📚 Decoding the AQA A-Level Physics Unit 2 Mark Scheme (January 2021) | 解读AQA A-Level物理Unit 2评分方案（2021年1月）
The January 2021 AQA A-Level Physics Unit 2 paper tested mechanics, materials, and waves. Understanding its mark scheme is not just about checking answers — it reveals how examiners award marks for reasoning, notation, and precise language.
2021年1月AQA A-Level物理Unit 2试卷考查了力学、材料与波动。理解其评分方案不仅仅是核对答案——它揭示了考官如何为推理、符号使用和语言精确性分配分数。
1. Anatomy of the Mark Scheme | 评分方案的结构
The AQA mark scheme groups marks into three types: ‘M1’ for method, ‘A1’ for a correct final answer, and ‘B1’ for independent standalone marks (often definitions or plotted points). A mark labelled ‘C’ is a condition-linked mark, awarded only if a previous step is correct.
AQA评分方案将分数分为三类：’M1’表示方法分，’A1’表示最终正确答案分，’B1’表示独立的单独得分点（通常是定义或绘图点）。标记为’C’的分数是条件关联分数，仅在前一步正确时才授予。
For each question, the mark scheme lists ‘allow’ (acceptable alternatives), ‘ignore’ (material not penalised), and ‘not allowed’ (common errors). Learning this vocabulary helps you predict which parts of your answer will earn credit.
对于每道题，评分方案列出’allow’（可接受的替代答案）、’ignore’（不予扣分的内容）和’not allowed’（常见错误）。学会这些用语有助于你预测答案中哪些部分能得分。
2. Mechanics: ‘Show That’ Questions | 力学：“证明”类题目
In Unit 2 mechanics, ‘show that’ questions require you to demonstrate a stated result. The mark scheme typically awards one mark for the correct equation, one for substitution, and one for a matching conclusion. Writing only the final number often loses the method marks.
在Unit 2力学中，“证明”类题目要求你演示一个给定的结论。评分方案通常为正确的方程给1分，代入给1分，结论匹配给1分。只写最终数字往往会失去方法分。
For example, if asked to show that the maximum height reached is 45 m, you must quote v² = u² + 2as, substitute v = 0, u = 30 m s⁻¹, a = −9.81 m s⁻², then state s ≈ 45.9 m ≈ 46 m. The mark scheme allows a small rounding tolerance, but the equation and substitution are mandatory.
例如，若要求证明最大高度为45 m，你必须写出v² = u² + 2as，代入v = 0、u = 30 m s⁻¹、a = −9.81 m s⁻²，然后说明s ≈ 45.9 m ≈ 46 m。评分方案允许微小的四舍五入容差，但方程和代入是必须的。
3. Newton’s Third Law: Precision of Language | 牛顿第三定律：语言的精确性
The January 2021 mark scheme penalised vague statements like ‘equal and opposite forces cancel out’. For full marks, you must specify: the forces act on different bodies, are of the same type, and exist simultaneously. The scheme lists all three features as separate B marks.
2021年1月的评分方案对“等大反向的力相互抵消”这种模糊表述扣分。要得满分，你必须明确：力作用于不同物体、属于同一性质、并且同时存在。评分方案将这三个特征列为各自的B分。
A model answer might read: ‘The force exerted by the ball on the wall is equal in magnitude and opposite in direction to the force exerted by the wall on the ball; the two forces act on different objects and are of the same type (normal contact).’
一个模范答案可以是：“球对墙施加的力与墙对球施加的力大小相等、方向相反；这两个力作用在不同物体上，且属于同一类型（法向接触力）。”
4. Momentum Conservation in Collisions | 碰撞中的动量守恒
When applying conservation of momentum to a two-body collision, the mark scheme requires a clear sign convention. For example, taking the initial direction of the moving object as positive, write m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ with signs inserted before substitution.
在将动量守恒应用于两体碰撞时，评分方案要求明确的符号约定。例如，以运动物体的初始方向为正，写出m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂，并在代入前插入符号。
Examiner reports note that many students lost marks by omitting the direction of the final velocity. The mark scheme awards the final A mark only when both magnitude and direction are stated, e.g., ‘3.2 m s⁻¹ in the opposite direction to the initial motion’.
考官报告指出，许多学生因省略末速度的方向而失分。评分方案仅在同时给出大小和方向时才授予最终A分，例如“3.2 m s⁻¹，方向与初始运动方向相反”。
5. Stress-Strain Curves: Reading Graphs | 应力-应变曲线：读图
Questions on the stress-strain curve frequently ask you to identify the elastic limit, yield point, and breaking stress. The mark scheme awards B marks for reading values from the graph to the correct precision — usually one decimal place matching the graph grid.
关于应力-应变曲线的问题常要求你识别弹性极限、屈服点和断裂应力。评分方案对从图中读取数值并达到正确精度（通常与网格匹配的一位小数）的答案授予B分。
For the calculation of Young’s modulus, you must choose a point on the linear region, read the stress and strain, then compute stress ÷ strain. The mark scheme penalises using a point beyond the proportional limit, as the gradient there no longer represents Young’s modulus.
对于杨氏模量的计算，你必须在线性区域选取一点，读取应力和应变，然后计算应力÷应变。评分方案对使用比例极限之后的点扣分，因为该处的梯度不再代表杨氏模量。
6. Uncertainties and Significant Figures | 不确定度与有效数字
The January 2021 mark scheme reinforced the rule: a calculated value must be stated to the same number of significant figures as the least precise data supplied. If you are given values to 3 s.f., your answer must not be given to 2 s.f. or 4 s.f. without justification.
2021年1月的评分方案重申了规则：计算值必须与所给数据中精度最低者的有效数字位数一致。如果给定值为3位有效数字，你的答案不应无缘无故写成2位或4位有效数字。
When calculating fractional uncertainty, the mark scheme expects the equation Δq/q = Δx/x + Δy/y for products or quotients. For a repeated measurement, use range ÷ 2 as the absolute uncertainty, then convert to a percentage.
在计算分数不确定度时，评分方案期望对乘除关系使用方程Δq/q = Δx/x + Δy/y。对于重复测量，使用极差÷2作为绝对不确定度，然后转换为百分比。
7. Waves: Phase Difference and Path Difference | 波动：相位差与路程差
A classic Unit 2 question links phase difference to path difference. The mark scheme awards marks for writing the relationship as:
Unit 2的经典题目将相位差与路程差联系起来。评分方案为写出以下关系授予分数：
phase difference = (2π ÷ λ) × path difference
where λ is the wavelength. If the phase difference is expressed in degrees, the scheme allows 360° × path difference ÷ λ. The key credited detail is explicitly naming the wavelength as the one in the medium, not in air.
其中λ是波长。若相位差以度为单位，评分方案允许360° × 路程差 ÷ λ。关键的得分细节是明确指出波长是介质中的波长，而不是空气中的波长。
For constructive interference, the mark scheme requires the condition path difference = nλ (n = 0, 1, 2, …), while destructive interference requires path difference = (n + ½)λ. A common error is swapping these — the scheme deliberately orders them to test this distinction.
对于相长干涉，评分方案要求条件路程差 = nλ（n = 0, 1, 2, ……），而相消干涉要求路程差 = (n + ½)λ。常见错误是将两者互换——评分方案特意以此区分来考察这一点。
8. Stationary Waves: Nodes and Antinodes | 驻波：波节与波腹
In the stationary wave questions, the mark scheme distinguishes between a node (point of zero displacement) and an antinode (point of maximum displacement). A diagram-based question awards B marks for marking at least two nodes and two antinodes correctly on a given string or pipe.
在驻波问题中，评分方案区分波节（位移为零的点）和波腹（位移最大的点）。基于图的问题要求你正确标出给定弦或管上的至少两个波节和两个波腹，才能获得B分。
For a closed pipe, the first harmonic has length L = λ/4; for an open pipe, L = λ/2. The mark scheme penalises students who treat both pipes as identical. A precise statement of the boundary conditions — displacement node at a closed end, antinode at an open end — is often a credited B mark.
对于闭管，基波的管长L = λ/4；对于开管，L = λ/2。评分方案对将两种管等同对待的学生扣分。准确描述边界条件——闭端为位移波节、开端为波腹——通常是可得的B分。
9. Diffraction and Interference: Double Slit | 衍射与干涉：双缝
The double-slit equation w = λD/s (fringe spacing = wavelength × slit-to-screen distance ÷ slit separation) appears in nearly every Unit 2 session. The mark scheme requires you to state the units of each quantity and to convert mm to m before substitution.
双缝方程w = λD/s（条纹间距 = 波长 × 缝屏距离 ÷ 缝间距）几乎出现在每次Unit 2考试中。评分方案要求你说明每个量的单位，并在代入前将毫米换算为米。
Explaining why interference fringes are observed requires the concept of coherence: the two slits act as coherent sources because they are illuminated by the same wavefront. The mark scheme specifically lists ‘same wavelength’, ‘constant phase difference’, and ‘single source used’ as three separate B marks.
解释为何观察到干涉条纹需要相干性的概念：两个缝作为相干光源，因为它们被同一波前照亮。评分方案将“同波长”、“恒定相位差”和“使用单一光源”列为三个独立的B分。
10. Experimental Design: The ‘Describe How’ Question | 实验设计：“描述如何”题
In the required practical-style question, the mark scheme uses a grid of marks: apparatus (1), method (3–4), measurements (2), and analysis (2). To score the method marks, you must write a sequential procedure with one sentence per step, not a paragraph of continuous prose.
在基于必修实验的题目中，评分方案使用一个分数网格：装置（1分）、方法（3–4分）、测量（2分）和分析（2分）。要获得方法分，你必须按步骤写一个顺序程序，每步一句话，而不是连续段落式的叙述。
Example: ‘1. Set up the ripple tank and level it using a spirit level. 2. Use a stroboscope to slow the wave pattern. 3. Measure the distance between ten wavefronts using a ruler. 4. Divide by nine to obtain the wavelength. 5. Repeat three times and average.’ Each numbered step maps directly to a mark in the scheme.
示例：“1. 搭建波纹槽并用水平仪调平。2. 使用频闪仪使波形减慢。3. 用尺子测量十个波前之间的距离。4. 除以九得到波长。5. 重复三次并取平均值。”每个编号步骤直接对应方案中的一个分数。
11. Calculation Presentation: The Three-Line Method | 计算题呈现：三行写法
The mark scheme consistently rewards calculations written in a three-line format: equation → substitution → result with unit. For example, using the wave equation: v = fλ; then v = 250 × 1.36; then v = 340 m s⁻¹. This structure guarantees that even if the final answer is wrong, the method mark is secured.
评分方案始终鼓励用三行格式书写计算题：方程 → 代入 → 带单位的结果。例如，使用波动方程：v = fλ；然后v = 250 × 1.36；最后v = 340 m s⁻¹。这种结构保证即使最终答案错误，方法分仍然可得。
Examiner comments from January 2021 note that many students combined the first two lines into ‘v = 250 × 1.36 = 340 m s⁻¹’, which is acceptable but risks losing a mark if the substitution is incorrect. Separating the lines isolates each skill.
2021年1月的考官评论指出，许多学生将前两行合并为“v = 250 × 1.36 = 340 m s⁻¹”，这虽然可接受，但如果代入有误则可能失分。分开展示各步骤可以隔离每个技能。
12. Maximising Marks from ‘Explain’ Questions | 从“解释”类题目中最大化得分
The final type of question that separates high achievers is the three-mark ‘explain’ item. The mark scheme allocates one mark per correct, independent physical idea. Common credited ideas include: ‘energy is transferred as work is done’, ‘the force acts through a distance’, and ‘friction converts kinetic energy into internal energy’.
最后一种区分高分段学生的题是3分“解释”题。评分方案为每个正确的、独立的物理概念分配1分。常见的得分点包括：“做功时能量被转移”、“力作用了一段距离”、“摩擦力将动能转化为内能”。
To maximise marks, write three short sentences, each pointing to a different physical law or quantity. Avoid repeating the same idea in different words — the scheme does not award double marks for paraphrasing. If the question specifies a direction, such as ‘in terms of energy’, ensure every sentence references energy explicitly.
为最大化得分，请写三个短句，每句指向不同的物理定律或量。避免用不同的词复述同一个概念——评分方案不会对同义改写重复给分。如果问题指定了角度，如“从能量角度”，请确保每个句子都明确提到能量。
Published by TutorHao | Physics Revision Series | aleveler.com
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