📚 Year 8 WJEC Physics Case Study Practice | 八年级 WJEC 物理案例实战演练
Welcome to your WJEC Year 8 Physics revision session, where we apply key ideas to realistic case studies. Each scenario challenges you to use forces, energy, electricity, waves and more – just like in your exam. By working through these examples, you will strengthen your ability to identify variables, perform simple calculations, and explain physical phenomena clearly.
欢迎来到 WJEC 八年级物理复习课堂，我们将核心概念应用于真实的案例研究中。每个情景都要求你动用力、能量、电学、波等知识——就像考试中一样。通过练习这些案例，你会提升识别变量、进行简单计算以及清楚解释物理现象的能力。
1. Analysing a Car's Stopping Distance | 分析汽车停车距离
A car travelling on a dry road must stop when the driver spots a hazard. The total stopping distance is the sum of the thinking distance (distance moved during the driver’s reaction time) and the braking distance (distance travelled while the brakes slow the car to a halt).
一辆汽车在干燥路面上行驶，当司机看见危险时必须停下。总停车距离是思考距离（司机反应时间内行驶的距离）与制动距离（刹车使汽车减速直至停止期间行驶的距离）之和。
total stopping distance = thinking distance + braking distance
总停车距离 = 思考距离 + 制动距离
Case study: The car is moving at 13 m/s. The driver' ion time = 13 m/s × 0.7 s = 9.1 m. Hence, total stopping distance = 9.1 m + 14 m = 23.1 m.
案例研究：汽车以13 m/s的速度行驶，司机的反应时间为0.7秒。制造商说明在此速度下的制动距离为14米。思考距离 = 速度 × 反应时间 = 13 m/s × 0.7 s = 9.1 m。因此，总停车距离 = 9.1 m + 14 m = 23.1 m。
Factors that increase thinking distance include tiredness, alcohol, mobile phone use and poor visibility. Factors that increase braking distance include wet or icy roads, worn tyres, worn brake pads and extra passengers (greater mass).
增加思考距离的因素有疲劳、酒精、使用手机和能见度差。增加制动距离的因素包括潮湿或结冰路面、磨损的轮胎、磨损的刹车片以及额外的乘客（质量更大）。
| Speed (m/s) | Thinking Distance (m) | Braking Distance (m) | Total Stopping Distance (m) |
|---|---|---|---|
| 8 | 5.6 | 6 | 11.6 |
| 13 | 9.1 | 14 | 23.1 |
| 20 | 14 | 32 | 46 |
The data show that doubling the speed more than doubles the braking distance. This is because the car's kinetic energy increases with the square of speed, requiring much more work from the brakes to stop.
数据表明，速度加倍会使制动距离增加不止一倍。这是因为汽车的动能随速度的平方增加，需要刹车做更多的功才能停下来。
2. Troubleshooting a Household Circuit | 家庭电路故障排查
An electric oven suddenly stops working. The user checks the plug and finds the 13 A fuse has blown. When a fuse melts, it breaks the circuit and prevents excessive current from causing overheating or fire.
一台电烤箱突然停止工作。用户检查插头，发现13 A保险丝熔断了。保险丝熔断时会断开电路，防止过大电流导致过热或火灾。
se the current slightly exceeds the fuse rating, a sustained overload would blow the fuse.
为了找出原因，我们计算电流：烤箱额定功率=3000 W，市电电压=230 V。电流 I = P ÷ V = 3000 W ÷ 230 V ≈ 13.04 A。由于电流略高于保险丝额定值，持续过载会使保险丝熔断。
Domestic circuits are connected in parallel. This means that if the oven fuse blows, other appliances on the same ring main continue to work. A parallel arrangement also ensures each device receives the full 230 V.
家庭电路采用并联连接。这意味着即使烤箱保险丝熔断，同一环形电路上的其他电器仍能工作。并联结构还能确保每个设备都获得完整的230 V电压。
A short circuit (when live and neutral wires touch due to damaged insulation) can also cause a sudden current surge. In both cases, the fuse acts as a safety device, preventing the wires from overheating.
短路（火线与零线因绝缘损坏而接触）也会导致电流突然剧增。在这两种情况下，保险丝都起到安全装置的作用，防止导线过热。
3. Keeping Drinks Hot: A Flask Investigation | 保温瓶探秘：热传递
A vacuum flask keeps hot soup warm for hours. Understanding heat transfer helps explain its design. Heat can travel by conduction, convection and radiation, but the flask minimises all three.
保温瓶能让热汤保温数小时。理解热传递有助于解释其设计。热量可以通过传导、对流和辐射传播，但保温瓶将这三种方式降到最低。
The double-walled glass container has a vacuum between the walls. Without any particles, conduction and convection cannot take place across the vacuum. The silvered inner surface reflects infrared radiation back into the liquid, reducing radiative heat loss. The stopper prevents convection and conduction at the top.
双层玻璃容器在器壁之间是真空的。没有粒子，传导和对流都无法穿过真空。镀银的内表面将红外辐射反射回液体中，减少辐射散热。瓶塞则阻止了顶部的对流和传导。
acity of water c = 4200 J/(kg°C), temperature change Δθ = energy / (m × c) = 4200 J / (0.5 kg × 4200 J/(kg°C)) = 2 °C drop. The flask keeps the change small.
案例任务：假设瓶内有0.5 kg汤散热，估算温度下降。假设损失了4200 J能量。使用水的比热容 c = 4200 J/(kg°C)，温度变化 Δθ = 能量 / (m × c) = 4200 J / (0.5 kg × 4200 J/(kg°C)) = 2 °C 下降。保温瓶使温度变化很小。
4. Analysing a Cyclist's Journey | 分析自行车骑行旅程
A student records her cycle ride to school. The distance-time data are shown below. From this we can calculate average speed and describe the journey stages.
一位学生记录了她骑车上学的过程。距离-时间数据如下所示。我们可以据此计算平均速度并描述旅程的各个阶段。
| Time (s) | Distance (m) |
|---|---|
| 0 | 0 |
| 60 | 300 |
| 120 | 600 |
| 180 | 800 |
| 240 | 1000 |
| 300 | 1200 |
60 s, where speed = 300 m ÷ 60 s = 5.0 m/s. Between 120 s and 180 s the speed is only (800–600) m / 60 s ≈ 3.3 m/s, perhaps because of a hill or traffic.
平均速度 = 总距离 ÷ 总时间 = 1200 m ÷ 300 s = 4.0 m/s。最快的路段是在0 s到60 s之间，速度 = 300 m ÷ 60 s = 5.0 m/s。在120 s到180 s之间速度仅为(800–600) m / 60 s ≈ 3.3 m/s，也许是因为上坡或交通原因。
On a distance-time graph, a steeper slope means a higher speed. A flat (horizontal) section would indicate the cyclist is stationary. In this journey there are no flat sections, so the cyclist kept moving.
在距离-时间图上，斜率越大表示速度越快。平直部分（水平段）表示骑车人静止。在此次旅程中没有平直部分，所以骑行者一直在移动。
5. Energy Transfers on a Roller Coaster | 过山车上的能量转换
A roller coaster car of mass 500 kg descends from a height of 20 m. At the top it has gravitational potential energy (GPE) which converts to kinetic energy (KE) as it drops, assuming friction is negligible.
一辆质量为500 kg的过山车从20 m高处下降。在顶端它具有重力势能（GPE），下落时势能转换为动能（KE），假设摩擦可以忽略。
GPE = mgh = 500 kg × 10 N/kg × 20 m = 100 000 J
势能 = mgh = 500 kg × 10 N/kg × 20 m = 100 000 J
nd speed v: ½ × 500 × v² = 100 000 J → v² = (100 000 × 2) / 500 = 400 → v = √400 = 20 m/s.
在最低点，所有势能都转化为动能。利用 KE = ½ m v²，可求得速度 v：½ × 500 × v² = 100 000 J → v² = (100 000 × 2) / 500 = 400 → v = √400 = 20 m/s。
In reality, some energy is transferred to thermal energy by friction and air resistance, so the actual speed would be slightly lower. Engineers design the track to minimise these losses and keep the ride exciting but safe.
实际上，一部分能量会因摩擦和空气阻力转化为热能，所以实际速度会稍低。工程师设计轨道时会尽量减小这些损失，让过山车刺激而安全。
6. Lightning and Thunder: A Wave Speed Mystery | 闪电与雷鸣：波速之谜
During a thunderstorm, you see a flash of lightning and, after a delay, hear the thunder. This happens because light travels much faster
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