📚 Year 7 WJEC Engineering: Unit Test Mock Exam Paper Walkthrough | Year 7 WJEC 工程：单元测试模拟卷解析
This walkthrough breaks down a typical Year 7 WJEC Engineering unit test mock paper, covering key topics such as materials, forces, mechanisms, the design process, safety, measurement, technical drawing and CAD/CAM. Each section explains a sample question, gives the correct answer with reasoning, and highlights common mistakes to help you revise effectively.
本解析将一份典型的 Year 7 WJEC 工程单元测试模拟卷拆解开来，涵盖材料、力、机构、设计流程、安全、测量、技术图纸和 CAD/CAM 等关键主题。每个小节讲解一道样题，给出正确答案及推理过程，并指出常见错误，帮助你高效复习。
1. Materials and Properties | 材料与性质
Mock Question: A designer wants to make a strong yet lightweight bicycle frame. Which property is the most important when choosing the material? A) High density B) High strength-to-weight ratio C) High melting point D) High electrical resistance
The correct answer is B) High strength-to-weight ratio. This means the material is strong but not too heavy, which is exactly what you need for a bicycle frame. Aluminium alloy and carbon fibre are often chosen for this reason. High density would make the bike too heavy. A high melting point is not critical for normal use, and electrical resistance is irrelevant here.
正确答案是 B) 高强度重量比。这意味着材料强度高且重量轻，正是自行车车架所需要的。铝合金和碳纤维常因这一特性被选用。高密度会使自行车过重；高熔点对日常使用不重要；电阻在此无关。
Key concept: Engineering materials are selected by matching properties – tensile strength, compressive strength, hardness, toughness, density and corrosion resistance – to the job they must do.
核心概念：工程材料的选用是根据其性能——抗拉强度、抗压强度、硬度、韧性、密度和耐腐蚀性——与实际用途相匹配。
2. Forces in Structures | 结构中的力
Mock Question: A crane lifts a heavy container. The steel cable experiences a pulling force. Which type of force is this? A) Compression B) Torsion C) Tension D) Shear
The answer is C) Tension. Tension is a force that stretches a material. Cables and ropes are always in tension when lifting loads. Compression is a pushing or squashing force. Torsion is twisting, and shear is a cutting or sliding force.
答案为 C) 拉力 (Tension)。拉力是拉伸材料的力，吊起负载时钢缆始终承受拉力。压缩力是推挤力，扭力是扭转力，剪切力是切割或滑动力。
A good way to remember: Tension = stretching, Compression = squashing, Torsion = twisting, Shear = cutting. Structures often combine these forces – a beam may have compression on top and tension on the bottom.
一种记忆方法：Tension = 拉伸，Compression = 挤压，Torsion = 扭转，Shear = 剪切。结构通常同时承受多种力——一根梁可能上部受压、下部受拉。
3. Bridge Types and Load Distribution | 桥梁类型与载荷分布
Mock Question: A simple beam bridge is supported at both ends. When a weight is placed in the middle, which side of the beam experiences the greatest compressive force?
In a simply supported beam with the load at the centre, the top surface of the beam experiences compression (squashing), while the bottom surface experiences tension (stretching). The answer is therefore the top surface. This is why concrete beams are often reinforced with steel bars on the tension side.
对于两端支撑且载荷作用于中部的简支梁，梁的上表面承受压缩力，而下表面承受拉力。因此答案是上表面。这也是混凝土梁常在受拉侧配钢筋的原因。
Different bridge types handle forces differently: an arch bridge carries mainly compression, a suspension bridge uses cables in tension, and a truss bridge spreads loads through a triangular framework.
不同桥型受力方式不同：拱桥主要承受压力，悬索桥利用受拉的缆索，桁架桥通过三角形构架分散载荷。
4. Levers and Mechanical Advantage | 杠杆与机械效益
Mock Question: A lever is used to lift a 200 N load. The distance from the fulcrum to the load is 0.2 m, and from the fulcrum to the effort is 0.8 m. What is the mechanical advantage of this lever system?
÷ Effort arm, OR Effort arm ÷ Load arm, depending on the definition. In WJEC Year 7, we use: MA = Effort Arm ÷ Load Arm. So here, MA = 0.8 m ÷ 0.2 m = 4 d is only a quarter of the load (200 N ÷ 4 = 50 N). The lever multiplies force by 4 times.
机械效益 (MA) = 力臂 ÷ 载荷臂。本题中 MA = 0.8 m ÷ 0.2 m = 4。这意味着所需作用力仅为载荷的四分之一（200 N ÷ 4 = 50 N）。该杠杆将力放大了4倍。
Common mistake: students sometimes swap the two distances. Remember, if the effort arm is longer, MA > 1 – you gain force but must move the effort a longer distance.
常见错误：学生有时会颠倒两个距离。请记住，若力臂更长，则 MA > 1——你会省力，但作用力移动的距离更长。
5. Gears and Gear Ratios | 齿轮与传动比
Mock Question: A motor drives a 12-tooth gear, which meshes with a 36-tooth gear. Calculate the gear ratio and explain its effect on speed and torque.
Gear ratio = Driven teeth ÷ Driver teeth = 36 ÷ 12 = 3:1. This is a reduction gear system. The output gear rotates three times slower than the input, but its torque (turning force) is approximately three times greater. This is used in bicycles climbing hills or in electric screwdrivers.
传动比 = 从动齿轮齿数 ÷ 主动齿轮齿数 = 36 ÷ 12 = 3:1。这是一个减速齿轮系统。输出齿轮转速是输入的三分之一，但扭矩（转动力）大约是输入的三倍。这用于爬坡的自行车或电动螺丝刀。
When a small gear drives a large gear, speed decreases and torque increases. If the large gear drives the small one, speed increases and torque decreases.
小齿轮带动大齿轮时，速度降低、扭矩增大；大齿轮带动小齿轮时，速度提高、扭矩减小。
6. The Design Process | 设计过程
Mock Question: List the first four stages of the engineering design process in the correct order.
The typical early stages are: 1. Identify the problem (or need), 2. Research and gather information, 3. Generate possible ideas (brainstorming), 4. Develop and select a solution. After these come modelling, testing, building and evaluation. The order is vital because you must understand the need before designing.
典型的前四个阶段为：1. 识别问题（或需求），2. 调研并收集信息，3. 提出各种想法（头脑风暴），4. 制定并选择解决方案。之后是建模、测试、制造和评价。顺序至关重要，因为只有理解需求才能开展设计。
Marking tip: many students confuse research with testing. Research comes before idea generation; testing comes after a physical model is made.
评分提示：许多学生将调研与测试混淆。调研应在产生想法之前；测试应在制作出实物模型之后。
7. Health and Safety Signs | 健康与安全标志
Mock Question: A blue circle with a white pictogram of a hard hat means: A) Danger – wear a hard hat B) You must wear a hard hat C) Hard hats are prohibited D) First aid station
The correct answer is B) You must wear a hard hat. Blue circles are mandatory signs – they tell you what you must do. Yellow triangles are warnings. Red circles with a cross are prohibitions. Green squares show emergency routes or first aid.
正确答案是 B) 你必须戴安全帽。蓝色圆形是强制标志——告诉你必须做什么。黄色三角形是警告，红色带斜杠的圆形是禁止，绿色方形表示紧急通道或急救。
An easy way to remember: Blue says do, yellow says careful, red says don't, green says where to go.
一个简单的记忆法：蓝色是“必须做”，黄色是“要小心”，红色是“禁止做”，绿色是“去哪里”。
8. Measurement and Accuracy | 测量与精度
Mock Question: A steel rule is used to measure the length of a plastic part. The reading is taken between the 3 cm and 4 cm marks, and the millimetre division shows 6 mm past the 3 cm line. Write the measurement in both millimetres and centimetres.
.6 cm. In engineering, measurements are often given in millimetres to avoid confusion with decimals. Always state the unit.
总长度 = 3 cm + 6 mm = 36 mm，即 3.6 cm。工程中常用毫米为单位以避免小数点混淆。务必标明单位。
ent. Accuracy of a steel rule is typically ±0.5 mm, while a micrometer can reach ±0.01 mm.
使用游标卡尺时，读取主尺和游标刻度的对齐位置。钢尺的精度一般为 ±0.5 mm，而千分尺可达 ±0.01 mm。
9. Isometric and Orthographic Drawings | 等距图与正交投影图
Mock Question: An orthographic projection of a step block shows three views. What are they called, and why is a third view needed?
The three views are front elevation, side elevation, and plan (top view). The third view is needed because two views cannot always show all hidden details or confirm the exact shape. For example, a cylinder looks like a rectangle from the front and a circle from the top; you need both to fully describe it.
三个视图分别是正视图（前视图）、侧视图和俯视图（平面图）。需要第三个视图是因为两个视图有时无法显示所有隐藏细节或确认确切形状。例如，圆柱体从正面看是矩形，从上面看是圆；需要两者才能完整表达。
Isometric drawings show a 3D view using 30° angles. They are useful for visualising the object but do not show true lengths along all axes directly – that is what orthographic drawings for manufacturing provide.
等距图以 30° 角展示三维视图，有助于可视化物体，但并非所有轴向的尺寸均为真实长度——这正是制造中需要正交投影图的原因。
10. Introduction to CAD and CAM | CAD/CAM 基础
Mock Question: A school uses a 3D printer to make a prototype gear. Explain the role of CAD and CAM in this process.
CAD (Computer-Aided Design) is used to create a digital 3D model of the gear on a computer. The file is then exported to CAM (Computer-Aided Manufacturing) software, which generates the toolpath and instructions for the 3D printer. The printer follows these instructions to build the part layer by layer. CAD helps design and simulate; CAM controls the manufacturing machine.
CAD（计算机辅助设计）用于在计算机上创建齿轮的数字三维模型。然后将文件导入CAM（计算机辅助制造）软件，生成刀具路径和 3D 打印机的指令。打印机依照指令逐层构建零件。CAD 帮助进行设计与模拟；CAM 控制生产设备。
Common mistake: thinking the 3D printer is CAD. The printer is the machine; the design on screen is CAD. In industry, CNC mills and laser cutters also use CAM.
常见错误：以为 3D 打印机就是 CAD。打印机是设备，屏幕上的设计才是 CAD。工业中，数控铣床和激光切割机同样使用 CAM。
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