M. Ibbotson - Professional english in use engineering (794233), страница 19
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This acts vertically downwards (see Unit 9). Gravity is exerted on every atom of anobject. However, to simplify their calculations, engineers assume that gravity is exerted onone imaginary point called the centre of gravity. Depending on the shape of the object, thispoint may be inside the object's cross-sectional area, or outside it (see the diagrams below) .•-Frames and trussesA frame (or framework) is made from a number of relatively thin members.
Examples are bicycleframes and steel-frame buildings. The members of a frame can form a complex shape- often withmany triangles -called a lattice. The advantage of triangles is that they are stiff- they stronglyresist deformation. Triangular assemblies can therefore be used to stiffen (or brace) structures.1 simple structure2 structure deformsunder load3 adding cross-members (orbraces) stiffens the structureIn diagram 2, the structure deforms because the joints between the thin members are weak andcan flex easily.
Joints which flex- because they are weak or because they are designed to do so-are called pin joints. Lattices which are entirely pin-jointed, and which therefore need crossmembers to stiffen them, are called trusses. A truss contains two types of member. Membersthat resist tension are called ties, and members that resist compression are called struts. A jointwhich does not flex is called a rigid joint (or moment connection).
Rigid joints are often thickand securely joined- for example, by welds.70Professional English in Use Engineering32.1Complete the exp lanation using words and expressions from A opposite. You will need tochange the form of one word .In (1) ................................,each load acting on the struct ure is (2) ................................ by a(3) ................................ . This means the structure always remains (4) ................................
.32.2Complete the text using words and expressions from A, B and C opposite.To analyze the roof structure shown in Diagram 1 , an engineer draws Diagram 2 tohelp make some initial , simpl e calculations . In Diagram 2 , the engineer uses t hefollowing assumptions and information .tThe three (1) ................................
forces from the loads act as a single (2) .............................. ..force on the top of the frame.tAll the joints in the frame are (3) ................................................................ (can flex) , so the framebehaves as a (4) ................................ .tThe (5) ................................ ................................................................ of the roof structure (shown in red)- not including the satellite dish - is exactly halfway between the two supports.loads from 3 supportscarrying a largesatellite dishDiagram 1Diagram 232.3 In Diagram 2 above, which of the members (a-c) are ties, and which are struts?32.4 Choose the correct words from the brackets to complete the sentences. Look at C opposite tohelp yo u.1 Trusses are (frameworks I ties).2 A connection between members which flexes is called a (pin joint I rigid joint).3 Struts are designed to resist (compression I tension) .4 By adding cross-members, a frame is (braced I pin-jointed), making it stiffer.Ove,r ..f-o tjou{J1Think about a structure or assembly you 're familiar with.
Answer the fol lowi ng questions:•What are the different component forces that act on it?•How is it prevented from deforming?•How is it affected by gravity?Professional English in Use Engineering71IIJ Motion and simple machines-Acceleration and motionIf an object is at rest (not moving) and is free to move (not fixed), an external force- aforce from outside -will make the object accelerate.
This means the velocity of the object(the speed of its movement in a given direction) will increase. Velocity is measured in metresper second (m/s). If acceleration is constant- that is, if the rate of acceleration remains thesame- it is measured as the increase in velocity (in metres per second) that is achieved eachsecond. The unit of acceleration is therefore metres per second per second - stated as metresper second squared (m/s 2 ).If an object is in motion (moving) and is subjected to an opposing force- that is, one actingon it in the opposite direction- the object will decelerate. As with acceleration, decelerationis measured in m/s 2 • If something moves in a straight line, we say its movement is linear- acar accelerating and driving along a straight road is an example of linear acceleration andlinear motion.On earth, acceleration due to gravity is roughly 10 m/s 2 • In other words, if an object isdropped and left to free fall, its velocity will increase by 10 m/s every second (not allowingfor air resistance).
Acceleration and deceleration, such as that generated by aircraft and cars,can be compared with acceleration due to gravity. This relative measure is called G-force (Gstands for gravity). An acceleration of 10 m/s 2 is measured as 1 G, 20 m/s 2 as 2 G (or 2 Gs),and so on.~InertiaThe greater the mass of an object (see Unit 9), the greater the external force required to causeit to accelerate or decelerate.
Resistance to acceleration or deceleration, due to the mass ofan object, is called inertia. When an object is in motion, its resistance to deceleration, due toinertia, is often called momentum.-Simple machinesThe word machine generally refers to an assembly which has parts that move. However, asimple machine can be a very basic device. A simple machine is something which provides amechanical advantage- that is, the load generated by the machine (the force it puts out, oroutput) is greater than the effort (the force put in, or input) required to generate the load.An example of a simple machine is a lever, which is used with a fulcrum- a point which acts asa support, and allows the lever to pivot (turn around the support). If the lever is placed so thatthe distance between the effort and the fulcrum is greater than the distance between the loadand the fulcrum, a mechanical advantage is created.In general language, the turning force generated by a lever is called leverage.
In engineering, aturning force is called a turning moment (or moment). Moments are calculated by multiplyingthe distance from the fulcrum, in metres, by the magnitude of the force, in newtons. They aremeasured in newton metres (Nm).Note: See Appendix VI on page 108 for more on moments.BrE: spanner; ArnE: wrenchA spanner is an example of a lever.72Professional English in Use Engineering33.1Complete the article about the Titanic, taken from a popular science magazine. Look at Aand B opposite to help you.It's been suggested that the passenger liner Titanic wouldn't have sunk after colliding with an icebergin 1912, if it had hit the obstacle head on and damaged only the front of the ship.
As history tells, thecrew tried to turn to avoid the iceberg, and 1,5171ives were lost. But how severe would a frontal impacthave been for the passengers? The answer depends on several questions:***The ship tried to slow while turning. Would (1) ................................ deceleration have been moreeffective, allowing the ship to (2) ................................ more rapid ly?Based on t his (3) ................................ of deceleration (and assuming the ship would not have stopped intime), what wou ld the (4) ................................
of t he sh ip have been at the moment of impact?What was the (5) ................................ of the iceberg? Calculating the approximate number of kilogramsof ice would allow the (6) ................................ of the iceberg to be compared with the momentum of theTitanic. This would show whether the impact wou ld have caused the iceberg to (7) ............................... .to any significant degree, and so absorb some of the shock as it was pushed forward.Clearly, the above questions depend on numerous unknown variables. So let's make a rough estimate.Let's assume the impact would have occurred at a pretty fast 25 kilometres per hour- that's seven(8) .................................
And allowing for some shock abso rption from bending steel and crush ing ice, let'ssay the ship would have stopped w ithin three seconds (although it wou ld probably have taken longer).This wou ld have resulted in a deceleration of 2.3 (9) ................................. Expressed as a(1 0) ................................ ,that gives 0.23 -less than one-third of the dece leration generated by a carbraking heavily. So the impact probably wouldn't have caused too much of a shock to the passengers.Whether or not the ship would have sunk, however, is another question.33.2Replace the underlined words and expressions with alternative words and expressions fromA and B opposite.The first diagram below illustrates how a worker is able to apply a total (1) force of SO newtonsto the corner of a nut using just his fingers .
The distance from the centre of the nut- the pointaround which the nut (2) turns- and the corner of the nut is lOmm. This results in a (3) forceof leverage of O.S newton metres. This is insuffident to tighten the nut properly.The second diagram shows how a spanner can be used as a (4) tool to provide a(S) boost in force . Applying the same SO-newton force to the end of the 200 mm spanner,which acts as a (6) turning tool, generates 10 newton metres- a force 20 times greater, andenough to tighten the nut .SONspannernut~.w1:-f lO mm= 0.01m50 N x 0.01 m = 0.5 Nm50 N0V~(' .f-otjOUx''0.2 ill = 10 Nm200 mm =0.2 m(J1Think about a simp le machine you usc or arc fami liar with.
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