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2010 HSC Notes from the Marking Centre – Engineering Studies

Contents

Introduction

This document has been produced for the teachers and candidates of the Stage 6 course in Engineering Studies. It contains comments on candidate responses to the 2010 Higher School Certificate examination, indicating the quality of the responses and highlighting their relative strengths and weaknesses.

This document should be read along with the relevant syllabus, the 2010 Higher School Certificate examination, the marking guidelines and other support documents which have been developed by the Board of Studies to assist in the teaching and learning of Engineering Studies.

General comments

Teachers and candidates should be aware that examiners may ask questions that address the syllabus outcomes in a manner that requires candidates to respond by integrating their knowledge, understanding and skills developed through studying the course.

Candidates need to be aware that the marks allocated to the question and the answer space (where this is provided on the examination paper), are guides to the length of the required response. A longer response will not in itself lead to higher marks. Writing in excess of the space allocated may reduce the time available for answering other questions.

Candidates need to be familiar with the Board’s Glossary of Key Words which contains some terms commonly used in examination questions. However, candidates should also be aware that not all questions will start with or contain one of the key words from the glossary. Questions such as ‘how?’, ‘why?’ or ‘to what extent?’ may be asked or verbs may be used which are not included in the glossary, such as ‘design’, ‘translate’ or ‘list’.

Some marks are often awarded for a correct method, even when incorrect answers are given, so all working should be shown. Candidates are also advised that when drawings are labelled as being drawn ‘to scale’, they may be used for calculations or graphical solutions to problems. It is also acceptable to use the provided drawings as part of the solution.

Candidates are reminded that the approved equipment for the Engineering Studies examination is a Board approved calculator, a pair of compasses, a protractor and set squares. No other equipment is permitted in the examination room.

Section II

Question 11 – Historical and Societal Influences, and the Scope of the Profession

  1. In better responses, candidates correctly identified a modern material, not found in the early 1900s, and explained how or why it is now used in a component of lift construction. In mid-range responses, candidates only identified an appropriate material without linking it to any explanation or listed useful properties of a lift component without linking them to a material. In weaker responses, candidates did not name, used vague or incorrect nomenclature or chose an inappropriate material, without any explanation of properties or reasons for use.
  2. Better responses provided at least two outlines of engineering roles covering both the design of the lift and the safe operation of the lift, outlining roles such as calculating the size and shape of components, the testing and selection of materials, determining safe operating procedures, establishing maintenance schedules and supervising construction.

    Weaker responses generally focused only on one particular role or only provided a list of design or safety concerns for the engineer.

  3. In better responses, candidates successfully outlined the impact that passenger lifts have had on both the built environment and society in general, expanding on impacts such as enabling building heights to increase, reducing the footprint of the building, increasing population densities and providing improved access to groups such as the elderly and the disabled. In mid-range responses, candidates tended to expand on the impact in only one focus area while weaker responses generally lacked detail, providing only a limited list of possible impacts.
  4. In better responses, candidates successfully described the characteristics or features of a feasible hydraulic system typically involving a telescopic ram. Some candidates described a system that involved increasing the cross-sectional area of a 2.5 m long effort ram linked hydraulically to a ram of smaller cross-sectional area, giving an increase in lift distance. In weaker responses, candidates provided answers that lacked detail in their description or generally outlined methods that were neither practical nor feasible.

Question 12 – Civil Structures

    1. In better responses, candidates used ΣFH = 0 to find the horizontal reaction at A. The concept of taking moments to find one of the vertical forces was not well answered, as many candidates did not include the wind loading in their calculations. However, many candidates did consider that ΣFV = 0 and so they were able to have the structure in vertical equilibrium.
    2. Candidates could use a number of appropriate methods to solve this problem. Only the best responses correctly found the force in BJ, generally because many candidates did not consider the force in IJ or AJ to be needed in the solution. Whether candidates chose to use Method of Joints or Method of Sections, they did not incorporate all necessary forces as part of their solutions. Calculation of the angle at BJ was done, however weaker responses did not use the information correctly. Better responses realised that the horizontal component of BJ was equal to the force in AJ and calculated through to a correct answer.
    3. In better responses, candidates sketched and described that adding extra diagonal members would make the truss more rigid. In weaker responses, candidates indicated that ties could be attached to the truss to brace it against the wind. However, they did not redesign the truss. They also considered lowering the structure to reduce the load effect of the wind but this did not retain the function in the truss.
    1. In some weaker responses, candidates selected the correct beam D, but were unable to supply a suitable reason for their choice. In better responses, candidates identified that beam D had the most resistance to bending because it had the highest Second Moment of Area, or described why the beam design of D resists bending.
    2. In better responses, candidates recognised the correct formula and identified the data from the table and correctly substituted into the equation. In weaker responses, candidates did not find the correct maximum bending moment for this loaded beam. A small number of candidates did not recognise that the beam was bending but did calculate a maximum shear stress.

Question 13 – Personal and Public Transport

    1. In better responses, candidates identified a range of aluminium alloys as suitable material for die casting the brake lever. In weaker responses, candidates chose a range of unsuitable materials such as cast iron, steel and a range of polymers.
    2. Most candidates identified some form of casting, however, there was some confusion between injection moulding of polymers with die casting of aluminium alloys. In some of the weaker responses, candidates demonstrated a poor understanding of the whole process with many only describing the melting of materials before injecting into a mould using pressure. Other weaker responses described the shell moulding or the sand casting processes.

      In better responses, candidates identified the key elements of the process by describing the preparation of permanent steel dies with lubrication, the injection of a molten alloy under pressure into the sealed cavity and maintaining this pressure until solidification occurred. They also described the opening of the dies and the ejection of the casting followed by the separation of scrap, including runners, gates and flash from the final product. In some weaker responses, candidates incorrectly stated that the metal was heated to just above the recrystallisation temperature before being forced into the die using hydraulic rams.

    1. In better responses, candidates calculated the power required to maintain constant velocity by determining the component of the weight force acting down the incline and the correct velocity or by calculating the work done by determining the change in potential energy and the time taken to travel the length of the incline as shown in the diagram. In weaker responses, candidates either used the ‘Normal’ force, or incorrectly used non-SI standard units for velocity, and applied it to the power equation or they simply misused the weight force shown on the diagram as total Work. Some candidates chose to use the energy method, using incorrect addition (Kinetic Energy not realising that the change in KE is zero when the velocity is constant). In weaker responses, candidates demonstrated little understanding of power and simply used figures from the question in unrelated equations.
    2. In better responses, candidates correctly substituted the data from the question into the formula given and manipulated the equation to calculate current. In weaker responses, candidates did not recognise the units of power and voltage (resulting in incorrect substitution) or incorrectly manipulated the formula given.
    3. In better responses, candidates demonstrated a good understanding of ‘ampere hours’ and how they influence the duration of the battery. They determined the ampere ratio and calculated the length of time that 45 amperes could be drawn from a 12-volt battery. This time was then correctly multiplied by the scooter’s velocity to determine the total distance travelled. Weaker responses attempted to calculate power using the current and voltage given in the question and then multiplying by the speed of the scooter or they incorrectly related ampere hours with amperes drawn.

Question 14 – Lifting devices

    1. In better responses candidates gave advantages and qualified their responses, making the link between many moving parts and losses in efficiency, increased maintenance and reliability. Some candidates identified safety issues. In weaker responses, candidates gave very simple or one-word answers that were difficult to interpret. Some candidates incorrectly stated that the hydraulic system could lift more weight or had a greater mechanical advantage without any explanation. In some weaker responses, candidates also assumed that the mechanical system was manually operated and the hydraulic system was automated.
    2. Better responses contained the correct formula and summed the two forces to equal the tension in the cable. In weaker responses, candidates only equated one force with tension in the cable, used an inappropriate formula, subtracted the two forces, or wrote down figures without a formula. The concept of summing the two forces to equal the tension in the cable was not well understood by candidates. Candidates are advised to write the formulae they are using in their responses and to include all working.
    1. In better responses, candidates demonstrated good understanding of the concepts and answered this question in a few steps, saving significant time. In weaker responses, candidates did not write down equations, poorly manipulated equations or did not calculate areas correctly. Candidates who used velocity ratio or mechanical advantage were usually unable to follow the concept to a correct response.
    2. In better responses, candidates suggested a correct material and related the properties of that material to the service properties of the hydraulic system. In weaker responses, candidates either suggested an incorrect material or did not understand the function of a hydraulic seal. Some candidates were unable to correctly link properties of the material to service properties, did not effectively justify their ideas, or simply gave incorrect properties.
  1. In better responses, candidates named an appropriate destructive test for the crane hook, described the test, demonstrated knowledge of results and were aware that these results would be compared to a standard to ensure quality control. In weaker responses, candidates suggested incorrect tests, did not correctly identify the intended test, used incorrect technical language, or mixed up the test with the description. They also wrote about a test without describing the test they named.

Question 15 – Aeronautical engineering

  1. In better responses, candidates demonstrated good understanding of the features/functions of recreational aircraft and provided reasons as to the suitability of piston-driven internal combustion engines for use in these aircraft.
  2. In better responses, candidates explained how the linear flow was disturbed and that the wing became less aerodynamic, increasing drag and reducing lift. Diagrams were sometimes used to aid explanation. A significant number of candidates described the brake as a spoiler adding ‘downforce’ to the wheels.
  3. In better responses, candidates included greater strength-to-weight ratios and the ability to be formed into complex shapes as advantages of composites over traditional materials such as aluminium alloys. Weaker responses often referred to lighter or stronger materials without any linking of the two. Impact resistance was claimed to be a superior property by many candidates.
  4. Age hardening was not well understood by most candidates. Most responses simply gave property changes as a result of heat, such as softer, more ductile, and weaker. In better responses, candidates explained over-aging with correct property changes.
  5. In better responses, candidates demonstrated understanding of the corrosion resistance provided by the pure aluminium outer layer. Weaker responses only referred to corrosion resistance or simply restated high strength as the advantage.
  6. Most candidates produced an assembled drawing. In better responses, candidates assembled the components and completed the drawing using appropriate sectioning standards. In weaker responses, candidates generally only assembled the components. Knowledge of AS1100 standards with reference to nuts, threads and sectioning techniques was limited. The propeller blade was incorrectly included in a significant number of responses.

Question 16 – Telecommunication

    1. In better responses, candidates understood that the term ‘unguided’ meant there was no permanent or fixed conduit for the microwave.
    2. In better responses, candidates identified that the microwave communication link required line of sight and that microwave transmission is susceptible to adverse weather conditions although its bandwidth enables larger quantities of data to be transmitted. Many candidates failed to respond to the example outlined in the question and diagram. In weaker responses, candidates confused the situation given in the question with satellite communication, or tried describing television and or radio transmission.
    3. Some candidates confused the correct frequency with VHF and UHF.
    1. Better responses demonstrated the link between a series of measurements being taken at regular intervals in time. The use of sketches by some candidates to help demonstrate the sampling process was quite common although this was not requested. Responses generally indicated a lack of understanding of the sampling process. Weaker responses incorrectly described the process of modulation or made reference to the measurement or value but did not link it to time.
    2. The quantisation process was not understood by many candidates. Weaker responses commonly described digitisation instead of quantisation.
    3. In better responses, candidates assigned values on the graph that represented the sampling rate requested. Weaker responses indicated a lack of understanding of waves formed by quantisation.
  1. In better responses, candidates visualised and sketched the shape and proportion of the antenna bracket. A variety of pictorial representations was used including isometric projection or oblique projection. Candidates who chose to use alternative pictorial projection methods found it difficult to sketch in a manner expected to AS1100. Candidates who did not recognise that the orthogonal drawing of the antenna bracket was done in third angle also found it hard to sketch an appropriate pictorial drawing.

Section III

Question 17 – Engineering and the Engineering Report

    1. In better responses, candidates understood the term selection criteria, provided a clear justification, and used engineering terminology in their response. In weaker responses, candidates mistakenly selected a particular pole as a solution.
    2. In better responses, candidates showed a comparison between the two materials by identifying properties and justifying one material over the other. In weaker responses, candidates did not compare the properties, but merely stated the properties of each material.
    3. In better responses, candidates identified a social impact and an environmental impact of power poles. Some candidates only identified one type of impact which indicated more careful reading of the question was required.
    1. In better responses, candidates identified different factors that could be included in an engineering report for different stakeholders. In weaker responses, candidates did not state what would be needed in an engineering report that was prepared for different stakeholders.
    2. In better responses, candidates stated that transmitting power at high voltages reduces the power lost. They showed an understanding of reduced current being the reason for this by using the relationship between power loss, voltage and current.

Question 18 – Engineering and the Engineering Report

    1. In better responses, candidates articulated at least two factors that an engineer would consider. A small number of candidates spoke at length about only one factor, but the question clearly asked for ‘factors’. In some better responses, selected factors related to wind and another area such as proximity to houses.
    2. In better responses, candidates named two appropriate materials for the blades of the wind turbine and then justified their choices. Many candidates failed to select two appropriate materials, selecting aluminium instead of an aluminium alloy. Many candidates selected composite as the material, which, although it is an engineering term, it is not specific enough.
    3. In better responses, candidates realised that the total torque was found by calculating the moment for one blade (300 Nm) and then multiplying this by three because there are three blades, hence giving the correct answer of 900 Nm. Some candidates failed to recognise that each blade provided a torque of 300 Nm, and failed to multiply by three. A small number of responses used the wrong distance.
    1. In the best responses, candidates demonstrated understanding that when the motor is loaded its rotational speed falls which means the back emf also reduces. This in turn allows the rotor current to increase. Many simply stated that both the rotor current and the back emf increase.
    2. Many candidates correctly calculated the voltage drop across the rotor to be 7.59 V, but then failed to subtract this from the supply voltage to determine the back emf (ER). Some candidates did realise that this question drew on information from the opening of part (b) (i).

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