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Mechanical Engineering Principles 3Rd Edition

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Mechanical Engineering Principles 3Rd Edition Glossary of Terms Acceleration: The amount by which the velocity of an object increases in a certain time. Acceleration of free fall: The acceleration experienced by bodies falling freely in the Earth’s gravitational field. It varies from place to place around the globe, but is assigned a standard value of 9.80665 m / s2 , called ‘g’. Ignoring air resistance, the acceleration does not vary with the size or shape of the falling body. The value of ‘g’ on the equator ≈ 9.78 m / s2 is less than its value at the poles, where g ≈ 9.83 . Anemometer: An instrument for measuring wind speed. It consists of three cups affixed to an upright length of metal, which in turn drives a mechanism that adjusts a dial. The cups are blown round by the wind, and the speed of the wind can be read from the dial. Angular acceleration: The rate of change of angular velocity. Angular momentum: The product of the moment of inertia I and the angular velocity ω of an object. Angular velocity: The rate of change of an object’s angular position relative to a fixed point. Archimedes’ principle: A body immersed in a fluid is pushed up by a force equal to the weight of the displaced fluid. Atmospheric pressure: The downward force exerted by the atmosphere because of its weight, (gravitational attraction to the Earth), measured by barometers, and usually expressed in units of millibars. Standard atmospheric pressure at sea level is 1013.25 mb. Bar: Unit of pressure – the pressure created by a column of mercury 75.006 cm high at 0 °C, or about 33.45 feet of water at 4 °C. It is equal to 105 Pascal. Standard atmospheric pressure (at sea level) is 1.01325 bar, or 1013.25 mb. Barometer: An instrument for measuring atmospheric pressure. There are two main types – the mercury barometer, and the aneroid barometer. 128 Bernoulli’s law: For a steadily flowing fluid (liquid or gas), the sum of the pressure, kinetic energy per unit volume and potential energy per unit volume is constant at any point in the fluid. Using this relationship, it is possible to measure the velocity of a fluid by measuring its pressure at two points, as with a manometer or Pitot tube. Boyle’s law: The volume of a gas at constant temperature is inversely proportional to the pressure. This means that as pressure increases, the volume of a gas decreases. Buoyancy: The upward pressure exerted on an object by the fluid in which it is immersed. The object is subjected to pressure from all sides, but the pressure on its lower part is greater because of the increasing depth of the fluid. The result of all these pressures is a force acting upwards that is equal to the weight of the fluid displaced. Calorie: A unit of heat. A calorie is the amount of heat required to raise 1 g of water by 1 °C between the temperatures of 14.5 °C and 15.5 °C. The SI system uses the joule (1 calorie = 4.184 joules) instead of the calorie. 1000 gram calories = 3.968 Btu (British thermal unit). 1 J = 1 N m. Celsius: The temperature scale based on the freezing point of water (0 °C) and the boiling point of water (100 °C). The interval between these points is divided into 100 degrees. The scale was devised by Anders Celsius. Centre of gravity: Point at which the weight of a body can be considered to be concentrated and around which its weight is evenly balanced. In a uniform gravitational field, the centre of gravity is the same as the centre of mass. Centripetal force: In circular or curved motion, the force acting on an object that keeps it moving in a circular path. For example, if an object attached to a rope is swung in a circular motion above a person’s head, the centripetal force acting on the object is the tension in the rope. Similarly, the centripetal force acting on the Earth as it orbits the sun is gravity. In accordance with Newton’s laws, the reaction to this can be regarded as a centrifugal force, equal in magnitude and opposite in direction. 129 Change of state: The change that takes place when matter turns from one physical phase (gas, liquid or solid) into another. Charles’ law: The volume of a gas at constant pressure is directly proportional to its absolute temperature. Coefficient of cubic expansion: The fractional increase in volume per unit temperature rise. Coefficient of friction: The number characterising the force necessary to slide or roll one material along the surface of another. If an object has a weight N and the coefficient of friction is µ, then the force F necessary to move it without acceleration along a level surface is F = µN. The coefficient of static friction determines the force necessary to initiate movement; the coefficient of kinetic friction determines the force necessary to maintain movement. Kinetic friction is usually smaller than static friction. Coefficient of linear expansion: The fractional increase in length per unit temperature rise. Coefficient of superficial expansion: The fractional increase in area per unit temperature rise. Conduction, thermal: The transfer of heat from a hot region of a body to a cold region. Conservation of energy, law of: States that energy cannot be created or destroyed. Convection: The transfer of heat by flow of currents within fluids due to kinetic theory. Couple: Two equal and opposite parallel forces, which do not act in the same line. The forces produce a turning effect or torque. Dalton’s law: The pressure exerted by each gas in a mixture of gases does not depend on the pressures of the other gases, provided no chemical reaction occurs. The total pressure of such a mixture is therefore the sum of the partial pressures exerted by each gas (as if it were alone in the same volume as the mixture occupies). Density: The ratio of mass to volume for a given substance expressed in SI units as kilograms per cubic metre. The symbol for density is ρ (Greek rho). Ductility: Ability of metals and some other materials to be stretched without being weakened. 130 Dynamics: The branch of mechanics that deals with objects in motion. Its two main branches are kinematics, which studies motion without regards to its cause, and kinetics, which also takes into account forces that cause motion. Efficiency: The work a machine does (output) divided by the amount of work put in (input), usually expressed as a percentage. For simple machines, efficiency can be defined as the force ratio (mechanical advantage) divided by the distance ratio (velocity ratio). Elasticity: Capability of a material to recover its size and shape after deformation by stress. When an external force (stress) is applied, the material develops strain (a change in dimension). If a material passes its elastic limit, it will not return to its original shape. Energy: The capacity for doing work; it is measured in joules. Equilibrium: A stable state in which forces acting on a particle or object negate each other, resulting in no net force. Expansion: A change in the size of an object with change in temperature. Most substances expand on heating, although there are exceptions – water expands when it cools from 4 °C to its freezing point at 0 °C. Fahrenheit: The temperature scale based on the freezing point of water (32 °F) and the boiling point of water (212 °F). The interval between these points is divided into 180 equal parts. Although replaced by the Celsius scale, the Fahrenheit scale is still sometimes used for non-scientific measurements. Fluid: Any substance that is able to flow. Of the three common states of matter, gas and liquid are considered fluid, while any solid is not. Force: A push, a pull or a turn. A force acting on an object may (1) balance an equal but opposite force or a combination of forces to maintain the object in equilibrium (so that it does not move), (2) change the state of motion of the object (in magnitude or direction), or (3) change the shape or state of the object. The unit of force is the Newton. 131 Force ratio: The factor by which a simple machine multiplies an applied force. It is the ratio of the load (output force) to the effort (input force). Free fall: The state of motion of an unsupported body in a gravitational field. Freezing point: The temperature at which a substance changes phase (or state) from liquid to solid. The freezing point for most substances increases as pressure increases. The reverse process, from solid to liquid, is melting; melting point is the same as freezing point. Friction: The resistance encountered when surfaces in contact slide or roll against each other, or when a fluid (liquid or gas) flows along a surface. Friction is directly proportional to the force pressing the surfaces together and the surface roughness. Before the movement begins, it is opposed by static friction up to a maximum ‘limiting friction’ and then slipping occurs. Fulcrum: Point about which a lever pivots. Gear wheel: Is usually toothed, attached to a rotating shaft. The teeth of one gear engage those of another to transmit and modify rotary motion and torque. The smaller member of a pair of gears is called a pinion. If the pinion is on the driving shaft, speed is reduced and turning force increased. If the larger gear is on the driving shaft, speed is increased and turning force reduced. A screw-type driving gear, called a worm, give the driven gear a greatly reduced speed.
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