded onto its inner surface. It runs over matching toothed pulleys or sprockets, when correctly tensioned, they have no slippage and are often used to transfer motion for indexing or timing purposes. They are often used in lieu of chains or gears, so there is less noise, toothed belts are used widely in mechanical devices, including sewing machines, photocopiers and many others. A major use of toothed belts is as the belt used to drive the camshafts within an automobile engine. As toothed belts can also deliver more power than a friction-drive belt and these include the primary drive of some motorcycles, notably later Harley-Davidsons. Also the supercharger used for dragsters, microlight aircraft driven by high-speed two-stroke engines such as the Rotax 532 use toothed belt reduction drives to allow the use of a quieter and more efficient slower-speed propeller. Some amateur built airplanes powered by automotive engines use cog belt reduction drive units and these belts are made of a flexible polymer over a fabric reinforcement. Originally this was rubber over a natural textile, but developments in science have had a substantial effect in increasing the lifetime of these belts. This included changes from natural to synthetic rubber and polyurethane and also the adoption of nylon, Kevlar or other aramid fibres, toothed belts have two failure modes, one gradual and one catastrophic. Both increase their risk over time, so it is common for highly-stressed belts to be given a service lifetime, one failure mode is gradual wear to the tooth shape, which may eventually lead to slippage over rounded teeth. The belt often continues to work, but the timing between shafts changes. The catastrophic failure mode is caused by delamination between the belt and its fabric reinforcement, although this may be caused by age and wear, it is often accelerated by mistreatment of the belt, often during initial installation. Another cause, particularly with natural rubber belts, is contamination by oil, especially to the edges where the fabric is exposed. It is extremely rare for a belt to break. More common is for the belt to delaminate, disconnecting the fabric strength member from the teeth that ride on the pulleys, the belt is then often thrown from the pulleys and may be further damaged, or cut. Although worn teeth may be detectable by careful inspection, internal deterioration is not considered to be reliably detectable and so the observance of service lifetimes is important
14.
Block and tackle
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A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads. The pulleys are assembled together to form blocks and then blocks are paired so that one is fixed, the rope is threaded, or rove, through the pulleys to provide mechanical advantage that amplifies that force applied to the rope. Hero of Alexandria described cranes formed from assemblies of pulleys in the first century, illustrated versions of Heros book on raising heavy weights show early block and tackle systems. A block is a set of pulleys or sheaves mounted on a single axle, an assembly of blocks with a rope threaded through the pulleys is called tackle. A block and tackle system amplifies the force in the rope to lift heavy loads. They are common on boats and sailing ships, where tasks are performed manually. A block and tackle is characterized by the use of a continuous rope to transmit a tension force around one or more pulleys to lift or move a load. Its mechanical advantage is the number of parts of the rope that act on the load, the mechanical advantage of a tackle dictates how much easier it is to haul or lift the load. An ideal block and tackle with a moving block supported by n rope sections has the advantage, M A = F B F A = n. Consider the set of pulleys that form the block and the parts of the rope that support this block. If there are n of these parts of the supporting the load FB. This means the force on the rope is FA=FB/n. Thus, the block and tackle reduces the force by the factor n. Ideal mechanical advantage correlates directly with velocity ratio, the velocity ratio of a tackle is the ratio between the velocity of the hauling line to that of the hauled load. A line with an advantage of 4 has a velocity ratio of 4,1. In other words, to raise a load at 1 metre per second, therefore, the mechanical advantage of a double tackle is 4. In this case the block and tackle is said to be rove to advantage, rove to advantage – where the pull on the rope is in the same direction as that in which the load is to be moved. The hauling part is pulled from the moving block, rove to disadvantage – where the pull on the rope is in the opposite direction to that in which the load is to be moved
15.
Energy
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In physics, energy is the property that must be transferred to an object in order to perform work on – or to heat – the object, and can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. Mass and energy are closely related, for example, with a sensitive enough scale, one could measure an increase in mass after heating an object. Living organisms require available energy to stay alive, such as the humans get from food. Civilisation gets the energy it needs from energy resources such as fuels, nuclear fuel. The processes of Earths climate and ecosystem are driven by the radiant energy Earth receives from the sun, the total energy of a system can be subdivided and classified in various ways. It may also be convenient to distinguish gravitational energy, thermal energy, several types of energy, electric energy. Many of these overlap, for instance, thermal energy usually consists partly of kinetic. Some types of energy are a mix of both potential and kinetic energy. An example is energy which is the sum of kinetic. Whenever physical scientists discover that a phenomenon appears to violate the law of energy conservation. Heat and work are special cases in that they are not properties of systems, in general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from, the distinctions between different kinds of energy is not always clear-cut. In contrast to the definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two, in 1807, Thomas Young was possibly the first to use the term energy instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described kinetic energy in 1829 in its modern sense, the law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat and these developments led to the theory of conservation of energy, formalized largely by William Thomson as the field of thermodynamics
16.
Compound lever
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Almost all scales use some sort of compound lever to work. Other examples include nail clippers and piano keys, a lever arm uses the fulcrum to lift the load using and intensifying an applied force. In these conditions, combinations of levers, called compound levers, are used. Compound levers can be constructed from first, second and/or third-order levers, in all types of compound lever, the rule is that force multiplied by the force arm equals the weight multiplied by the weight arm. The output from one lever becomes the input for the lever in the system. The figure on the left illustrates a compound formed from two first-class levers, along with a short derivation of how to compute the mechanical advantage. With the dimensions shown, the advantage, W/F can be calculated as 9 × 10/3 ×4 =7.5. This is not usually the goal of a lever system. The distances used in calculation of mechanical advantage are measured perpendicular to the force, in the example of a nail clipper on the right, because the effort is applied vertically, distances to the respective fulcrums are measured horizontally, instead of along the lever. In this example, W/F is × 6/1 × =6, note that cm =8 cm is the distance from the point of application of the effort to the fulcrum of the first lever, perpendicular to the applied effort. A few examples of the lever are the scale, train brakes. Another example is the elbow-joint press, which is used in printing, molding or handloading bullets, minting coins and medals, compound balances are used to weigh heavy items. These all use levers to magnify force to accomplish a specific purpose. The train brake translates the force of pushing back the stick to the levers and they rub against the wheels, using friction to slow and these are everyday applications of this mechanism. A piano key is a lever of the first-class, since the fulcrum is between the weight to be moved and the power. The purpose of this lever is to translate a small movement into a larger, the quality of the resulting tone depends on whether the final speed is brought about by gradual or sudden movement of the key. The earliest remaining writings regarding levers date from the 3rd century BC and were provided by Archimedes, give me a place to stand, and I shall move the earth with a lever is a remark attributed to Archimedes, who formally stated the correct mathematical principle of levers
17.
Simple machine
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A simple machine is a mechanical device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage to multiply force, ignoring friction losses, the work done on the load is equal to the work done by the applied force. The machine can increase the amount of the force, at the cost of a proportional decrease in the distance moved by the load. The ratio of the output to the force is called the mechanical advantage. Simple machines can be regarded as the building blocks of which all more complicated machines are composed. For example, wheels, levers, and pulleys are all used in the mechanism of a bicycle, the mechanical advantage of a compound machine is just the product of the mechanical advantages of the simple machines of which it is composed. The great variety and sophistication of modern machine linkages, which arose during the Industrial Revolution, is described by these six simple categories. By the late 1800s, Franz Reuleaux had identified hundreds of machine elements, Models of these devices may be found at Cornell Universitys Kinematic Models for Design website. The idea of a simple machine originated with the Greek philosopher Archimedes around the 3rd century BC, who studied the Archimedean simple machines, lever, pulley and he discovered the principle of mechanical advantage in the lever. Archimedes famous remark with regard to the lever, Give me a place to stand on, expresses his realization that there was no limit to the amount of force amplification that could be achieved by using mechanical advantage. Later Greek philosophers defined the five simple machines and were able to roughly calculate their mechanical advantage. For example, Heron of Alexandria in his work Mechanics lists five mechanisms that can set a load in motion, lever, windlass, pulley, wedge, and screw, and describes their fabrication and uses. However the Greeks understanding was limited to the statics of simple machines, the balance of forces, and did not include dynamics, in 1586 Flemish engineer Simon Stevin derived the mechanical advantage of the inclined plane, and it was included with the other simple machines. The complete dynamic theory of machines was worked out by Italian scientist Galileo Galilei in 1600 in Le Meccaniche. He was the first to understand that simple machines do not create energy and they were rediscovered by Guillaume Amontons and were further developed by Charles-Augustin de Coulomb. Although each machine works differently mechanically, the way they function is similar mathematically, in each machine, a force F in is applied to the device at one point, and it does work moving a load, F out at another point. Simple machines do not contain a source of energy, so they cannot do more work than they receive from the input force, a simple machine with no friction or elasticity is called an ideal machine. Similarly the power input from the force is equal to the velocity of the input point v in multiplied by the applied force P in = F in v in
18.
Mechanical advantage device
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A simple machine that exhibits mechanical advantage is called a mechanical advantage device - e. g. Lever, The beam shown is in static equilibrium around the fulcrum. This is due to the moment created by vector force A counterclockwise being in equilibrium with the moment created by vector force B clockwise, the relatively low vector force B is translated in a relatively high vector force A. The force is increased in the ratio of the forces A, B, which is equal to the ratio of the distances to the fulcrum b. This ratio is called the mechanical advantage and this idealised situation does not take into account friction. Wheel and axle motion, A wheel is essentially a lever with one arm the distance between the axle and the point of the wheel, and the other the radius of the axle. Typically this is a large difference, leading to a proportionately large mechanical advantage. A block and tackle of multiple pulleys creates mechanical advantage, by having the flexible material looped over several pulleys in turn, adding more loops and pulleys increases the mechanical advantage. Screw, A screw is essentially an inclined plane wrapped around a cylinder, the run over the rise of this inclined plane is the mechanical advantage of a screw. Consider lifting a weight with rope and pulleys, a rope looped through a pulley attached to a fixed spot, e. g. a barn roof rafter, and attached to the weight is called a single pulley. It has an mechanical advantage =1, moving no mechanical advantage however advantageous the change in direction may be, a single movable pulley has an MA of 2. Consider a pulley attached to a weight being lifted, a rope passes around it, with one end attached to a fixed point above, e. g. a barn roof rafter, and a pulling force is applied upward to the other end with the two lengths parallel. In this situation the distance the lifter must pull the rope becomes twice the distance the weight travels, by looping more ropes around more pulleys we can construct a block and tackle to continue to increase the mechanical advantage. Again note, if we add another pulley so that someone may stand on the ground and pull down, here are examples where the fixed point is not obvious, A velcro strap on a shoe passes through a slot and folds over on itself. The slot is a pulley and the MA =2. Two ropes laid down a ramp attached to a raised platform, a barrel is rolled onto the ropes and the ropes are passed over the barrel and handed to two workers at the top of the ramp. The workers pull the ropes together to get the barrel to the top, the barrel is a movable pulley and the MA =2. If there is enough friction where the rope is pinched between the barrel and the ramp, the point becomes the attachment point. This is considered a fixed attachment point because the rope above the barrel does not move relative to the ramp, alternatively the ends of the rope can be attached to the platform
19.
Chain drive
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Chain drive is a way of transmitting mechanical power from one place to another. It is often used to power to the wheels of a vehicle. It is also used in a variety of machines besides vehicles. The gear is turned, and this pulls the chain putting mechanical force into the system, another type of drive chain is the Morse chain, invented by the Morse Chain Company of Ithaca, New York, United States. Sometimes the power is output by simply rotating the chain, which can be used to lift or drag objects, in other situations, a second gear is placed and the power is recovered by attaching shafts or hubs to this gear. By varying the diameter of the input and output gears with respect to each other, for example, when the bicycle pedals gear rotate once, it causes the gear that drives the wheels to rotate more than one revolution. The oldest known application of a chain appears in the Polybolos. Two flat-linked chains were connected to a windlass, which by winding back and it is here that the flat-link chain, often attributed to Leonardo da Vinci, actually made its first appearance. The chain drive itself was given power via the works of Sus water clock tank and waterwheel. Roller chain and sprockets is an efficient method of power transmission compared to belts. Although chains can be stronger than belts, their greater mass increases drive train inertia. Drive chains are most often made of metal, while belts are often rubber, plastic, urethane, or other substances. Drive belts can slip unless they have teeth, which means that the side may not rotate at a precise speed. Wear on rubber or plastic belts and their teeth is often easier to observe, one problem with roller chains is the variation in speed, or surging, caused by the acceleration and deceleration of the chain as it goes around the sprocket link by link. It starts as soon as the line of the chain contacts the first tooth of the sprocket. This contact occurs at a point below the circle of the sprocket. As the sprocket rotates, the chain is raised up to the circle and is then dropped down again as sprocket rotation continues. This rising and falling of the line is what causes chordal effect or speed variation
20.
Belt (mechanical)
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A belt is a loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. Belts may be used as a source of motion, to transmit power efficiently, belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel. In a two pulley system, the belt can either drive the pulleys normally in one direction, or the belt may be crossed, as a source of motion, a conveyor belt is one application where the belt is adapted to carry a load continuously between two points. Belts are the cheapest utility for power transmission between shafts that may not be axially aligned, power transmission is achieved by specially designed belts and pulleys. The demands on a transmission system are large, and this has led to many variations on the theme. They run smoothly and with noise, and cushion motor and bearings against load changes. However, improvements in belt engineering allow use of belts in systems that only formerly allowed chains or gears. They are related as T1 T2 = e μ α, where, μ is the coefficient of friction, Belt drives are simple, inexpensive, and do not require axially aligned shafts. They help protect machinery from overload and jam, and damp and isolate noise and they need no lubrication and minimal maintenance. They have high efficiency, high tolerance for misalignment, and are of low cost if the shafts are far apart. Clutch action is activated by releasing belt tension, different speeds can be obtained by stepped or tapered pulleys. The angular-velocity ratio may not be constant or equal to that of the diameters, due to slip. However, this problem has largely solved by the use of toothed belts. Working temperatures range from −31 °F to 185 °F, adjustment of center distance or addition of an idler pulley is crucial to compensate for wear and stretch. Flat belts were used in the 19th and early 20th centuries in line shafting to transmit power in factories. Flat belts are used today, although not nearly as much as in the line-shaft era. The flat belt is a system of power transmission that was well suited for its day. It can deliver power at high speeds, in cases of wide belts
21.
Roller chain
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It consists of a series of short cylindrical rollers held together by side links. It is driven by a wheel called a sprocket. It is a simple, reliable, and efficient means of power transmission, though Hans Renold is credited with inventing the roller chain in 1880, sketches by Leonardo da Vinci in the 16th century show a chain with a roller bearing. There are actually two types of links alternating in the roller chain. The first type is inner links, having two inner plates held together by two sleeves or bushings upon which rotate two rollers. Inner links alternate with the type, the outer links. This has the advantage of removing one step in assembly of the chain, the roller chain design reduces friction compared to simpler designs, resulting in higher efficiency and less wear. This problem was solved by the development of bushed chains. This distributed the wear over an area, however the teeth of the sprockets still wore more rapidly than is desirable. There is even very low friction, as long as the chain is sufficiently lubricated, continuous, clean, lubrication of roller chains is of primary importance for efficient operation as well as correct tensioning. Many driving chains operate in environments, and thus the wearing surfaces are safe from precipitation and airborne grit. Some roller chains are designed to have built into the space between the outside link plate and the inside roller link plates. Chain manufacturers began to include this feature in 1971 after the application was invented by Joseph Montano while working for Whitney Chain of Hartford, Connecticut. O-rings were included as a way to improve lubrication to the links of power transmission chains and these rubber fixtures form a barrier that holds factory applied lubricating grease inside of the pin and bushing wear areas. Further, the rubber o-rings prevent dirt and other contaminants entering the inside of the chain linkages. There are also many chains that have to operate in dirty conditions, examples include chains on farm equipment, bicycles, and chain saws. Chains operating at speeds comparable to those on motorcycles should be used in conjunction with an oil bath. For modern motorcycles this is not possible, and most motorcycle chains run unprotected, thus, motorcycle chains tend to wear very quickly relative to other applications
22.
Bicycle chain
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A bicycle chain is a roller chain that transfers power from the pedals to the drive-wheel of a bicycle, thus propelling it. Most bicycle chains are made from carbon or alloy steel. Obsolete chain designs previously used on bicycles included the chain, the skip-link chain. The first chains were of a simple, bushingless design and these had inherent reliability problems and a bit more friction than modern chains. With these limitations in mind, the Nevoigt brothers, of the German Diamant Bicycle Company, designed the roller chain in 1898, which uses bushings, and it is the prevalent chain today. Whether it be singlespeed, fixed gear (such as track bikes and modern urban fixies, or multi-speeds with derailleurs, early examples of chain-driven bicycles include the 1869 Guilmet and Meyer, the 1879 Lawson, the 1884 McCammon, the 1884 Starley Rover, and the 1895 Diamant. Various linkage mechanisms were invented to raise the gear ratio. A bicycle chain can be energy efficient, one study reported efficiencies as high as 98. 6%. The study, performed in a laboratory environment, found that efficiency was not greatly affected by the state of lubrication. A larger sprocket will give an efficient drive, because moves the point of pressure farther away from the axle, placing less stress on the bearings. Higher chain tension was found to be efficient, This is actually not in the direction youd expect. How best to lubricate a bicycle chain is a debated question among cyclists. Liquid lubricants penetrate to the inside of the links and are not easily displaced, dry lubricants, often containing wax or Teflon, are transported by an evaporating solvent, and stay cleaner in use. The cardinal rule for long life is never to lubricate a dirty chain. Chains should be cleaned before lubrication, the chain should be wiped dry after the lubricant has had enough time to penetrate the links. An alternative approach is to change the very frequently, then proper care is less important. Some utility bicycles have fully enclosing chain guards, which virtually eliminate chain wear, on recumbent bicycles the chain is often run through tubes to prevent it from picking up dirt, and to keep the cyclists leg free from oil and dirt. On most upright bicycles, the chain loops through the rear triangle made by the right chain stay
23.
Bicycle gearing
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Bicycle gearing is the aspect of a bicycle drivetrain that determines the relation between the cadence, the rate at which the rider pedals, and the rate at which the drive wheel turns. On some bicycles, there is only one gear and the ratio is fixed. Many contemporary bicycles have multiple gears and thus multiple gear ratios, different gear ratios and gear ranges are appropriate for different people and styles of cycling. A cyclists legs produce power optimally within a narrow pedalling speed range, gearing can be optimized to use this narrow range as efficiently as possible. As in other types of transmissions, the ratio is closely related to the mechanical advantage of the drivetrain of the bicycle. On single-speed bicycles and multi-speed bicycles using derailleur gears, the ratio depends on the ratio of the number of teeth on the chainring to the number of teeth on the rear sprocket. For bicycles equipped with hub gears, the gear ratio depends on the internal planetary gears within the hub. For a shaft-driven bicycle the gear ratio depends on the bevel gears used at end of the shaft. For a bicycle to travel at the speed, using a lower gear requires the rider to pedal at a faster cadence. Conversely, a higher gear provides a speed for a given cadence. Different cyclists may have different preferences for cadence and pedaling force, there are at least four different methods for measuring gear ratios, gear inches, metres of development, gain ratio, and quoting the number of teeth on the front and rear sprockets respectively. The fourth method uses two numbers and is useful in comparing bicycles with the same drive wheel diameter. In the case of road bikes, this is usually around 670 mm, a 700c standard wheel has a 622mm rim diameter. The final wheel diameter depends on the tire but will be approximately 622mm plus twice the tire width. Front/rear measurement only considers the sizes of a chainring and a rear sprocket, gear inches and metres of development also take the size of the rear wheel into account. Gain ratio goes further and also takes the length of a pedal crankarm into account, gear inches and metres of development are closely related, to convert from gear inches to metres of development, multiply by 0.08. The methods of calculation which follow assume that any hub gear is in direct drive, multiplication by a further factor is needed to allow for any other selected hub gear ratio. Gear inches = Diameter of drive wheel in inches ×, normally rounded to nearest whole number
24.
Transmission (mechanics)
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A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term refers simply to the gearbox that uses gears and gear trains to provide speed. In British English, the term refers to the whole drivetrain, including clutch, gearbox, prop shaft, differential. In American English, however, the term more specifically to the gearbox alone. The most common use is in vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a high rotational speed, which is inappropriate for starting, stopping. The transmission reduces the engine speed to the slower wheel speed. Transmissions are also used on bicycles, fixed machines. Often, a transmission has multiple gear ratios with the ability to switch between them as speed varies and this switching may be done manually or automatically. Directional control may also be provided, single-ratio transmissions also exist, which simply change the speed and torque of motor output. The output of the transmission is transmitted via the driveshaft to one or more differentials, while a differential may also provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds as it changes the direction of rotation. Conventional gear/belt transmissions are not the mechanism for speed/torque adaptation. Alternative mechanisms include torque converters and power transformation, automatic transmissions use a valve body to shift gears using fluid pressures in conjunction with an ecm. Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, in support of pumping, milling, most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft. This means that the shaft of a gearbox rotates at a slower rate than the input shaft. A gearbox can be set up to do the opposite and provide an increase in speed with a reduction of torque. Some of the simplest gearboxes merely change the rotational direction of power transmission. Many typical automobile transmissions include the ability to select one of several gear ratios, in this case, most of the gear ratios are used to slow down the output speed of the engine and increase torque
25.
On the Equilibrium of Planes
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On the Equilibrium of Planes is a treatise by Archimedes in two volumes. The first book establishes the law of the lever, and locates the centre of gravity of the triangle, according to Pappus of Alexandria, Archimedes work on levers caused him to remark, Give me a place to stand on, and I will move the Earth. The second book, which contains ten propositions, examines the centres of gravity of parabolic segments, book one contains fifteen propositions with seven postulates. In proposition six Archimedes establishes the law of the lever, concluding that Magnitudes are in equilibrium at distances proportional to their weights. In propositions ten and fourteen, respectively, Archimedes locates the centre of gravity of the parallelogram, additionally, in proposition 15, he establishes the centre of gravity of the trapezium. The second book, which contains ten propositions, studies parabolic segments exclusively and it examines these segments by substituting them with rectangles of equal area, an exchange made possible by results obtained in the Quadrature of the Parabola. Archimedes proof of the law of the lever is executed within proposition six and it is for commensurable magnitudes only, and relies upon propositions four, and five, and on postulate one. In postulate one Archimedes states that Equal weights at equal distances are in equilibrium, now, double the length of ED by duplicating the longer arm on the left, and the shorter arm on the right. Note, additionally, that EH carries the common divisor N, a number of times, as does EC. It remains then to prove that A applied at E, and B applied at D, will have their centre of gravity at C. Therefore, as the ratio of LH to HK is not four to three, but eight to six, similarly divide the magnitudes A and B, and align them as per the diagram opposite. Further, the system consists of an even number of equal weights equally distributed. Thus A applied at E, and B applied at D, have their centre of gravity at C, whilst the authenticity of book two is not doubted, a number of researches have highlighted inconsistencies within book ones presentation. Berggren, in particular, questions the validity of one as a whole, highlighting, inter alia. However, Berggren follows Dijksterhuis, in rejecting Machs criticism of proposition six, further, it should be noted that proposition seven is incomplete in its current form, so that book one demonstrates the law of the lever for commensurable magnitudes only
26.
International Standard Book Number
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The International Standard Book Number is a unique numeric commercial book identifier. An ISBN is assigned to each edition and variation of a book, for example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, the method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country. The initial ISBN configuration of recognition was generated in 1967 based upon the 9-digit Standard Book Numbering created in 1966, the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108. Occasionally, a book may appear without a printed ISBN if it is printed privately or the author does not follow the usual ISBN procedure, however, this can be rectified later. Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines, the ISBN configuration of recognition was generated in 1967 in the United Kingdom by David Whitaker and in 1968 in the US by Emery Koltay. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108, the United Kingdom continued to use the 9-digit SBN code until 1974. The ISO on-line facility only refers back to 1978, an SBN may be converted to an ISBN by prefixing the digit 0. For example, the edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has SBN340013818 -340 indicating the publisher,01381 their serial number. This can be converted to ISBN 0-340-01381-8, the check digit does not need to be re-calculated, since 1 January 2007, ISBNs have contained 13 digits, a format that is compatible with Bookland European Article Number EAN-13s. An ISBN is assigned to each edition and variation of a book, for example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, a 13-digit ISBN can be separated into its parts, and when this is done it is customary to separate the parts with hyphens or spaces. Separating the parts of a 10-digit ISBN is also done with either hyphens or spaces, figuring out how to correctly separate a given ISBN number is complicated, because most of the parts do not use a fixed number of digits. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for country or territory regardless of the publication language. Some ISBN registration agencies are based in national libraries or within ministries of culture, in other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded. In Canada, ISBNs are issued at no cost with the purpose of encouraging Canadian culture. In the United Kingdom, United States, and some countries, where the service is provided by non-government-funded organisations. Australia, ISBNs are issued by the library services agency Thorpe-Bowker
27.
OCLC
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The Online Computer Library Center is a US-based nonprofit cooperative organization dedicated to the public purposes of furthering access to the worlds information and reducing information costs. It was founded in 1967 as the Ohio College Library Center, OCLC and its member libraries cooperatively produce and maintain WorldCat, the largest online public access catalog in the world. OCLC is funded mainly by the fees that libraries have to pay for its services, the group first met on July 5,1967 on the campus of the Ohio State University to sign the articles of incorporation for the nonprofit organization. The group hired Frederick G. Kilgour, a former Yale University medical school librarian, Kilgour wished to merge the latest information storage and retrieval system of the time, the computer, with the oldest, the library. The goal of network and database was to bring libraries together to cooperatively keep track of the worlds information in order to best serve researchers and scholars. The first library to do online cataloging through OCLC was the Alden Library at Ohio University on August 26,1971 and this was the first occurrence of online cataloging by any library worldwide. Membership in OCLC is based on use of services and contribution of data, between 1967 and 1977, OCLC membership was limited to institutions in Ohio, but in 1978, a new governance structure was established that allowed institutions from other states to join. In 2002, the structure was again modified to accommodate participation from outside the United States. As OCLC expanded services in the United States outside of Ohio, it relied on establishing strategic partnerships with networks, organizations that provided training, support, by 2008, there were 15 independent United States regional service providers. OCLC networks played a key role in OCLC governance, with networks electing delegates to serve on OCLC Members Council, in early 2009, OCLC negotiated new contracts with the former networks and opened a centralized support center. OCLC provides bibliographic, abstract and full-text information to anyone, OCLC and its member libraries cooperatively produce and maintain WorldCat—the OCLC Online Union Catalog, the largest online public access catalog in the world. WorldCat has holding records from public and private libraries worldwide. org, in October 2005, the OCLC technical staff began a wiki project, WikiD, allowing readers to add commentary and structured-field information associated with any WorldCat record. The Online Computer Library Center acquired the trademark and copyrights associated with the Dewey Decimal Classification System when it bought Forest Press in 1988, a browser for books with their Dewey Decimal Classifications was available until July 2013, it was replaced by the Classify Service. S. The reference management service QuestionPoint provides libraries with tools to communicate with users and this around-the-clock reference service is provided by a cooperative of participating global libraries. OCLC has produced cards for members since 1971 with its shared online catalog. OCLC commercially sells software, e. g. CONTENTdm for managing digital collections, OCLC has been conducting research for the library community for more than 30 years. In accordance with its mission, OCLC makes its research outcomes known through various publications and these publications, including journal articles, reports, newsletters, and presentations, are available through the organizations website. The most recent publications are displayed first, and all archived resources, membership Reports – A number of significant reports on topics ranging from virtual reference in libraries to perceptions about library funding
28.
John Tzetzes
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John Tzetzes was a Byzantine poet and grammarian, known to have lived at Constantinople during the 12th century. His most important work is the Chiliades, Tzetzes was Greek on his fathers side and Iberian on his mothers side. Tzetzes was described as vain, seems to have resented any attempt at rivalry, owing to a lack of written material, he was obliged to trust to his memory, therefore caution has to be exercised in reading his work. However, he was learned, and made a contribution to the furtherance of the study of ancient Greek literature. The most important of his works is considered to be the Book of Histories. The whole production suffers from a display of learning, the total number of authors quoted being more than 400. The author subsequently brought out an edition with marginal notes in prose and verse. His collection of 107 Letters addressed partly to fictitious personages, and partly to the men and women of the writers time. Tzetzes supplemented Homers Iliad by a work that begins with the birth of Paris, in the Antehomerica, Tzetzes recalled the events taking place before Homers Iliad. This work was followed by the Homerica, covering the events of the Iliad, all three are currently available in English translations. For the other works of Tzetzes see J. A. Fabricius, Bibliotheca graeca, xi.228, litt. monograph by G. Hart, De Tzetzarum nomine, vitis, scriptis, in Jahns Jahrbucher für classische Philologie. This article incorporates text from a now in the public domain, Chisholm, Hugh, ed. Tzetzes. Gautier, Paul, La curieuse ascendance de Jean Tzetzes, revue des Études Byzantines,28, 207-20
