📚 Case Study Practice for Year 7 Physics | Year 7 物理案例分析实战演练
Physics case studies are a brilliant way to sharpen your understanding by applying concepts to everyday situations. Each case study in this article focuses on a key topic from the Year 7 CIE curriculum. You will be guided through the scenario, the physics behind it, and a clear solution. Use these worked examples to build confidence for your assessments.
物理案例分析是将概念应用于日常情境、从而深化理解的绝佳方法。本文每个案例都聚焦于 Year 7 CIE 课纲的核心主题，带你经历情景、背后的物理原理和清晰的解答过程。通过这些解题范例，你可以为考试建立信心。
1. Introduction to Physics Case Studies | 物理案例分析简介
A strong case study answer follows three steps: understand what the question is asking, identify the relevant physics idea, and then apply it logically. Always read the scenario carefully, pick out the variables, and check whether you need to measure, calculate, compare or explain.
要给出一个扎实的案例分析答案，需要三个步骤：理解题目在问什么，识别相关的物理概念，然后有逻辑地应用。一定要仔细阅读情景，提取变量，并判断是需要测量、计算、比较还是解释。
For example, a question about a ball rolling down a slope is really about energy changes and forces. Once you spot that, you can use the ideas of gravitational potential energy converting to kinetic energy and friction slowing it down.
例如，一个关于球从斜坡滚下的问题，本质上是关于能量变化和力。一旦你发现这一点，就可以运用重力势能转化为动能、以及摩擦力使其减速的概念。
In Year 7, we focus on practical investigations: circuits, forces, sound, light, energy, and properties of materials. The following eight cases cover common exam-style contexts and show you exactly how to structure your answers.
在 Year 7 阶段，我们重点关注实践探究：电路、力、声音、光、能量和材料特性。以下八个案例涵盖了常见的考试题型，并展示了如何组织答案。
2. Case 1: Measuring Length and Accuracy | 案例一：测量长度与准确度
A student wants to measure the height of a laboratory bench. She uses a metre ruler, placing it vertically on the floor. Her reading is 72.4 cm. However, when her partner checks, the reading is 71.8 cm. They notice a gap between the ruler and the bench leg because the floor is slightly uneven.
一名学生想测量实验台的高度。她把一把米尺垂直立在地板上，读数为 72.4 cm。但是她的同伴检查后，读数是 71.8 cm。他们发现由于地面稍有不平，尺子与实验台腿之间存在缝隙。
Why did the two readings differ? The main reason is parallax error. When viewing the scale at an angle, the measurement changes because your eye is not directly in line with the mark. Additionally, the ruler was not flush against the bench, introducing a systematic error.
为什么两次读数会不同？主要原因是视差误差。当从某个角度观察刻度时，测量值会改变，因为眼睛没有与刻线正对。另外，尺子没有贴紧台面，引入了系统误差。
To minimise such errors, the student should place the ruler firmly against the bench with its zero mark exactly at the base. She should read the scale with one eye closed and her line of sight perpendicular to the ruler. A second person can also confirm the reading. A digital laser measure might be used for greater precision, but here the key was good technique.
为了减少这些误差，学生应将尺子紧贴台面，让零刻线对准底部。她应单眼观察，视线与尺子垂直，并请第二人确认读数。若需要更高精度，可使用数字激光测距仪，但这里的核心是正确的方法。
In your answer, always state the cause (parallax and misalignment) and the improvement (align zero, eye level at right angle, repeat measurement).
在作答时，总要指出原因（视差和未对准）以及改进方法（对齐零点、眼睛水平且垂直、重复测量）。
3. Case 2: Simple Electric Circuits | 案例二：简单电路
Lucy builds a series circuit with a battery, a switch and two bulbs. When she closes the switch, one bulb lights up brightly but the other remains off. She swaps the bulbs around and the same position in the circuit still gives no light. The bulb that was dark before now lights up in the other socket.
露西用电池、开关和两个灯泡搭建了一个串联电路。她闭合开关后，一个灯泡很亮，另一个却不亮。她把两个灯泡对调，电路中同一个位置的灯泡仍然不亮，而之前暗的那个在新的灯座里亮了。
This tells us the ‘dark’ bulb itself is not faulty. The problem lies in the socket or the connecting wire at that position. In a series circuit, a break anywhere stops current flow only if the break is the only path. Here the bright bulb still lights, so the circuit is complete but the second socket must have a poor connection or an internal break.
这告诉我们那个“不亮”的灯泡本身没有故障。问题出在那个位置的灯座或连接导线上。在串联电路中，如果某一处断开而它又是唯一通路，电流才会中断。这里亮着的灯泡仍然发光，说明电路是通的，但是第二个灯座接触不良或内部断开了。
The student should inspect the holder, tighten the screws or check for corrosion. In an exam, you might be asked to predict what happens if a third bulb is added in series: all bulbs become dimmer because the total resistance increases, reducing the current.
她应该检查灯座，拧紧螺丝或查看是否生锈。考试中可能问：如果再串联一个灯泡，会发生什么？所有灯泡都会变暗，因为总电阻增大，电流减小。
Thus, the case reinforces that a component not working in a series circuit can be traced by swapping parts and checking connections, and that current is the same everywhere in a series loop.
这个案例强调，串联电路中不工作的元件可以通过更换部件和检查连接点来排查，并且串联回路中各处电流相等。
4. Case 3: Effects of Forces on a Trolley | 案例三：力对小车的作用
A dynamics trolley is released from rest on a ramp. The ramp is set at three different heights: 10 cm, 20 cm and 30 cm. The trolley travels along the floor and stops. The distances travelled after leaving the ramp are 45 cm, 120 cm and 210 cm respectively.
一辆动力小车从斜坡上静止释放。斜坡高度分别设置为 10 cm、20 cm 和 30 cm。小车在平面上滑行一段距离后停下。离开斜坡后行驶的距离分别为 45 cm、120 cm 和 210 cm。
The question is: why does a greater ramp height give a longer stopping distance? At a higher start, the trolley has more gravitational potential energy, which converts to kinetic energy at the bottom. A larger kinetic energy means a higher speed. The friction force from the floor is roughly constant, so a higher speed takes longer to decelerate, resulting in a longer distance.
问题是：为什么斜坡越高，停车距离越长？起点越高，小车的重力势能就越大，在斜坡底部转化成更大的动能。动能越大，速度就越快。地板的摩擦力大致恒定，所以速度越大，减速所需时间越长，从而距离更远。
This is a classic energy-and-forces case. You should explain: ‘Gravitational potential energy increases with height; more kinetic energy at the bottom means greater speed; friction acts against motion and stopping distance increases with speed.’
这是一个经典的能与力的案例。你应该解释：“重力势能随高度增加；底部动能更大意味着速度更大；摩擦力阻碍运动，停车距离随速度增大而增大。”
To test this, you could add a rough surface (like carpet) and predict the distance becomes shorter because the friction force is larger. Always link energy, speed, force and distance.
为了验证，可以增加粗糙表面（如地毯），并预测距离会变短，因为摩擦力更大。始终要将能量、速度、力和距离关联起来。
5. Case 4: Energy Changes in a Bouncing Ball | 案例四：弹跳球的能量变化
A student drops a tennis ball from 1.5 m and measures the height of the first bounce: 0.9 m. She drops it again from the same height and records the second bounce: 0.54 m. The ball eventually stops bouncing.
一名学生从 1.5 m 高处抛下一个网球，测量第一次弹起高度为 0.9 m。她再次从同样高度抛下，记录第二次弹起高度为 0.54 m。球最终停了下来。
Why does the bounce height decrease? As the ball falls, gravitational potential energy converts to kinetic energy. On impact with the ground, the ball deforms and some energy transfers to thermal energy in the ball and the ground, and to sound. Not all kinetic energy turns back into useful gravitational potential energy. Each bounce loses more energy until the mechanical energy is dispersed.
为什么弹起高度会下降？球下落时，重力势能转化为动能。撞击地面时，球发生形变，一部分能量转化为球和地面的热能以及声音。并非所有动能都能重新变回有用的重力势能。每次弹跳都会损失能量，直到机械能耗散殆尽。
→ kinetic → thermal and sound (dissipated). The maximum height decreases because energy is ‘wasted’. The ball does not reach the original height unless additional energy is supplied.
回答时要说明能量储存的转移：重力势能 → 动能 → 热能和声能（耗散）。最大高度下降是因为能量被“浪费”了。除非提供额外能量，否则球达不到原来的高度。
This case also helps you understand the principle of conservation of energy: total energy remains the same, but it spreads out and becomes less useful.
这个案例还有助于理解能量守恒原理：总能量保持不变，但能量会分散并变得不太有用。
6. Case 5: Light Reflection and Image Formation | 案例五：光的反射与成像
A ray of light hits a plane mirror at an angle. The student measures the angle between the incident ray and the mirror surface as 30°. She then measures the angle between the reflected ray and the mirror surface and finds it is also 30°.
一束光线以某个角度照射到平面镜上。学生测得入射光线与镜面之间的夹角为 30°。她接着测量反射光线与镜面的夹角，发现也是 30°。
This seems correct, but what is the angle of incidence? he ? The angle of incidence is defined as the angle between the incident ray and the normal (a line perpendicular to the mirror). Similarly, the angle of reflection is measured from the normal. Since the ray is 30° from the mirror surface, it is 90° – al. So the angles of incidence and reflection are both 60°, obeying the law of reflection.
这看起来没错，但入射角是多少呢？入射角的定义是入射光线与法线（垂直于镜面的线）之间的夹角。同样，反射角是从法线量起的。由于光线与镜面成 30°，它与法线成 90° – 30° = 60°。因此入射角和反射角都是 60°，遵守反射定律。
In an experiment, students often confuse the surface angle with the normal. Remember: always measure from the normal. The image formed in a plane mirror is virtual, upright, same size, and laterally inverted.
在实验中，学生常常混淆镜面角与法线。记住：总是从法线测量。平面镜所成的像是虚像、正立、等大且左右颠倒的。
distance = image distance. This case tests careful reading and correct use of terminology.
如果题目要求你预测镜后像的位置，利用物距等于像距。这个案例考查的是仔细审题和正确使用术语。
7. Case 6: Sound and Vibration | 案例六：声音与振动
A wooden ruler is held firmly against a table with about 25 cm overhanging. When the free end is plucked, it vibrates and produces a low-pitched hum. The student shortens the overhanging part to 15 cm and plucks it again. This time the sound is higher pitched.
将一把木尺紧压在桌边，留出约 25 cm 悬空。拨动自由端，尺子振动并发出低沉的声音。学生把悬空部分缩短到 15 cm，再次拨动，声音的音调变高了。
The pitch of a sound is determined by the frequency of vibrations. A shorter ruler vibrates faster because it is stiffer and the moving part has less mass – it can oscillate more times per second. Thus, higher frequency gives a higher pitch. The length of the vibrating part and frequency are inversely related.
声音的高低由振动频率决定。较短的尺子振动更快，因为它更坚硬且振动部分质量更小——每秒可以振动更多次。因此，频率越高，音调越高。振动部分的长度与频率成反比关系。
What about volume? The loudness depends on the amplitude of the vibration. If you pluck harder, the displacement is larger and the sound is louder. This case can be turned into a full investigation by measuring frequency with a microphone and oscilloscope. But for a written case, you must link length, frequency and pitch, and amplitude to loudness.
音量呢？响度取决于振动的幅度。拨动越用力，位移越大，声音越响。这个案例可通过使用麦克风和示波器测量频率，变成完整的探究。但就书面案例而言，你必须将长度、频率和音调联系起来，并将振幅与响度联系起来。
8. Case 7: Thermal Conductors and Insulators | 案例七：热导体与绝缘体
Four spoons made of different materials – metal, plastic, wood and glass – are placed in a cup of hot water. A small blob of butter is placed on each handle. The student times how long it takes for the butter to melt.
将用四种不同材料制成的勺子——金属、塑料、木材和玻璃——放入一杯热水中。每个勺柄上都放一小块黄油。学生记录黄油融化所需的时间。
The metal spoon’s butter melts first, within 30 seconds. The glass spoon takes about 2 minutes, the wood spoon 5 minutes, and the plastic spoon’s butter barely softens. This informs us about thermal conductivity. Metal is a good conductor: heat travels quickly from the hot water along the spoon to the butter. Wood and plastic are poor conductors (good insulators), so heat transfers slowly.
金属勺上的黄油最先在 30 秒内融化。玻璃勺大约用了 2 分钟，木勺 5 分钟，而塑料勺上的黄油几乎没怎么变软。这告诉我们关于热导率的信息。金属是良导体：热量从热水沿勺子迅速传到黄油。木材和塑料是热的不良导体（好的绝缘体），因此热传递很慢。
Thus, conductors allow heat to flow through them easily, while insulators slow down the transfer. In your answer, always use the terms ‘thermal conductor’ and ‘thermal insulator’. Explain that the metal’s free electrons help carry energy, whereas wood and plastic lack these mobile charge carriers.
因此，导体容易让热量通过，而绝缘体则减缓热量传递。回答时要使用“热导体”和“热绝缘体”这些术语。解释金属中的自由电子有助于传递能量，而木材和塑料缺少这些可移动的带电粒子。
This case study can also link to keeping houses warm: insulating materials reduce heat loss by conduction.
这个案例还可以联系到房屋保暖：绝缘材料能够减少热传导带来的热量损失。
9. Case 8: Floating and Sinking (Density) | 案例八：浮与沉（密度）
A student places a small wooden block, a metal key, a wax candle and a piece of pumice stone into a tank of water. The wood floats, the metal key sinks, the candle floats and the pumice stone floats. She then pushes the wooden block under water; when released, it bobs straight back up.
一名学生将小木块、金属钥匙、蜡烛和一块浮石放入水槽中。木块浮起来，金属钥匙沉下去，蜡烛浮起来，浮石也浮着。然后她把木块按到水下，一松手它又马上冒了出来。
Whether an object floats or sinks depends on its density compared to the density of water (1 g/cm³). Wood, wax and pumice all have a density less than water, so they experience an upthrust greater than their weight when fully submerged, causing them to float. The metal key is denser than water, so its weight is greater than the upthrust, and it sinks.
物体浮沉取决于其密度与水的密度（1 g/cm³）的对比。木头、蜡和浮石的密度都小于水，因此当完全浸没时，它们受到的向上浮力大于自身重量，因而上浮。金属钥匙密度大于水，自身重量大于浮力，所以下沉。
The upthrust is equal to the weight of the fluid displaced. A floating object displaces a volume of water whose weight equals the object’s weight. A sinking object cannot displace enough water weight to match its own weight. Therefore, density is the key concept.
浮力等于物体排开流体的重量。漂浮的物体排开的水的重量等于其自身重量。下沉的物体无法排开足够重量的水来匹配自身重量。因此，密度是关键概念。
ity = mass / volume if data is provided. This kind of case often appears with data tables.
回答此类案例题时，可先计算或比较密度，然后说明哪个力更大。如果提供了数据，使用公式：密度 = 质量 / 体积。这类案例常以数据表的形式出现。
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