📚 Case Study Practice in KS3 AQA Physics | KS3 AQA 物理案例分析实战演练
Physics is not just about memorising facts; it is about applying concepts to real-world situations. This article presents a series of case studies that test your understanding of Key Stage 3 AQA Physics topics such as forces, energy, electricity, waves, and magnetism. Each scenario is designed to strengthen your analytical skills and prepare you for exam-style questions.
物理不只是记忆知识点，更在于将概念应用到真实情境中。本文通过一系列案例分析，检验你对 KS3 AQA 物理中力、能量、电学、波和磁学等主题的理解。每个场景都旨在强化你的分析能力，为考试题型做好准备。
1. Forces in a Tug of War | 拔河中的力
Two teams are pulling on a rope. Team A exerts a force of 520 N to the right, while Team B pulls with 480 N to the left. The rope itself has a small weight but we can ignore it for this analysis.
两队正在拔河。A 队向右施加了 520 N 的力，B 队向左施加了 480 N 的力。绳子本身重量很小，本次分析可以忽略不计。
The forces are horizontal and act in opposite directions. To find the resultant force, we subtract the smaller force from the larger one and take the direction of the larger force.
两个力沿水平方向且方向相反。求合力时，用较大的力减去较小的力，方向与较大力的方向一致。
Resultant force = 520 N – 480 N = 40 N to the right
合力 = 520 牛顿 – 480 牛顿 = 40 牛顿，方向向右
Because the resultant force is not zero, the forces are unbalanced. According to Newton’s First Law, the rope (and both teams) will accelerate to the right. If the resultant force were zero, the rope would stay still or move at constant speed.
由于合力不为零，这些力是不平衡的。根据牛顿第一定律，绳子（以及两队人）将会向右加速。如果合力为零，绳子将保持静止或匀速直线运动。
This case shows that it is the net force, not individual forces, that decides motion. In a real tug of war, friction between feet and ground also plays a key role in providing the pulling force.
这个案例说明，决定运动状态的是合力而非单个力。在真实的拔河比赛中，脚与地面之间的摩擦力同样在提供拉力方面起着关键作用。
2. Electrical Circuits in the Home | 家庭电路
A living room has three ceiling lamps connected to the mains. They are all controlled by a single wall switch. If one lamp’s filament breaks, will the other two stay on?
客厅里有三盏天花板灯连接在家庭电路中，均由一个墙壁开关控制。如果其中一盏灯的灯丝断了，另外两盏灯还会亮吗？
In a UK home, lighting circuits are wired in parallel. Each lamp receives the full mains voltage (230 V), and there is more than one path for the current. When the filament in one lamp breaks, that path is broken, but the other two independent paths remain complete. Hence the other lamps stay on.
在英国的家庭中，照明电路采用并联连接。每盏灯都能分到全部市电电压（230 伏），并且电流有不只一条路径。当一盏灯的灯丝烧断时，该路径断开，但另外两条独立路径仍然完好，因此其他灯会继续亮。
A parallel arrangement also has a safety advantage: if too much current flowed through a single series loop, overheating could cause a fire. In parallel, each lamp draws a smaller share of the total current, and a fuse or circuit breaker can protect the entire circuit.
并联连接还有一个安全优势：如果所有灯串联在同一个回路中，电流过大会导致过热并可能引发火灾。并联时，每盏灯消耗的电流较小，同时保险丝或断路器可以保护整个电路。
Knowing the difference between series and parallel helps explain why household appliances are not connected one after another.
了解串联与并联的区别有助于解释为什么家用电器不会一个接一个地串联连接。
3. Energy Transfers on a Roller Coaster | 过山车中的能量转移
Imagine a roller coaster car at rest at the top of a 30 m high hill. Its gravitational potential energy (GPE) is at a maximum. As it descends, the GPE decreases and kinetic energy (KE) increases. Ignoring friction, the sum of GPE and KE stays constant.
想象一辆过山车静止在 30 米高的坡顶。此时它的重力势能（GPE）最大。当它下降时，重力势能减少，动能（KE）增加。若忽略摩擦，GPE 与 KE 的总和保持不变。
GPE = m × g × h = mass (kg) × 10 N/kg × height (m)
重力势能 = 质量 × 重力场强度 × 高度 = 质量 (kg) × 10 牛/千克 × 高度 (m)
At the bottom, nearly all the GPE has changed into KE. In reality, some energy is transferred to thermal energy because of air resistance and wheel friction. This ‘wasted’ energy heats the track and surroundings, but the total energy in the universe is still conserved.
到达底部时，几乎所有 GPE 都转化成了 KE。实际上，由于空气阻力和车轮摩擦，一部分能量会转化为热能。这些“浪费”的能量使轨道和周围环境变热，但宇宙中的总能量仍然是守恒的。
Understanding energy transfers helps engineers design safer, more efficient rides that manage speed without exceeding structural limits.
理解能量转移有助于工程师设计更安全、更高效的游乐设施，在不超过结构极限的情况下控制速度。
4. Sound Waves and Hearing | 声波与听觉
An astronaut on the Moon cannot hear the blast of a nearby rocket unless they are inside a pressurised cabin. Why? Sound is a longitudinal wave that travels by causing particles in a medium to vibrate. On the Moon there is practically no atmosphere — it is a vacuum — so there are very few particles to carry the vibration.
月球上的宇航员无法听到附近火箭的爆炸声，除非他们身处加压舱内。为什么？声音是一种纵波，它通过使介质中的粒子振动来传播。月球上几乎没有大气层——那里是真空——因此几乎没有粒子可以传递振动。
Sound travels at about 340 m/s in air. If a flash of lightning is seen 5 seconds before the thunder is heard, the lightning struck roughly 1700 m away.
声音在空气中的传播速度约为 340 米/秒。如果看到闪电 5 秒后才听到雷声，那么闪电发生处大约在 1700 米之外。
This time-lag method is called echolocation, also used by bats and by ships’ sonar. For hearing, our eardrums convert the vibrations into electrical signals that the brain interprets.
这种利用时间差的方法叫做回声定位，蝙蝠和船只的声呐也会使用这一原理。在听觉过程中，我们的耳膜将振动转化为电信号，再由大脑进行解读。
5. Light and Reflection | 光的反射
A periscope uses two plane mirrors placed parallel and at 45° to the light path. Light from an object strikes the top mirror, reflects downwards, hits the bottom mirror, and then enters the eye. The law of reflection states that the angle of incidence equals the angle of reflection, measured from the normal.
潜望镜使用两面彼此平行且与光路成 45° 角的平面镜。来自物体的光线照射到顶部镜子上，向下反射，击中底部镜子，然后进入眼睛。反射定律指出，入射角等于反射角，均从法线开始测量。
Because the mirrors are parallel, the light ray exits parallel to its original direction but shifted down. This allows a submarine crew to see above the water while staying submerged. The image is upright and virtual.
由于两面镜子平行，出射光线与原始方向平行，只是向下平移了一段距离。这使潜艇人员能够在水下看到水面上方的景象。所成的像是正立的虚像。
If one of the mirrors is slightly tilted, the final image will shift, demonstrating how sensitive optical instruments are to alignment.
如果其中一面镜子轻微倾斜，最终的像就会移动，这表明光学仪器对校准是多么敏感。
6. Pressure in Fluids | 流体压强
A diver descends to a depth of 15 m in a freshwater lake. The pressure around her body increases with depth because the weight of the water above pushes down. Liquid pressure depends on depth, fluid density, and gravitational field strength.
一位潜水员下潜到淡水湖中 15 米的深度。由于上方水的重量向下挤压，她身体周围的压强随着深度增加而增大。液体压强取决于深度、液体密度和重力场强度。
Pressure ΔP = ρ × g × h = 1000 kg/m³ × 10 N/kg × 15 m = 150 000 Pa
压强 ΔP = 密度 × g × 深度 = 1000 千克/米³ × 10 牛/千克 × 15 米 = 150 000 帕斯卡
This 150 000 Pa is the pressure due to the water alone. At the surface, atmospheric pressure already pushes with about 100 000 Pa, so the total absolute pressure at 15 m is about 250 000 Pa. Her body must equalise internal pressure to avoid injury.
这 150 000 帕斯卡仅是水造成的附加压强。在水面，大气压强已经约为 100 000 帕斯卡，因此 15 米深处的绝对总压强约为 250 000 帕斯卡。她的身体必须平衡内外压强，以避免受伤。
Gases are also fluids, so the same principles explain why our ears ‘pop’ when ascending in an aeroplane.
气体也是流体，因此相同的原理也解释了为什么在飞机上升时我们的耳朵会“发闷”。
7. Speed and Distance-Time Graphs | 速度与距离-时间图
A delivery van travels from a depot. Between 0 and 10 minutes, it moves 4 km at a steady speed. Between 10 and 15 minutes, the distance does not change — the van is stationary. From 15 to 25 minutes, it covers a further 6 km.
一辆送货车从仓库出发。在 0 到 10 分钟内，它以稳定的速度行驶了 4 公里。在 10 到 15 分钟之间，距离没有变化——货车静止不动。从 15 到 25 分钟，它又行驶了 6 公里。
On a distance-time graph, a straight sloping line means constant speed; a horizontal line means stationary. The speed in the first segment is:
在距离-时间图中，一条倾斜的直线代表匀速运动，一条水平线代表静止。第一段旅程的速度为：
v = Δd / Δt = 4 km / (10/60) h = 24 km/h
速度 = 距离 / 时间 = 4 公里 除以 (10/60) 小时 = 24 公里/小时
In the last segment the speed is higher: 6 km in 10 minutes gives 36 km/h. The overall average speed for the whole journey (10 km in 25 minutes) is 24 km/h. Notice that a steeper slope always indicates a greater speed.
在最后一段旅程中速度更高：6 公里花费 10 分钟，得出 36 公里/小时。全程（10 公里 / 25 分钟）的平均速度为 24 公里/小时。注意，斜率越陡，速度越大。
8. Magnetism and Electromagnets | 磁性与电磁铁
A scrapyard uses a large electromagnet attached to a crane to lift iron and steel cars. When the switch is closed, a current flows through coils of wire wrapped around a soft iron core, magnetising the core. When the switch is opened, the magnetic field collapses and the load drops.
废品回收站使用连接在起重机上的大型电磁铁来吊起铁质和钢质汽车。当开关闭合时，电流流过缠绕在软铁芯上的线圈，使铁芯磁化。当开关断开时，磁场消失，被吊起的物体便落下。
Electromagnets are useful because their strength can be increased by adding more turns of wire, increasing the current, or using a pure iron core. They are also switchable, unlike permanent magnets.
电磁铁之所以有用，是因为可以通过增加线圈匝数、增大电流或使用纯铁芯来增强其磁场强度。而且与永磁体不同，它们是可以被开关控制的。
Other applications include electric bells, loudspeakers, and MRI scanners. In a relay, a small current in an electromagnet closes a switch for a larger, more dangerous circuit, keeping the user safe.
其他应用还包括电铃、扬声器和核磁共振成像仪。在继电器中，电磁铁中的小电流会闭合一个大电流的危险电路的开关，从而保障使用者的安全。
9. Friction and Air Resistance | 摩擦力与空气阻力
When a skydiver jumps from a plane, she accelerates downwards due to gravity. As her speed increases, upward air resistance (drag) builds up. Eventually drag equals her weight, the resultant force becomes zero, and she falls at a constant speed called terminal velocity.
当跳伞者从飞机上跳下时，她因重力而向下加速。随着速度增加，向上的空气阻力（曳力）逐渐增大。最终阻力等于她的体重，合力变为零，她便以恒定速度下落，这个速度称为终端速度。
Opening a parachute greatly increases the surface area, which increases air resistance dramatically. The upward force now greatly exceeds weight, so she decelerates to a new, much lower terminal velocity, allowing a safe landing.
打开降落伞会大幅增加表面积，从而使空气阻力急剧增大。此时向上的力远大于体重，因此她减速到一个新的、低得多的终端速度，从而安全着陆。
Friction between solids is also vital: walking, braking, and holding objects all depend on the grip provided by friction.
固体之间的摩擦力同样至关重要：走路、刹车和握住物体都依赖于摩擦力提供的抓握力。
10. Gravity and Weight | 重力与重量
The mass of an object is the amount of matter it contains, measured in kilograms (kg), and it stays the same wherever you go. Weight is the force of gravity on that mass, measured in newtons (N). On Earth, the gravitational field strength g is about 10 N/kg.
物体的质量是它所含物质的多少，用千克（kg）衡量，无论人在哪里，质量都保持不变。重量则是作用于该质量上的重力，用牛顿（N）衡量。在地球上，重力场强度 g 约为 10 牛/千克。
A 2 kg bag of sugar has a weight of:
一袋 2 千克的糖，其重量为：
W = m × g = 2 × 10 = 20 N
重量 = 质量 × g = 2 × 10 = 20 牛顿
On the Moon, g is only about 1.6 N/kg, so the same bag would weigh just 3.2 N, although its mass remains 2 kg. This is why astronauts can jump higher on the Moon — weight is less, mass unchanged.
在月球上，g 仅有约 1.6 牛/千克，因此同一袋糖的重量只有 3.2 牛顿，尽管它的质量依然是 2 千克。这就是宇航员在月球上能跳得更高的原因——重量变小了，但质量没变。
Published by TutorHao | Physics Revision Series | aleveler.com
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