📚 Year 7 Cambridge Physics: Unit Test Mock Paper Walkthrough | 剑桥七年级物理：单元测试模拟卷解析
This mock paper walkthrough is designed to guide Year 7 students through a typical Cambridge Physics unit test. By analysing common question types, you will improve your understanding of key concepts, learn the exam technique that earns marks, and boost your confidence for the real assessment.
本模拟卷解析旨在帮助七年级学生熟悉典型的剑桥物理单元测试。通过分析常见题型，你将加深对核心概念的理解，掌握能够拿分的应试技巧，并为真实考试建立信心。
The walkthrough follows the paper question by question. Each section shows the correct method, the scientific reasoning behind it, and the mistakes that cost students the most marks every year.
本解析按试卷题目顺序逐一展开。每一节都会展示正确的解题方法、背后的科学原理，以及每年最容易让学生失分的典型错误。
1. Overview of the Mock Paper | 模拟卷概览
The mock paper includes 10 questions that cover three main topic areas: Forces and Energy, Waves, and Electricity and Magnetism. Most questions require calculations, short written answers or diagram drawing.
模拟卷包含10道题目，涵盖三大主题领域：力与能量、波、以及电与磁。大多数题目要求计算、简答或画图。
You will have 45 minutes to complete the paper, which is worth 40 marks. That works out at roughly one minute per mark, leaving a few minutes at the end to check your units and your working.
你将有45分钟完成试卷，满分为40分。这大约等于每1分用1分钟，最后还能留出几分钟检查单位和解题步骤。
The marks are distributed roughly as follows:
分值大致分布如下：
| Topic | 主题 | Questions | 题号 | Approx. marks | 约分值 |
| Forces and Energy | 力与能量 | Q1 - Q4 | 18 |
| Waves | 波 | Q5 - Q7 | 12 |
| Electricity and Magnetism | 电与磁 | Q8 - Q10 | 10 |
Read each question carefully, underline the numbers and units given, and always show your working to earn partial marks even when the final answer is wrong.
仔细阅读每道题，把题目给出的数字和单位划线标出，并且始终写出解题步骤——即使最终答案错了，也能拿到过程分。
2. Mark Scheme and Command Words | 评分标准与指令词
Cambridge questions use specific command words. Knowing what each one demands tells you how much detail to write and how many marks are available.
剑桥考题会使用特定的指令词。弄清每个指令词的要求，你就知道该写多少细节、能拿几分。
- State / Give - a single fact, no explanation. Usually 1 mark. | 说明/给出——写一个事实即可，不需要解释，通常1分。
- Describe - say what happens, in order. | 描述——按顺序说出发生了什么。
- Explain - say what happens and why, using a scientific reason or the word 'because'. | 解释——说出发生了什么以及为什么，要给出科学理由或用"因为"连接。
- Calculate - write the equation, substitute, then give the answer with a unit. | 计算——写公式、代入数值、给出带单位的答案。
- Draw / Label - use a sharp pencil and a ruler; add arrows and names. | 画出/标注——用削尖的铅笔和直尺；加上箭头和名称。
Marks are awarded for the physics, not for beautiful handwriting. A correct number without a unit often loses the final mark, and a correct answer with no working may lose the method mark.
分数给的是物理内容，而不是字迹是否漂亮。只写数字不写单位常常会丢掉最后一分；只写正确答案却不写步骤，则可能丢掉方法分。
A quick rule for written answers: one mark equals roughly one clear scientific point, so a 2-mark 'explain' question needs two linked ideas.
简答题的快速法则：1分大约等于一个清晰的科学要点，所以一道2分的"解释"题需要两个相互关联的观点。
3. Question 1: Speed Calculation | 速度计算
The first question asks you to calculate the average speed of a cyclist who travels 400 metres in 50 seconds.
第一题要求计算一名自行车手的平均速度，他50秒内骑行了400米。
speed = distance ÷ time
Substitute the values: speed = 400 m ÷ 50 s = 8 m/s. Always include the unit 'm/s' (metres per second).
代入数值：速度 = 400 米 ÷ 50 秒 = 8 米/秒。务必带上单位"米/秒"。
The same equation can be rearranged, and the exam may test any of the three forms:
同一个公式可以变形，考试可能考其中任何一种形式：
- speed = distance ÷ time | 速度 = 路程 ÷ 时间
- distance = speed × time | 路程 = 速度 × 时间
- time = distance ÷ speed | 时间 = 路程 ÷ 速度
at answer is far too slow for a cyclist, so a quick sanity check on the size of your answer will catch the error.
常见错误是把路程和时间弄反，算出 50 ÷ 400 = 0.125 米/秒。对自行车手来说这个速度实在太慢，所以对答案大小做一个常识性检查就能发现错误。
e km/h into m/s, divide by 3.6; to go the other way, multiply by 3.6. So 36 km/h = 10 m/s.
还要注意单位换算。把千米/小时换成米/秒要除以 3.6；反过来则乘以 3.6。因此 36 千米/小时 = 10 米/秒。
4. Question 2: Distance-Time Graphs and Acceleration | 距离-时间图与加速度
The second question shows a distance-time graph and asks you to describe the motion and calculate a speed from the gradient.
第二题给出一个距离-时间图，要求你描述运动情况，并通过斜率计算速度。
On a distance-time graph the gradient gives the speed. A steeper line means a faster object, a flat line means the object is stationary, and a straight sloping line means constant speed.
在距离-时间图上，斜率代表速度。线越陡表示物体越快，水平线表示物体静止，倾斜的直线表示匀速运动。
To find the gradient, pick two points on the line and divide the change in distance by the change in time:
求斜率的方法是：在直线上取两点，用路程变化量除以时间变化量：
speed = change in distance ÷ change in time
If the line rises from 0 m at 0 s to 100 m at 20 s, then speed = 100 m ÷ 20 s = 5 m/s.
如果直线从 0 秒时的 0 米上升到 20 秒时的 100 米，那么速度 = 100 米 ÷ 20 秒 = 5 米/秒。
Some questions use a speed-time graph instead. Here the gradient gives acceleration and the area under the line gives the distance travelled.
有些题目改用速度-时间图。此时斜率代表加速度，图线下的面积代表通过的路程。
acceleration = change in velocity ÷ time taken
on is measured in metres per second squared because it is the change in speed each second.
例如，一辆汽车在6秒内从 0 米/秒加速到 12 米/秒，加速度为 12 ÷ 6 = 2 米/秒²。加速度的单位是"米每二次方秒"，因为它表示每秒速度的变化量。
Do not forget to describe the whole journey in part (a) style questions: mention starting from rest, moving at constant speed, then stopping or returning.
在描述整段运动的题目中，别忘了写全：从静止出发、匀速运动、然后停下或返回。
5. Question 3: Forces, Weight and Mass | 力、重力与质量
The third question asks for the weight of a 5 kg box on Earth and then on the Moon, and asks you to explain why the mass stays the same.
第三题要求计算一个 5 千克的箱子在地球上和在月球上的重力，并解释为什么质量保持不变。
weight = mass × gravitational field strength (W = m × g)
arth, g = 9.8 N/kg (your course may round this to 10 N/kg). So W = 5 kg × 9.8 N/kg = 49 N.
在地球上，g = 9.8 牛/千克（你的课程可能取整为 10 牛/千克）。所以 W = 5 千克 × 9.8 牛/千克 = 49 牛。
e Moon, g = 1.6 N/kg, so W = 5 kg × 1.6 N/kg = 8 N. The weight is smaller because the Moon's gravitational pull is weaker.
在月球上，g = 1.6 牛/千克，所以 W = 5 千克 × 1.6 牛/千克 = 8 牛。重力变小是因为月球的引力更弱。
- Mass is the amount of matter in an object and is measured in kilograms (kg). It does not change with location. | 质量是物体所含物质的多少，单位是千克（kg），不随位置改变。
- Weight is the force of gravity on an object and is measured in newtons (N). It changes with the gravitational field strength. | 重力是作用在物体上的引力，单位是牛顿（N），会随重力场强度改变。
Forces are drawn as arrows. The length of the arrow shows the size of the force, and the arrowhead shows the direction. When forces on an object are balanced, the resultant force is zero and the object stays still or keeps moving at a steady speed.
力可以用箭头表示。箭头长度表示力的大小，箭头方向表示力的方向。当物体受力平衡时，合力为零，物体保持静止或持续匀速运动。
A typical mistake is writing 'the weight is 5 kg'. Weight is a force, so it must be in newtons. Also remember the resultant force, not just one force, decides how the motion changes.
典型错误是写成"重力是 5 千克"。重力是力，必须用牛顿作单位。还要记住：决定运动如何改变的是合力，而不是某一个力。
6. Question 4: Energy Stores, Transfers and Conservation | 能量储存、转移与守恒
The fourth question asks you to name the energy stores involved when a ball is dropped and to explain why it does not bounce back to its original height.
第四题要求你指出小球下落过程中涉及的能量储存形式，并解释为什么它弹不回原来的高度。
Useful stores to know at Year 7 level include kinetic, gravitational potential, elastic potential, thermal, chemical and nuclear. Energy can be transferred by heating, by mechanical work, by electrical work or by radiation.
七年级需要掌握的能量储存形式包括：动能、重力势能、弹性势能、热能、化学能和核能。能量可以通过加热、机械做功、电做功或辐射来转移。
kinetic energy = ½ × mass × speed² (Eₖ = ½mv²)
gravitational potential energy = mass × g × height (Eₚ = mgh)
Eₖ = ½ × 2 × 3² = ½ × 2 × 9 = 9 J. For the same ball raised 5 m: Eₚ = 2 × 9.8 × 5 = 98 J.
一个 2 千克的球以 3 米/秒运动：Eₖ = ½ × 2 × 3² = ½ × 2 × 9 = 9 焦。同一个球被举高 5 米：Eₚ = 2 × 9.8 × 5 = 98 焦。
As the ball falls, gravitational potential energy is transferred to the kinetic store. When it hits the floor, some energy is transferred to the thermal store of the ball and the floor, and some to the surroundings by sound. That is why the ball never returns to its original height.
小球下落时，重力势能被转移到动能储存中。撞到地面时，一部分能量转移到球和地面的热能储存中，一部分以声音的形式转移到周围环境。这就是球永远回不到原来高度的原因。
Energy cannot be created or destroyed, only transferred between stores. This is the principle of conservation of energy, and it is worth a mark on its own.
能量既不能创造也不能消灭，只能在不同的储存之间转移。这就是能量守恒原理，本身就可以直接得一分。
7. Question 5: Wave Speed, Frequency and Wavelength | 波速、频率与波长
The fifth question gives a wave with a frequency of 50 Hz and a wavelength of 2 m, and asks for the wave speed.
第五题给出一个频率为 50 赫兹、波长为 2 米的波，要求计算波速。
wave speed = frequency × wavelength (v = f × λ)
requency is measured in hertz (Hz), wavelength in metres (m), and wave speed in metres per second (m/s).
代入得：v = 50 赫兹 × 2 米 = 100 米/秒。频率的单位是赫兹（Hz），波长的单位是米（m），波速的单位是米/秒（m/s）。
- Amplitude - the maximum displacement from the rest position; it shows how much energy the wave carries. | 振幅——偏离平衡位置的最大距离，反映波携带的能量大小。
- Wavelength - the distance from one crest to the next crest. | 波长——相邻两个波峰之间的距离。
- Frequency - the number of complete waves passing a point each second. | 频率——每秒通过某一点的完整波的个数。
- Period - the time for one complete wave, T = 1 ÷ f. For 50 Hz, T = 0.02 s. | 周期——完成一个完整波所需的时间，T = 1 ÷ f。50 赫兹对应 T = 0.02 秒。
Waves can also be described as transverse or longitudinal. In a transverse wave the vibrations are at right angles to the direction of travel (light, water ripples). In a longitudinal wave the vibrations are along the direction of travel (sound).
波还可以分为横波和纵波。横波的振动方向与传播方向垂直（如光、水波）；纵波的振动方向与传播方向一致（如声波）。
Do not confuse wavelength with amplitude when labelling a diagram. Wavelength is measured crest to crest; amplitude is measured from the middle line to a crest.
在图上标注时不要把波长和振幅弄混。波长从波峰量到波峰；振幅从中间线量到波峰。
8. Question 6: Reflection and Ray Diagrams | 反射与光线图
The sixth question shows a ray of light hitting a plane mirror and asks you to complete the diagram and state the law of reflection.
第六题给出一束光射到平面镜上的情形，要求你补全光路图并说明反射定律。
angle of incidence = angle of reflection
Both angles are measured from the normal, which is a dashed line drawn at 90° to the mirror surface at the point where the ray hits it.
两个角都要从法线量起。法线是在光线入射点处垂直于镜面画出的虚线。
- Draw the normal as a dashed line, at exactly 90° to the mirror. | 法线用虚线画出，与镜面成严格的 90°。
- Draw the reflected ray with a ruler, on the other side of the normal. | 用直尺在法线另一侧画出反射光线。
- Add an arrow on each ray to show the direction light travels. | 在每条光线上加箭头，表示光的传播方向。
- Label the incident ray, reflected ray, angle of incidence (i) and angle of reflection (r). | 标注入射光线、反射光线、入射角（i）和反射角（r）。
If the light passes into a denser medium such as glass, it slows down and bends towards the normal. Coming out into air it speeds up and bends away from the normal.
如果光进入玻璃等较密介质，速度变慢，会向法线偏折。从玻璃射出到空气时速度变快，会远离法线偏折。
The two most common errors are measuring the angle from the mirror surface instead of from the normal, and forgetting the arrows so that the diagram has no direction.
最常见的两个错误是：从镜面而不是从法线量角；以及忘记画箭头，导致图中没有方向信息。
9. Question 7: Electric Circuits - Current, Voltage and Resistance | 电路——电流、电压与电阻
The seventh question shows a series circuit with a 6 V battery and a 3 Ω resistor and asks for the current.
第七题给出一个串联电路，电源电压 6 伏，电阻 3 欧，要求计算电流。
voltage = current × resistance (V = I × R)
I = V ÷ R = 6 V ÷ 3 Ω = 2 A. Current is measured in amperes (A) with an ammeter connected in series; voltage is measured in volts (V) with a voltmeter connected in parallel across a component.
变形得 I = V ÷ R = 6 伏 ÷ 3 欧 = 2 安。电流用安培表测量，必须串联在电路中；电压用伏特表测量，必须并联在元件两端。
Circuit rules you should be able to quote:
你需要能够准确说出的电路规律：
- In a series circuit the current is the same at every point. | 串联电路中，各处的电流相同。
- In a series circuit the supply voltage is shared between the components. | 串联电路中，电源电压由各元件分担。
- In a parallel circuit the current from the supply is the sum of the currents in each branch. | 并联电路中，干路电流等于各支路电流之和。
- In a parallel circuit the voltage across each branch is the same. | 并联电路中，各支路两端的电压相同。
In parallel, each branch gets the full 6 V and carries 2 A, so the supply current is 4 A.
若电源 6 伏、两个相同的 3 欧电阻串联，每个电阻分得 3 伏，电流为 6 ÷ 6 = 1 安。若并联，每条支路都获得 6 伏、各通过 2 安，干路电流为 4 安。
Remember that resistance is measured in ohms (Ω) and that a longer or thinner wire has more resistance. Adding resistors in series increases the total resistance; adding them in parallel decreases it.
记住电阻的单位是欧姆（Ω），导线越长或越细电阻越大。串联更多电阻会使总电阻增大；并联更多电阻会使总电阻减小。
10. Question 8: Magnetism and Electromagnets | 磁性与电磁铁
The eighth question asks you to describe what happens when two bar magnets are brought together and to suggest two ways of making an electromagnet stronger.
第八题要求你描述两块条形磁铁靠近时会发生什么，并提出两种增强电磁铁的方法。
- Like poles repel (N-N or S-S). | 同名磁极相互排斥（N-N 或 S-S）。
- Unlike poles attract (N-S). | 异名磁极相互吸引（N-S）。
- Magnetic materials include iron, steel, cobalt and nickel. Aluminium and copper are not magnetic. | 磁性材料包括铁、钢、钴和镍。铝和铜不是磁性材料。
The magnetic field is strongest at the poles, and field lines run from the north pole to the south pole outside the magnet. The closer the lines, the stronger the field.
磁场在两极最强。在磁体外部，磁感线从北极出发指向南极。磁感线越密，磁场越强。
An electromagnet is made from a coil of wire carrying a current, usually wrapped around an iron core. Unlike a permanent magnet, it can be switched on and off and its strength can be changed.
电磁铁由通电线圈绕在铁芯上制成。与永久磁铁不同，它可以开关，强度也可以改变。
To make an electromagnet stronger you can:
增强电磁铁的方法有：
- Increase the current through the coil. | 增大通过线圈的电流。
- Increase the number of turns on the coil. | 增加线圈的匝数。
- Use a soft iron core instead of an air core. | 用软铁芯代替空心线圈。
Electromagnets are used in scrapyard cranes, electric bells, relays and loudspeakers because their magnetism can be controlled.
电磁铁被用于废车场吊车、电铃、继电器和扬声器，因为它们的磁性可以人为控制。
11. Question 9 and 10: Practical Skills and Data Handling | 实验技能与数据处理
The last two questions are practical skills questions, testing variables, measurements and graphs. These are easy marks if you follow a routine.
最后两题是实验技能题，考查变量、测量和图像处理。只要按固定套路作答，这些分数很好拿。
- Independent variable - the one you change. | 自变量——你主动改变的量。
- Dependent variable - the one you measure. | 因变量——你测量得到的量。
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