01Units physical0213.docx

1、01Units physical0213Units, physical fundamental termsWhen measuring it is compared with fixed measuring units. Lengths are measured with a metering rule in centimetres and millimetre, units of weight with a scale in kilogrammes and grammes, time is measured with a clock in hours and minutes and temp

2、erature is measured with a thermometer in degrees.These units are defined in the “international unit system” (SI).The following basic units have a relevance for mechanical measurements: meter (m) for the length, kilogramme (kg) for the weight, second (s) for the time.Furthermore, there are basic uni

3、ts for electrical amperage (Ampere), voltage (Volt), temperature (Kelvin) and for intensity of light 光强(Candela烛点).Further measuring units can be derived from the basic units.These are either decimal units or larger units, e.g. decimetre公寸, millimetre毫米, kilometre etc. gramme克, tonne etc. minute, ho

4、ur etc.Length unitsMeter (m) is the basic length unit.1 m = 10 dm = 100 cm = 1 000 mm = 1 000 000 m (micrometer)Inches are used in Great Britain and the USA.1 Inch = 25,4 mmWeight unitsKilogramme (kg) is the unit of weight.1 kg = 1 000 gNewton (N) is the unit of gravity重力.1 N = 0,1 kgSI-Basic unitsT

5、ermsSymbolUnitCommentsTermsSymbolUnitCommentsNameAbbreviationNameAbbreviationLength长度LMetremTorqueMNewtonmetreNmM = F lWidth 宽BMass 质量mKilogrammekg1 kg = 1000 gHeight, depth 高，深HWeight 重量GrammegRadius 半径RTonnet1 t = 1000 kgDiametre 直径DTemperatureTKelvinK0 K = -273CDistance 距离StCelsiusC0C = 273 KArea

6、ASquare metremCapacity 功率PWattWkWTimeTSecondsPressure pPascalPa100000 Pa =PeriodeMinutemin1 min = 60 sbar1 barHourh1 h = 60 minElectrical voltageUVoltVU = R IDurationDayd1 d = 24 hYearaElectrical currentIAmpreAVelocity 速度VMetre/secondm/s1 m/s = 3,6 km/hElectricalresistanceROhmKilometre/Hourkm/hAccel

7、arationASquareMetre/secondm/sWork做 功WJouleJAcceleration of fallGm/s9,81 m/sEnergy 能量E, W1 J = 1 Ws= 1 NmForce 力FNewtonNF = m aGravity 重力GG = m gMultiple and parts of unitsTask: Enter the derived decimal or the multiple units in the chart below!NameSymbolFactorExampleTaskGiga十亿G1 000 000 000 1 GW = 1

8、 000 000 000 W0,02 GW兆瓦 =20 000 kWMega百万M1 000 0001 MW = 1 000 000 W50 MW =50 000 kWKilo千k1 0001 kN = 1 000 N350 N =0,35 kNHecto百h1001 hPa = 100 Pa5500 Pa =55 hPaDeca十da101 daN = 10 N1000 daN =10000 NDecid0,11 dm = 0,1 m600 m =6000 dmCentic0,011 cm = 0,01 m253 m =25 300 cmMillim0,0011 mm = 0,001 m56

9、3 mV =0,563 VMicro0,000 0011 m = 0,000 001 m20 cm =200 000 mNanon0,000 000 0011 nm = 0,000 000 001 m33 nm =0,000 003 3 cmForcesDifferent forces are made use of in the mechanics机械学. The oldest force is the use of human and animal muscular strength. Today muscular strength still plays an important rol

10、e although by modern technology machines are replacing it more and more.If a force operates on an object, it can either cause a deformation, acceleration or an equal counterpoise平衡 of/to the object.AT_00_0056AT_00_0054Force = mass acceleration F = m aThe derived SI-unit of force is Newton (N).On the

11、 earths surface dominates an acceleration force of a = 9,81 m/s2.重力加速度A force is clearly defined bypoint of application - size direction of the line of action.The force is shown by a drawn arrow. The dimension of the force results from the length of the arrow according to the chosen force measuring

12、unit.Should a force be transferred and distributed in two directions the dimension of the partial forces can also be determined by the use of the parallelogram平行四边形 of forces.力的定义力是物体之间的相互作用，能使物体的运动状态发生改变，或使物体变形。在理论力学中讨论的主要对象为质点或刚体，则力的作用效果只改变其运动状态。在国际单位制中，力的单位为kg m/s2，或称为牛顿(N)。AT_00_0055Lever and To

13、rqueThe lever possesses two lever arms with a common pivot支点. If the pivotal point is situated位于 between the two forces then it concerns a double-sided lever.The turning point受力点 of a one sided lever lies at the end of the lever.A torque is being produced on both lever arms by both forces.A lever is

14、 balanced when the anticlockwise moment力矩 is equal to the clockwise moment.In physics every body is called lever on which a force causes a rotating action e.g. crowbars撬棍, pliers and toothed wheels. The rotating action is called torque.The torque grows with the increasing force and the length of the

15、 lever arm.AT_00_0052AT_00_0053Torque = force lever arm M = F rThe newtonmetre (Nm) is the derived SI-unit of the torque.The properties of torques appear扭矩产生的情况 e.g. whilst tightening a screw with a spanner as well as on crankshafts, handwheels方向盘, toothed wheels etc. The force always operates on a

16、lever arm. This is the effective radius on rotary objects.M=rF改变匀速定轴转动的运动状态的作用就是力矩，作用力作用于转动物体，从而改变它的转动状态，是通过力矩的形式进行的。AT_00_0051Task:Which torque acts upon the screw?80 N 0,25 m = 20 NmEnergy, WorkIn power machines, the input energy is transformed into a form of energy that is necessary for a certain

17、 function of the machine.The combustion engine is a power machine in which the chemical energy of the fuel is being transformed into kinetic energy（内燃机是一个做功机械将化学能转为动能） which is needed for the drive of the vehicle.Mechanical work is performed when a body under the influence of a force puts a way back

18、 to the force direction.在我们周围的环境中，我们接触的都是一些彼此之间以某种力（弹性力、万有引力、摩擦力以及其他各种力）互相作用着的物体所以，在这种情况下，物体的位置改变，只有在力的作用下才能实现因此，自然就有表明力使物体移动的作用特性的必要性在力学中，作为这种特性，所取的是这样一个量：沿移动方向的分力和这分力的作用点移动的距离愈大，这个量也愈大这个量叫做功”这一段话的前一半说明功的概念是如何产生的它包含着两个重要因素：其一就是物体与物体之间的相互作用；其二就是在相互作用之下物体位置改变的过程所具有的一个共同的特性，这一特性就是功即 ，其中s表示力的作用点移动的距离（即

19、移位的大小）， 表示力在位移方向的分力AT_00_0062.1Mechanical work (w) growswith the size of the force (F) and W = F swith the length of the distance (s).The derivative SI-unit for mechanical work is the newtonmetre (Nm) or joule焦耳 (j) and also wattsecond瓦 (ws).1 newtonmetre is the mechanical work which is performed wh

20、en the point of application of the force of 1 n is displaced by 1 m in the direction of the force.Types of mechanical work:1瓦等于1N力使物体移动一米（物体延作用力方向） Lifting work = weight force lifting height, Braking work = braking force braking distance, Acceleration work = acceleration force acceleration distance.

21、 1马力=0。735千瓦The same work can be either performed with a larger force at short distance or with a small force at long distance.For this purpose, an appliance is needed to allow “Saving force at the expense of the distance“. Thereby, the given work can never be larger than the used work.A conversion转

22、换 of force, which is also called a conversion of torque can result from the according leverage杠杆原理. 杠杆 定义：一根硬棒，在力的作用下如果能绕着固定点转动，这根硬棒就叫做杠杆。In an automobile this is achieved by the transmission of toothed wheels or belts.The higher torque is operating on the bigger toothed wheel (bigger lever arm, mor

23、e teeth) at a toothed wheel pairing.The leverage corresponds to the proportion of teeth of the driven toothed wheel to the driving cogwheel.AT_00_0060.1AT_00_0060.2AT_00_0060.3Transmission ratio (i) is the proportionof the torque of the driven wheel (M2)to the torque of the driving wheel (M1). i = t

24、ransmission ratio Driving toothed wheel = uneven numbers Driven toothed wheel = even numbersIf the distribution force in a gear unit or a belt drive, e.g. lies over two toothed wheel pairs, they result in two partial transmissions.AT_00_0050.1i complete = i1 i2Capacity, Efficiency factorIn machines,

25、 the energy is transformed and made useful.Energy losses occur with every energy transformation, which is caused by friction, heat abstraction热损失, and thermal radiation热辐射.The result is that the output capacity of a machine is always smaller than the input capacity.To compare the machines to their c

26、apability, the following efficiency factor is used (eta).AT_00_0066Efficiency factor效率 is the proportion of output capacity Pout to input capacity Pin. total = 1 2 3 .1. Pin = Input capacity; 2. PV = Loss capacity3. Pout = Output capacityAs the output capacity is always smaller than the input capaci

27、ty, the efficiency factor is always lower than 1 or lower than 100%. The input capacity always equals to 100%.The efficiency factor of electrical machines is the best whereas the worst efficiency factor is in heat engines e.g. gasoline and diesel engines.For comparisonEfficiency factor gasoline engi

28、ne approx. 35% resp. 0,35Efficiency factor diesel engine approx. 45% resp. 0,45Efficiency factor electrical motor approx. 90% resp. 0,9AT_00_0065 total = 0.3*0.95*0.9*0.9= 0.231During the combustion of fuel in the engine, only a small part of the produced energy is transformed into effective power t

29、hat is passed on to the gearbox.The largest part of the energy cannot be used for the power engine.Task:The circular diagram besides shows the energy distribution for a gasoline engine. Complete the table!Insert the acquired value for the effective power in the table! Complete the circular diagram with the calculated percentage for the usability.Radiant heat热辐射4%Friction, auxiliary drive摩擦损失6%Effective power有效34%Exhaust heat排气热量35%Coolant heat冷却热量21%Tasks concerning transformation:1. Toothed wheel drive:The missing values are to be