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2009 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 2009 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 2009 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.

Teachers and students are advised that, in December 2008, the Board of Studies approved changes to the examination specifications and assessment requirements for a number of courses. These changes will be implemented for the 2010 HSC cohort. Information on a course-by-course basis is available on the Board’s website.

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 a guide to the length of the required response. A longer response will not in itself lead to higher marks. Writing far beyond the indicated space 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’.

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 also reminded the approved equipment for Engineering Studies is a Board approved calculator, a pair of compasses, a protractor and set squares. Any other equipment is not permitted in the examination room and should not be taken in.

Section II

Question 11 – Historical and societal influences, and the scope of the profession

  1. Better responses listed more than one responsibility of the site engineer, such as OHS issues, quality control, environmental restrictions and planning construction tasks. Weaker responses addressed general engineering responsibilities, but then failed to link these to the role of a site engineer constructing a bridge.
  2. Responses generally displayed a strong knowledge of innovations in construction methods, such as improved lifting devices and electrical or pneumatically powered tools.
    1. Most responses recognised the need to preserve the materials used in bridge X and described the issues involved, such as sourcing appropriate replacement materials. Better responses also considered the reduction of loads and stresses placed on the bridge by modern day use.
    2. Many responses listed different fields of engineering and provided a brief explanation of what each did. However, they failed to link these with an explanation of how and why they collaborated on a project. Better responses combined the need for collaboration of engineers from different fields, named those specific fields, and concluded their explanation with a reason as to why collaboration between them was important.

Question 12 – Civil structures

  1. Better responses indicated the changes in grain structure and that a change in properties such as an increase in directional strength and lower ductility resulted. These responses often used a small diagram to aid the description of the structural changes that occur due to cold rolling.
    1. Correct responses to this question involved taking moments about the fixed support B to find the vertical reaction at support A. This then allowed finding of the vertical and horizontal reactions at support B by summing vertical and horizontal forces. Common errors included leaving the 8kN horizontal force out of the moment calculation, or substituting incorrect distances into the moment equation.

    2. Better responses correctly summed the vertical forces at joint B, that is the vertical component of member CB and the vertical reaction calculated from part (i). Other correct responses involved resolving the forces at the joint (method of joints) or using a method of sections.

      Candidates who calculated an incorrect value for the vertical and horizontal reactions at joint B in part (i) were not penalised again for using those values in this question. Many candidates failed to attempt this question.

    1. Better responses indicated that the best position for the hole was at A, as it was on the neutral axis of the beam and as a result had less effect on the bending stresses the beam may encounter.
    2. Better responses indicated that rounded corners to the hole would lessen the effect of stress concentration and the resultant cracking that would occur at sharp corners.

Question 13 – Personal and public transport

    1. Better responses named a suitable non-destructive test, such as dye-penetrant testing or ultrasonic testing and then provided a detailed description of the testing procedure. Poorer responses named a suitable test but then provided a partial description of the procedure. Weaker responses did not provide a description of the non-destructive test or named and described an inappropriate destructive test.
    2. Correct responses named ‘hardening’ as the heat treatment process used to produce Martensite in the surface of the head, and correctly described the appropriate heating and cooling ranges/rates necessary. Poorer responses named and described the correct heat treatment process needed to form Martensite but failed to apply it to the surface of the head. Weaker responses failed to name a suitable heat treatment process, did not provide suitable heating and cooling ranges or provided descriptions of incorrect heat treatment processes.
  2. Better responses described the structure of laminated glass and then explained how the polymer sheet prevented the glass shards from injuring passengers. Poorer responses placed the polymer laminate on the outside of the glass and then correctly explained how the binding properties of the laminate provided protection for passengers. Weaker responses described the safety features of toughened glass or provided a limited description of laminated glass without linking it to the protection of passengers during an accident.
  3. Correct responses placed the three inputs in the appropriate sequence. Weaker responses placed the provided inputs in the wrong order or inserted a variety of non-related inputs.

Question 14 – Lifting devices

    1. Better responses used a moment equation to correctly determine the mass, M. A significant number of candidates failed to read the question properly and left their answer as a force, in Newtons. Many candidates failed to show any working and arrived at an incorrect response. Candidates are encouraged to show all working as marks can be awarded to them even if the answer provided is incorrect.

      The concept of a moment (couple) and a reaction was not well understood. The reaction at point B required a vertical force combined with a moment to prevent the crane from rotating.

      In the vertical reaction section, better responses summed the two masses and converted to the appropriate units (kN). Weaker responses failed to convert correctly or left their answer in tonnes.

      In the moment section, better responses took moments about a consistent point, B, and were able to convert to MNm. Weaker responses either confused a moment equation or demonstrated a poor understanding of units.

      Candidates are encouraged to show all working and indicate the point for summing moments. In addition, candidates who wrote out powers of ten in their formulae were more successful in manipulating units correctly than candidates who displayed limited working.

    1. The concept of the stress/strain diagram was well understood. Better responses indicated a reason A was used, such as it being in the elastic range for deformation, and why B was not considered. Poorer responses involved a description of the diagram without explanation of the significance of A and B or only considered one of the points.
    2. Stress, load and area, and the manipulation of the appropriate formulae were well understood. Better responses showed working and correct manipulation of the formulae to calculate the correct diameter. Poorer responses left the answer as an area, used the wrong area formula for the bolt or mismanaged units in finding the cross-sectional area. Many candidates failed to convert to SI base units.
  3. The concept of pulse width modulation (PWM) was not well understood. Better responses demonstrated an understanding of switching the current on and off in pulses to change the average voltage and varying the pulse width or switching time to change the speed of the motor. Many candidates knew that changing the width of the pulse varied the speed but did not explain how this occurred. Some candidates tried to reword the question.

Question 15 – Aeronautical engineering

    1. Better responses described the internal functions of a jet turbine and then linked the internal function to the turning of the propeller. Mid-range responses described the internal workings of a jet turbine while weaker responses wrote that the propeller simply turned and generated a force behind it.
    2. Better responses linked the change in airflow caused by the change in direction of the elevators and then elaborated on how this affected the pitch of the aircraft. Mid-range responses wrote about change in pressures due to changes in elevator movement. Weaker responses wrote in non-specific terms about airflow.
  2. Better responses gave a characteristic and feature of an advantage and a disadvantage, using a number of appropriate terms such as ‘polymer adhesives give a smoother surface which helps to reduce the drag of an aircraft’. Mid-range and weaker responses tended to only list one or two advantages or disadvantages. The use of incorrect engineering terms like ‘stronger’ is still a concern.
  3. Better responses analysed the relationship between the lift to drag ratio and applied this to the lifting force being altered by the angle of decline. Mid-range responses did not take into account the 15˚ and calculated the drag by dividing the weight force by eight or did not see the relevance of the lift to drag ratio.
  4. Better responses calculated correctly the force using the formula for pressure and the appropriate SI base units. Mid-range responses used the correct formula but either used the incorrect formula for area or did not use SI base units in the equations. Conversion to base units is necessary to complete calculations correctly.
  5. In better responses, candidates sketched the cable mount using either isometric or oblique techniques to complete the pictorial sketch. Oblique projection of the cable mount provided the easiest solution this year. A common error included not drawing isometric or oblique curves in the manner expected to AS1100 drawing standards.

Question 16 – Telecommunication

  1. Correct responses indicated a good knowledge of the different properties of the different types of cable. A number of candidates incorrectly referred to the ‘speed of the signal’ instead of the ‘rate of data transfer’ as speed is similar for all cables.
  2. Most candidates displayed knowledge of multiplexing, generally mentioning the fact that it is the transmission of multiple signals simultaneously, but did not add the other important feature – on different carrier frequencies to enhance efficiency of transmission. A common error stated that multiplexing uses multiple strands of wire or multiple mediums, such as optical fibre and coaxial cable to convey the signal.
  3. Responses indicated a lack of understanding of FM modulation. The frequency of the carrier wave increased as the amplitude of the message signal increased. A common error was showing the carrier wave with variable amplitude.
  4. Many candidates had difficulty with this question. However, correct responses indicated that both television sound (FM) and television picture (AM) message signals were modulated to the carrier signal. Many responses simply compared the AM and FM modulation results without reference to the carrier wave.
  5. Knowledge of AS1100 standards with reference to nuts, threads and sectioning techniques is still limited. Most candidates were able to represent the parts assembled in correct sequence but did not understand the rotation of a PCD hole in a sectional view.

Section III

Question 17

    1. Better responses linked the use of crash barriers to a wide range of social benefits, such as less pressure on police, ambulance and hospital services, and lower legal and financial costs. Many responses, however, discussed the function of the barriers without analysing the social benefits or restricted the discussion to the occupants of a car hitting the barrier.
    2. Candidates were given marks for identifying that the crash barrier will contain an accident to a specific area. Other good responses described how the barriers slowed vehicles by dissipating energy and therefore reducing injury to the occupants or damage to the vehicle.
    1. Correct responses nominated criteria, such as materials to be used, visibility, impact resistance and energy absorption as a means of comparing the barriers, with some also indicating a need to analyse historical accident data. Better responses provided a range of criteria and were not restricted to those related to mechanical properties of materials.
    2. Most responses identified reasons for using steel (good elastic properties, high tensile strength and toughness) and polypropylene (corrosion resistance and light weight) in the crash barrier. Explanations were brief.
  3. Better responses correctly substituted values provided into the formulae for stress and Modulus of Elasticity and used SI standards for the values used. A large number of candidates used the correct formulae but failed to convert values so that the order of magnitude of their final answer was incorrect.

Question 18

  1. In better responses, candidates provided more than two valid criteria, such as maintenance costs, pedestrian mobility, or improved safety and visibility. Some candidates identified only one criterion, but then explained it well. Weaker responses listed criteria that were often irrelevant or inappropriate for the situation. Some candidates incorrectly interpreted the stimulus material as depicting a city scene and a suburban one and then described the different pole arrangements.
  2. Better responses compared polycarbonate and aluminium alloys by referring to properties relating to electrical insulation and methods of manufacture for each. They also correctly provided similarities of the two in areas of good strength to weight ratio and resistance to electrochemical corrosion. Weaker responses often listed properties for each without actually comparing the two materials. Candidates are reminded that responses such as strength should be qualified: tensile strength or impact strength for example.
    1. Better responses identified the voltage drop across the resistor as 4.5 V and then correctly manipulated the formulae V = IR to determine the value of the resistor. Many responses incorrectly used a value of 15 for the current. Candidates are reminded that conversion of quantities to SI units, for example 15mA to 0.015A is essential in calculations.

    2. The best responses tended to choose circuit 2, then justified it in terms of the improved safety to motorists and traffic management caused by the inherent redundancy allowed for in the series-parallel arrangement of components.

      A large number of responses selected circuit 2 but then discussed the advantages over a series circuit without relating these to the use of it, namely traffic lights and the safety of people. A small group of responses selected circuit 1 and justified their choice based on a mixture of simplicity, ease of manufacturing and lower cost. However, they then failed to include the effects on the traffic lights.

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