📚 Year 7 CAIE Physics Unit Test Mock Exam Analysis | Year 7 CAIE 物理：单元测试模拟卷解析
This article provides a step-by-step walkthrough of a mock unit test for Year 7 CAIE Physics. Each question is explained in detail, covering key scientific concepts, common mistakes, and effective revision strategies. Use this analysis to strengthen your understanding and boost your confidence before the real test.
本文针对 Year 7 CAIE 物理单元测试模拟卷进行逐步解析。每道题都详细解释了核心科学概念、常见错误和有效的复习策略。利用这份解析来加深理解，在实际测试前增强信心。
1. How to Use This Mock Exam Analysis | 如何使用这份模拟卷解析
The best way to use a mock paper analysis is to treat it as a guided practice session rather than a reading exercise. Cover the worked answer with a piece of paper, attempt the question yourself first, then compare your method line by line with the explanation given here. Pay special attention to the units, the number of significant figures, and the way each step is written down, because Year 7 CAIE unit tests award marks for clear working as well as for the final answer.
使用模拟卷解析的最佳方式，是把它当作一次有引导的练习，而不是一篇阅读材料。先用纸遮住答案，自己动手做一遍，再逐行与解析中的方法对照。要特别留意单位、有效数字，以及每一步的书写方式，因为 Year 7 CAIE 单元测试不仅给最终答案分数，也给清晰的解题步骤分数。
Before you start, gather the equipment you will need in a real test: a sharpened pencil, a 30 cm rule, a scientific calculator, and an eraser. Working under realistic conditions trains your brain to recall the method quickly when the pressure is on.
开始之前，请准备好在真实测验中会用到的文具：削好的铅笔、一把 30 cm 直尺、一台科学计算器和一块橡皮。在接近真实的条件下练习，能训练大脑在紧张的考试环境中快速回忆解题方法。
- Read every question twice before writing anything. | 动笔前把每道题读两遍。
- Write the formula, then substitute, then calculate. | 先写公式，再代入，最后计算。
- Always attach units to every measurement. | 每个测量结果都要带单位。
- Mark your own work honestly with a red pen. | 用红笔诚实地给自己批改。
2. Measuring Length Accurately | 准确测量长度
A student measures a pencil using a ruler marked in centimetres and millimetres. The tip of the pencil is placed exactly at the 0 cm mark. The other end lies between 12.3 cm and 12.4 cm, much closer to 12.4 cm. What is the length in centimetres and in metres?
一位学生用一把标记厘米和毫米的尺子测量铅笔的长度。铅笔尖端恰好对准 0 cm 刻度，另一端落在 12.3 cm 和 12.4 cm 之间，非常靠近 12.4 cm。该铅笔长度是多少厘米？合多少米？
The correct reading is 12.4 cm, because we always estimate one more digit beyond the smallest scale division on the instrument. The smallest division on this ruler is 1 mm, which equals 0.1 cm. The end of the pencil falls clearly nearer to the 12.4 cm mark than to the 12.3 cm mark, so the length is recorded as 12.4 cm. To convert centimetres into metres, divide by 100, since there are 100 cm in 1 metre.
正确读数是 12.4 cm，因为我们总是要估读到仪器最小分度值的下一位。这把尺子的最小分度值是 1 mm，即 0.1 cm。铅笔末端明显更接近 12.4 cm 刻度而非 12.3 cm 刻度，因此长度记为 12.4 cm。要把厘米换算成米，除以 100 即可，因为 1 米等于 100 厘米。
length = 12.4 cm ÷ 100 = 0.124 m
Notice that the metre value is smaller than the centimetre value, which is exactly what we expect when converting a smaller unit into a larger one. If your answer for a metre conversion ever comes out larger than the centimetre number you started with, you have multiplied instead of divided.
注意，化成米之后的数值比厘米数值更小，这是把较小单位换算成较大单位时必然的结果。如果你的米制答案反而比原来的厘米数值更大，说明你乘错了，应该除。
A second marker point that examiners look for is the choice of instrument and the avoidance of parallax error. The eye must be directly above the mark being read; if you look from an angle, the reading drifts and the answer becomes inaccurate. The pencil should also be placed at the 0 cm mark rather than at the end of a worn ruler, otherwise a zero error spoils every measurement you take.
考官另一个关注要点是仪器的选择以及视差误差的避免。眼睛必须正对读数刻度线的正上方；如果从斜角看，读数就会偏移，结果就不准确。铅笔还应从 0 cm 刻度开始放，而不是从磨损尺子的末端开始，否则零点误差会破坏你每一次测量的结果。
3. Converting Units With Confidence | 自信地完成单位换算
Unit conversion questions appear in almost every physics unit test, and they are among the easiest marks to gain if you know the multipliers. The golden rule is simple: when you change from a small unit to a large unit you divide, and when you change from a large unit to a small unit you multiply. Learn the common conversions as a table rather than as isolated facts.
单位换算题几乎出现在每一次物理单元测试中，只要掌握倍数关系，这就是最容易拿到的分数之一。黄金法则很简单：由小单位换算成大单位要除，由大单位换算成小单位要乘。要把常见换算当作一张表格整体记忆，而不是孤立地记零散事实。
| Quantity | 物理量 | Units | 单位 | Conversion | 换算关系 |
| Length 长度 | mm, cm, m, km | 1 cm = 10 mm; 1 m = 100 cm; 1 km = 1000 m |
| Volume 体积 | cm³, ml, l, m³ | 1 cm³ = 1 ml; 1 l = 1000 cm³; 1 m³ = 1 000 000 cm³ |
| Mass 质量 | g, kg | 1 kg = 1000 g |
| Density 密度 | g/cm³, kg/m³ | 1 g/cm³ = 1000 kg/m³ |
| Time 时间 | s, min, h | 1 min = 60 s; 1 h = 3600 s |
Compare the pencil question with a typical follow-up: convert 250 cm³ into m³. Because a cubic metre is a million times bigger than a cubic centimetre, you divide by 1 000 000, giving 0.00025 m³. The same logic applies in reverse: 0.5 kg becomes 500 g, and 3.6 km becomes 3600 m.
把铅笔题与常见的延伸题对比：把 250 cm³ 换算成 m³。由于 1 立方米比 1 立方厘米大一百万倍，要除以 1 000 000，得到 0.00025 m³。反向同理：0.5 kg 变成 500 g，3.6 km 变成 3600 m。
4. Reading Mass and Volume Scales | 读取质量和体积刻度
Mass is measured with a top-pan balance, and the reading is taken once the display or pointer becomes steady. A common trap is forgetting to check whether the balance reads zero with an empty pan. If it does not, the zero must be reset before the object is placed on the pan, otherwise every reading is shifted by the same amount.
质量用托盘天平（或电子天平）测量，待显示数值或指针稳定后读数。常见陷阱是忘记检查空盘时天平是否显示为零。如果没有归零，必须先归零再放物体，否则每次读数都会被同一个数值所偏移。
Volume of a liquid is measured with a measuring cylinder, and the reading is taken at the bottom of the curved surface, called the meniscus. This is a favourite exam point: reading the top of the meniscus gives a value that is too large. The cylinder must also sit on a flat bench and the eye must be level with the scale.
液体体积用量筒测量，读数要读液面弯曲的最低点，也就是凹液面的底部。这是考试非常喜欢的考点：读凹液面的顶部会让结果偏大。量筒还必须放在水平桌面上，眼睛与刻度线齐平。
For an irregular solid such as a small stone, the displacement method is used. Record the starting volume of water, lower the stone gently on a thread, and record the new volume. The volume of the stone is the difference between the two readings.
对于小块石头这类形状不规则的固体，要用排水法。先记录水的初始体积，用细线把石头轻轻放入水中，再记录新的体积。石头的体积就是两次读数之差。
Vsolid = Vfinal − Vinitial
If the water started at 40 cm³ and rose to 65 cm³, the stone has a volume of 25 cm³. Make sure the solid is fully submerged and that no water splashes out, because a single lost drop changes the answer by several cubic centimetres.
如果水的初始体积是 40 cm³，放入石头后升到 65 cm³，那么石头的体积是 25 cm³。要确保固体完全浸没，并且没有水溅出，因为溅出一滴水就可能让答案差好几立方厘米。
5. Measuring Time and Reducing Human Error | 测量时间与减少人为误差
Time in school experiments is usually measured with a digital stopwatch, which reads to 0.01 s. The instrument is far more precise than the student using it: human reaction time is roughly 0.2 s, so a single timing of a short event carries a large percentage error. The standard fix is to time many repetitions and divide.
学校实验中的时间通常用数字秒表测量，可读到 0.01 s。仪器本身远比使用者精确：人的反应时间大约是 0.2 s，所以对短时事件只测一次，误差百分比会很大。标准的解决办法是测量多次重复后再相除。
For a pendulum, the correct method is to time ten complete swings and divide by ten. If ten swings take 14.0 s, one swing takes 1.4 s. This reduces the reaction-time error ten times over, which is why examiners insist on the phrase "time ten oscillations and divide by ten".
对于单摆，正确的方法是测十次完整摆动的时间再除以十。如果十次摆动用时 14.0 s，则一次摆动为 1.4 s。这样就把反应时间误差缩小到十分之一，这也是考官坚持要求写出"测十次摆动后除以十"这一表述的原因。
Repeat readings also allow you to spot anomalies. Take three measurements, discard any value that is obviously far from the others, and average the rest. In a well-designed investigation, repeated readings should be close together; if they scatter widely, something in the method or the apparatus needs to be checked.
重复测量还能帮你发现异常数据。测三次，剔除明显偏离的其他数值，再对剩下的取平均。设计良好的探究中，重复读数应当彼此接近；如果分散很大，就说明方法或器材有问题需要检查。
6. Density Calculations From Measurements | 由测量数据计算密度
Density links mass and volume, and it is one of the most frequently examined calculations in Year 7 physics. The equation must be memorised, and every step must be shown with units.
密度把质量和体积联系起来，是 Year 7 物理中考查频率最高的计算之一。这个公式必须背熟，每一步都要带单位写出。
ρ = m ÷ V
where m is mass in grams or kilograms, V is volume in cm³ or m³, and ρ is density in g/cm³ or kg/m³. In a typical question a metal block has a mass of 54 g and a volume of 20 cm³. Substituting into the equation gives a density of 2.7 g/cm³, which is the accepted value for aluminium. Converting to SI units, multiply by 1000 to obtain 2700 kg/m³.
其中 m 是质量，单位克或千克；V 是体积，单位 cm³ 或 m³；ρ 是密度，单位 g/cm³ 或 kg/m³。在典型题目中，一块金属块质量为 54 g，体积为 20 cm³。代入公式得到密度 2.7 g/cm³，这正是铝的标准值。换算成国际单位，乘以 1000，得到 2700 kg/m³。
Density also explains floating and sinking. An object floats in water when its density is less than 1.0 g/cm³, and sinks when its density is greater. This is why a solid steel nail sinks while a large steel ship floats: the ship is mostly air, so its average density is far lower than that of water.
密度还能解释浮沉现象。当物体密度小于 1.0 g/cm³ 时在水中漂浮，大于时下沉。这就是实心钢钉会下沉、而巨大的钢船却能浮起的原因：船体内部大部分是空气，平均密度远小于水。
7. Common Mistakes in Measurement Questions | 测量题常见错误
Most lost marks in Year 7 measurement questions come from a small number of repeated errors rather than from a lack of knowledge. Knowing them in advance lets you check your own work before you hand the paper in.
Year 7 测量题中丢掉的分数，大多来自少数几个反复出现的错误，而不是知识缺失。提前了解这些错误，就能在交卷前自查。
| Mistake | 常见错误 | Correct practice | 正确做法 |
| Reading at the top of the meniscus | 读凹液面顶部 | Read at the bottom of the meniscus | 读凹液面底部 |
| Eye at an angle to the scale | 视线与刻度不垂直 | Eye level with the mark to avoid parallax | 视线与刻度齐平以避免视差 |
| Multiplying when converting cm to m | 厘米换米时用乘 | Divide by 100 when going to a larger unit | 换成大单位要除以 100 |
| Leaving out units in the answer | 答案不写单位 | Every measurement must carry a unit | 每个测量值都必须带单位 |
| Not checking for a zero error | 不检查零点误差 | Reset the balance or note the offset | 天平归零或记录偏差值 |
| Timing a single short event | 只测一次短时事件 | Repeat and average the readings | 重复测量并取平均值 |
A useful habit is to write the unit beside every number as soon as you write it down. This slows you down slightly during practice, but in the test it prevents the most common single mark loss of all, and it forces you to check that the units in the question and the units in your formula actually match.
一个有用的习惯是：每写下一个数字，就立刻在旁边写上单位。练习时这会稍微减慢速度，但在考试中它能避免最常见的一分丢失，同时也会促使你检查题目中的单位与自己公式里的单位是否真正一致。
8. Forces and Motion Basics in Unit Tests | 单元测试中的力与运动基础
Year 7 CAIE physics also tests the relationship between speed, distance and time. The equation is straightforward, but rearranging it correctly is where many students slip. Write the triangle or the formula out every single time rather than trying to do it mentally.
Year 7 CAIE 物理还会考查速度、路程和时间之间的关系。公式本身很直接，但很多学生错在变形出错。每次都把公式或三角图写出来，不要只凭心算。
speed = distance ÷ time
If a cyclist travels 150 m in 30 s, the average speed is 5 m/s. To convert to km/h, multiply by 3.6, giving 18 km/h. Notice how the unit of the answer follows directly from the units you put in: metres divided by seconds must give metres per second.
如果一名骑车人在 30 s 内行驶了 150 m，平均速度就是 5 m/s。换算成 km/h 要乘以 3.6，得到 18 km/h。注意答案的单位直接由代入的单位决定：米除以秒必然得到米每秒。
Forces are measured in newtons, and the key idea is that a resultant force changes motion. Balanced forces mean constant speed or rest; unbalanced forces mean acceleration, deceleration or a change of direction. Weight is the force of gravity on a mass, and on Earth the gravitational field strength is taken as 10 N/kg at this level.
力的单位是牛顿，核心概念是合力会改变运动状态。受力平衡意味着匀速运动或静止；受力不平衡意味着加速、减速或改变方向。重量是重力作用在质量上的力，在这一阶段，地球上的重力场强度取 10 N/kg。
W = m × g
A 5 kg bag therefore has a weight of 50 N on Earth, but the same bag would weigh only about 8 N on the Moon, while its mass of 5 kg stays the same everywhere. Mass, measured in kilograms, is the amount of matter; weight, measured in newtons, depends on where you are.
所以一个 5 kg 的包在地球上重 50 N，同一个包在月球上只有大约 8 N，而它 5 kg 的质量在任何地方都不变。用千克测量的质量表示物质的多少；用牛顿测量的重量则取决于你所处的位置。
9. Energy, Light and Sound Quick Checks | 能量、光与声的概念速查
Energy questions usually ask you to name a store and describe a transfer rather than to calculate. The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another. In a torch, energy is transferred from a chemical store in the battery to a light store and a thermal store in the surroundings.
能量题通常要求你说出一个能量库并描述能量的转移，而不是进行运算。能量守恒定律指出：能量既不会被创造，也不会被消灭，只会从一种能量库转移到另一种。在手电筒中，能量从电池的化学能量库转移到光能量库以及周围环境的热能量库。
Light travels in straight lines, which is why shadows have sharp edges and why a ray diagram uses a ruler. The law of reflection states that the angle of incidence equals the angle of reflection, both measured from the normal, the imaginary line perpendicular to the mirror surface.
光沿直线传播，这就是影子边缘清晰、光路图要用直尺作图的原因。反射定律指出：入射角等于反射角，两者都是从法线量起，法线是垂直于镜面的那条假想线。
Sound is produced by vibrating
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