📚 Year 7 SQA Physics: Case Study Practice | 七年级 SQA 物理：案例分析实战演练
Physics is not just a collection of formulas and facts; it is the study of how the world around us works. Case studies bridge the gap between textbook theory and real-life situations, helping you see physics in action. In SQA Year 7 Physics, you will often encounter questions that ask you to apply your knowledge to everyday scenarios. This article will guide you through a series of case study exercises, from braking bicycles to the colour of the sky. By the end, you will be able to tackle any case study question with confidence and clear reasoning.
物理不仅仅是公式和知识的堆砌，更是研究我们周围世界如何运转的学科。案例分析能架起课本理论与现实情境之间的桥梁，让你看到物理学如何在生活中发挥作用。在 SQA 七年级物理中，你经常会遇到需要将知识运用到日常场景中的题目。本文将通过一系列案例分析练习，从自行车刹车到天空的颜色，带你逐步掌握分析方法。读完全文，你将能够自信而清晰地应对任何案例分析题。
1. Why Case Studies in Physics? | 为什么要在物理中学习案例分析？
Case studies train you to think like a scientist. Instead of simply recalling a definition, you learn to observe a situation, identify the relevant physics, and explain what is happening using correct terminology. This skill is essential for the SQA assessments, where marks are awarded for applying concepts, not just stating them.
案例分析能训练你像科学家一样思考。你不是简单地回忆一个定义，而是学会观察一个情境，识别其中涉及的物理原理，并用准确的术语解释发生的过程。这项技能在 SQA 评估中至关重要，因为得分点在于你能否应用概念，而不仅仅是复述它们。
2. The 5-Step Case Study Method | 案例分析五步法
Step 1: Identify the key physics concepts. Read the scenario and underline words that link to topics you have studied – for example, ‘friction’, ‘energy’, ‘circuit’, ‘wave’. This will tell you what area of physics to focus on.
第一步：识别关键的物理概念。阅读情境描述，把与学习主题相关的词汇划出来，例如”摩擦力”、”能量”、”电路”、”波”。这会告诉你要重点关注物理学的哪个领域。
Step 2: Gather the data given. Carefully note down all the numbers, units, diagrams, and conditions provided. A common mistake is to miss a vital piece of information hidden in the text.
第二步：收集给出的数据。仔细记下所有提供的数字、单位、图表和条件。一个常见的错误就是忽略了隐藏在文本中的关键信息。
Step 3: Apply the relevant principles. Choose the correct relationship or model that fits the scenario. For instance, if the case talks about a moving object stopping, think about energy changes and friction.
第三步：应用相关的原理。选择适合该场景的正确关系或模型。例如，如果案例谈到一个运动的物体停下来，就要想到能量转化和摩擦力。
Step 4: Show your working. If a calculation is needed, write the equation, substitute the numbers, and calculate the answer. Always include the correct unit. For explanations, use complete sentences that link cause and effect.
第四步：展示你的解题过程。如果需要计算，写出公式，代入数字，算出结果。务必带上正确的单位。对于解释题，要用完整的句子写明因果关系。
Step 5: Evaluate and check. Look at your answer and ask: Does it make sense? Is the unit correct? Have I linked my answer back to the original question? This final check can catch silly errors.
第五步：评估并核查。看着你的答案，问自己：这符合常理吗？单位正确吗？我是否把答案和原问题联系起来了？最后这一查能避免粗心错误。
3. Case 1: Energy Changes When Braking | 案例 1：刹车时的能量转换
Scenario: A cyclist is riding along a flat road at a speed of 8 m/s. She sees a red light ahead and pulls the brake levers. The bicycle slows down and stops smoothly. Explain the energy changes that occur from the moment she applies the brakes until the bicycle comes to rest.
情境：一位骑自行车的人正以 8 m/s 的速度在平路上行驶。她看到前方红灯，于是捏紧刹车把手。自行车平稳减速并停下。请解释从她开始刹车到自行车停稳这个过程中所发生的能量转换。
Identify concepts: The keywords ‘speed’, ‘slows down’, ‘brakes’, and ‘stops’ point to kinetic energy and friction. The bicycle has kinetic energy because of its motion. When the brakes are applied, brake pads press against the wheel rims, creating friction.
识别概念：关键词”速度”、”减速”、”刹车”和”停下”指向动能和摩擦。自行车因运动而具有动能。当捏紧刹车时，刹车片压向轮圈，产生摩擦力。
Energy description: The kinetic energy of the bicycle and rider is not destroyed; it is transferred into thermal energy. The friction between the brake pads and the wheel rims causes the contact surfaces to heat up. Some of the kinetic energy also does work on the brake components and the tyres, but the vast majority becomes heat that dissipates into the surrounding air. By the time the bicycle stops, all its kinetic energy has been converted, mainly into thermal energy, with a tiny fraction producing sound.
能量描述：自行车和骑手的动能并没有被消灭，而是转化成了热（内）能。刹车片与轮圈之间的摩擦导致接触面温度升高。一部分动能还对刹车部件和轮胎做功，但绝大部分变成了热能，并散失到周围空气中。当自行车完全停下时，其动能全部被转化，主要是热能，还有极小一部分产生了声音。
Key equation (if needed for a quantitative question): To find the initial kinetic energy, you might later use kinetic energy = ½ × mass × speed². For now, simply knowing the types of energy involved is the goal.
关键公式（如果定量问题需要）：要求最初的动能，以后你可能会用到动能 = ½ × 质量 × 速度²。当前的目标只要知道涉及哪些能量形式就行。
4. Case 2: How a Vacuum Flask Keeps Drinks Hot | 案例 2：为什么保温瓶能保温？
Scenario: A vacuum flask has a double-walled glass container with a vacuum between the walls. The inner surfaces are silvered. A cup of hot tea is poured into the flask, and four hours later the tea is still warm. Using your knowledge of heat transfer, explain why the flask minimises energy loss.
情境：保温瓶有一个双层玻璃容器，两层之间是真空。内表面镀银。一杯热茶倒进保温瓶，四个小时后茶还是温的。请用热传递的知识解释为什么保温瓶能最大限度地减少能量损失。
Identify concepts: The design targets all three methods of heat transfer – conduction, convection, and radiation. The vacuum prevents conduction and convection because there are very few particles to pass on kinetic energy or to form convection currents. The silvered surfaces reflect thermal radiation back into the liquid or away from the cool outside, greatly reducing radiative heat loss. Finally, the stopper at the top is a poor conductor, which minimises conduction through the neck of the flask.
识别概念：保温瓶的设计针对了所有三种热传递方式——传导、对流和辐射。真空阻止了传导和对流，因为其中几乎没有粒子来传递动能或形成对流循环。镀银表面将热辐射反射回液体内部或阻止外部冷辐射进入，大大减少了辐射散热。此外，瓶口的塞子是热的不良导体，减少了通过瓶口的热传导。
Step-by-step reasoning: First, conduction is cut down because the vacuum gap lacks a medium. Second, convection cannot happen in the vacuum for the same reason; in the small air space near the stopper, convection is also limited. Third, radiation is reduced by the shiny silver coating, which is a poor radiator and good reflector. These features together keep the tea’s thermal energy inside for much longer.
一步步推理：首先，传导被抑制了，因为真空层缺乏介质。其次，同样的原因，对流也无法在真空中发生；靠近塞子的小空气层里对流也很有限。第三，光亮的镀银层是差的辐射体和好的反射体，减少了热辐射损失。这些特点共同使茶水的热能在保温瓶内停留更长时间。
5. Case 3: Designing a Simple Circuit | 案例 3：设计一个简单的电路
Scenario: You are given a battery, a switch, and a light bulb. Draw a circuit diagram that will allow the bulb to be switched on and off. Explain how the circuit works, and state the direction of conventional current.
情境：给你一个电池、一个开关和一个灯泡。请画出一个能让灯泡被点亮和熄灭的电路图。解释电路如何工作，并说明传统电流的方向。
Identify concepts: The keywords are ‘circuit’, ‘battery’, ‘switch’, and ‘current’. The circuit must be a complete loop for current to flow. The switch acts as a gap: when closed, the circuit is complete; when open, it breaks the loop. Conventional current flows from the positive terminal of the battery, through the switch and bulb, back to the negative terminal.
识别概念：关键词是”电路”、”电池”、”开关”和”电流”。电路必须是一个完整的回路，电流才能流动。开关相当于一个可控制的缺口：闭合时，回路完整；断开时，回路中断。传统电流从电池正极流出，经过开关和灯泡，回到电池负极。
Circuit description: One wire connects the positive terminal of the battery to one side of the switch. Another wire joins the other side of the switch to one terminal of the bulb. A final wire links the remaining bulb terminal to the negative battery terminal. When the switch is pressed, mobile electrons – though we talk about conventional current – drift around the circuit, transferring energy to the bulb filament, which heats up and emits light.
电路描述：一根导线连接电池正极和开关的一端。另一根导线连接开关另一端和灯泡的一个接线端。最后一根导线连接灯泡剩下的接线端与电池负极。当开关接通时，可移动的电子——尽管我们讨论的是传统电流——在电路中漂移，将能量传递给灯泡灯丝，使其发热发光。
6. Case 4: Measuring the Speed of Sound | 案例 4：测量声音的速度
Scenario: During a school sports day, a student sees the flash of the starter’s pistol 400 metres away. He hears the bang 1.2 seconds later. Use this data to calculate the approximate speed of sound in air. Explain the method and give your answer in metres per second (m/s).
情境：在学校运动会期间，一名学生看见 400 米外发令枪的火光。1.2 秒后，他听到了枪声。请利用这些数据计算声音在空气中的大致速度。解释方法，并以米每秒（m/s）为单位给出答案。
Identify concepts: The scenario involves the relationship between speed, distance and time for a wave, specifically sound. The flash travels almost instantly to the observer, so the time delay is the time taken by the sound to travel the distance. The relevant equation is:
识别概念：这个情境涉及波（具体为声波）的速度、距离与时间的关系。光几乎瞬间到达观察者，因此时间延迟就是声音传播这段距离所需的时间。相关公式为：
speed = distance / time
速度 = 距离 / 时间
Gather data: Distance d = 400 m, time t = 1.2 s.
收集数据：距离 d = 400 m，时间 t = 1.2 s。
Calculation: speed = 400 m / 1.2 s = 333.33… m/s. Rounded sensibly, the speed of sound is approximately 330 m/s.
计算：速度 = 400 m / 1.2 s = 333.33… m/s。合理取整，声速大约为 330 m/s。
Check: This is very close to the accepted value of about 340 m/s at room temperature. Small errors could arise from reaction time when starting the stopwatch. The case study shows how a simple field experiment can verify a well-known constant.
检查：该结果非常接近室温下公认的约 340 m/s。误差可能来自按下秒表时的反应时间。这个案例分析表明，一个简单的户外实验就能验证一个众所周知的常数。
7. Case 5: Why Is the Sky Blue? | 案例 5：天空为什么是蓝色的？
Scenario: On a clear day, the sky appears blue. Yet from space, astronauts see a black sky even when the Sun is shining. Using your knowledge of light scattering, explain why we perceive a blue sky from Earth’s surface.
情境：晴朗的日子，天空看起来是蓝色的。然而，宇航员从太空看，即使太阳高照，天空也是漆黑的。请运用光散射的知识，解释为什么我们从地球表面看到的天空是蓝色的。
Identify concepts: The key idea is Rayleigh scattering. Sunlight is made up of all the colours of the visible spectrum. When sunlight enters Earth’s atmosphere, it collides with gas molecules and tiny particles. The shorter wavelengths (blue and violet) are scattered much more strongly than the longer wavelengths (red and orange).
识别概念：核心概念是瑞利散射。太阳光由可见光谱中所有颜色的光组成。当太阳光进入地球大气层时，会与气体分子和微小粒子碰撞。较短波长（蓝光和紫光）的散射比长波长（红光和橙光）强烈得多。
Explanation: Although violet light is scattered even more than blue, our eyes are less sensitive to violet, and some of it is absorbed by the upper atmosphere. The scattered blue light reaches our eyes from all directions, making the sky look blue. The unscattered redder light continues more or less directly from the Sun, which is why the Sun itself looks yellowish-white. At sunset, the path through the atmosphere is longer; most blue light is scattered away, leaving the reds and oranges visible.
解释：虽然紫光比蓝光散射得更多，但我们的眼睛对紫光不太敏感，而且一部分紫光被高层大气吸收。散射的蓝光从四面八方进入我们的眼睛，使天空看起来呈蓝色。没有散射的红橙色光大致直接来自太阳，这就是为什么太阳本身看起来是黄白色。日落时，阳光穿过大气层的路径更长，大部分蓝光被散射掉，剩下红光和橙光可见。
8. Common Mistakes and Pitfalls | 常见错误与陷阱
Mistake 1: Confusing the forms of energy. Students sometimes say that kinetic energy ‘disappears’ when an object stops. Always remember the conservation of energy: energy is transferred, not destroyed. Name the output energy explicitly, such as thermal energy.
错误一：混淆能量形式。学生有时会说物体停止时动能”消失”了。务必记住能量守恒：能量是转化而不是被消灭。要明确说出转化后的能量形式，比如热能。
Mistake 2: Ignoring units in calculations. In case study questions, always write the unit next to the final number. If the question gives distance in metres and time in seconds, speed must be in m/s. Forgetting units can lose marks.
错误二：计算中忽略单位。在案例分析题中，务必在最终数字旁写下单位。如果题目给出的距离是米，时间是秒，速度就必须用 m/s。忘记单位会丢分。
Mistake 3: Not linking evidence to the explanation. In conduction questions, simply stating ‘the vacuum stops conduction’ is not enough. You must say why: because a vacuum has no particles to transfer kinetic energy.
错误三：没有把证据和解释联系起来。在传导类问题中，只说”真空阻止了传导”是不够的。你必须说明原因：因为真空里没有粒子来传递动能。
Mistake 4: Misapplying the current direction. Remember that conventional current flows from positive to negative, but electron flow is opposite. In most Year 7 SQA questions, stick to conventional current unless told otherwise.
错误四：弄错电流方向。记住传统电流从正极流向负极，而电子流动方向相反。在大多数七年级 SQA 题目中，除非另有说明，否则使用传统电流方向。
9. Exam Tips for Case Study Questions | 考试中的案例分析技巧
Read the question twice. The first time, get a general idea of the scenario. The second time, highlight or circle all the physics vocabulary, numbers and units. This activates your brain to recall the correct topic.
把题目读两遍。第一遍，大致了解情境。第二遍，标出或圈出所有物理词汇、数字和单位。这会激活你的大脑，回忆正确的知识点。
Think before you write. Spend a few seconds planning your answer. If it asks for an explanation, list the steps mentally. If it is a calculation, write the formula first. This prevents rambling and keeps your answer focused.
先想再写。花几秒钟规划答案。如果要求解释，在脑海里列出步骤。如果是计算题，先写出公式。这能避免漫无边际的叙述，让答案更聚焦。
Use ’cause and effect’ language. Phrases like ‘this means that…’, ‘because…’, ‘as a result…’ show the examiner that you understand the logical chain. Each sentence should build on the previous one.
使用”因果”语言。诸如”这意味着……”、”因为……”、”结果是……”等短句能向阅卷人展示你理解了逻辑链条。每句话都应该在前一句的基础上推进。
If a diagram is required, make it large and label it clearly with ruler-drawn lines. A well-labelled diagram can sometimes earn marks even if the written explanation is brief.
如果需要画图，画得大一些，并用尺子画线，清晰地标出各部分。一幅标注清晰的图有时即使文字解释简短，也能拿到分数。
10. Practice: Try It Yourself | 实战练习：自己试试
Case Study Challenge: A child slides down a smooth plastic slide on a warm day. The slide is 3 metres long and makes an angle with the horizontal. The child starts from rest and reaches the bottom with a speed of 4 m/s. Answer the following: (a) Name the main energy change from top to bottom. (b) The child’s mass is 25 kg. What is the kinetic energy at the bottom? (Use kinetic energy = ½ × mass × speed²). (c) Why is the output kinetic energy slightly less than the gravitational potential energy lost? (d) Suggest one way to design the slide to make the child go faster.
案例分析挑战：一个温暖的日子里，一个小孩从一个光滑的塑料滑梯上滑下。滑梯长 3 米，与水平面成一个角度。小孩从静止开始下滑，到达底部时速度为 4 m/s。回答以下问题：(a) 说出从顶到底主要的能量转换形式。(b) 小孩的质量为 25 kg。他在底部的动能是多少？（使用动能 = ½ × 质量 × 速度²）。(c) 为什么输出的动能略小于失去的重力势能？(d) 提出一种滑梯设计方案，好让小孩滑得更快。
Answers: (a) Gravitational potential energy is converted into kinetic energy (and a small amount of thermal energy). (b) kinetic energy = ½ × 25 × (4)² = ½ × 25 × 16 = 200 J. (c) Some energy is transferred to thermal energy due to friction between the child and the slide, and also by air resistance. This energy is not available as kinetic energy. (d) Make the slide steeper to increase the initial gravitational potential energy, or use a smoother material to reduce friction.
答案：(a) 重力势能转化为动能（以及少量的热能）。(b) 动能 = ½ × 25 × (4)² = ½ × 25 × 16 = 200 J。(c) 一部分能量因为小孩与滑梯之间的摩擦以及空气阻力转化成了热能。这部分能量没能变成动能。(d) 让滑梯更陡以增加初始重力势能，或者使用更光滑的材料来减小摩擦。
Reflect on this practice case. Notice how each answer directly uses the concepts and follows the structure: identify – gather – apply – state. With regular practice, you will find that all case studies can be unravelled in the same systematic way.
反思这个实战练习。注意每个答案都是如何直接运用概念，并遵循”识别 – 收集 – 应用 – 表述”这个结构的。只要经常练习，你就会发现所有案例分析题都可以用同样系统的方法来破解。
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