📚 Application Question Techniques from the Jan 2021 Unit 1 Examiner’s Report | 2021年1月单元1考官报告应用题技巧
The January 2021 International A-Level Physics Unit 1 exam assessed core topics in mechanics and materials. The examiner’s report highlighted common pitfalls and areas where candidates can improve their performance in application-style questions. This article distills key techniques directly from the report, offering bilingual guidance to help students avoid typical errors and score higher marks.
2021年1月的国际A-Level物理第一单元考试涵盖了力学和材料学的核心主题。考官报告指出了常见的失分点以及考生在应用题中可以提升的地方。本文直接从报告中提炼关键技巧，提供双语指导，帮助学生避免典型错误，取得更高分数。
1. Identifying Scalars and Vectors | 识别标量与矢量
One recurring issue was confusion between scalar and vector quantities. The examiner stressed that in motion questions, direction matters. Many candidates treated displacement as distance or speed as velocity. Always check whether the problem involves scalars like energy and mass, or vectors like force and momentum.
一个反复出现的问题是混淆标量和矢量。考官强调，在运动问题中，方向很重要。许多考生把位移当作路程，或把速率当作速度。务必检查题目涉及的是标量（如能量、质量）还是矢量（如力、动量）。
When a diagram shows a direction, assign positive and negative signs consistently. In projectiles, separate horizontal and vertical components immediately. After writing down a value, label it as scalar or vector; for vectors, denote with an arrow or define a sign convention.
当示意图显示方向时，要一致地设定正负号。在抛体运动中，立即将水平和竖直分量分开。写下数值后，标注它是标量还是矢量；对于矢量，用箭头表示或定义符号约定。
Top tip: use notation like v→ for velocity and keep signs consistent throughout the calculation. The examiner saw many sign errors in momentum questions simply because candidates swapped positive and negative after collisions.
顶级技巧：使用如 v→ 表示速度，并在整个计算过程中保持符号一致。考官在动量题中看到许多符号错误，仅仅因为考生在碰撞后交换了正负号。
2. Drawing Clear Free-Body Diagrams | 画清晰的受力分析图
Examiners noted that many students lost marks by omitting forces or including non-existent ones like “motion force” or “impetus.” A clear, labelled free-body diagram simplifies complex scenarios. Only draw forces acting on the body — weight, normal contact, friction, tension, etc.
考官指出，许多学生因遗漏力或画上不存在的力（如”动力”或”冲力”）而失分。一个清晰标注的受力图能简化复杂情景。只画出作用在物体上的力——重力、法向接触力、摩擦力、张力等。
In equilibrium problems, ensure that vector addition gives zero net force. Use arrows of proportional length and mark angles. For connected bodies, draw separate diagrams and then link them via Newton’s third law pairs: F→(A on B) = –F→(B on A).
在平衡问题中，确保矢量总和为零净力。使用长度成比例的箭头并标记角度。对于连接体，分别画受力图，然后通过牛顿第三定律配对联系起来：F→(A 对 B) = –F→(B 对 A)。
3. Selecting the Correct Kinematic Equations | 选择正确的运动学方程
Application questions often require the use of SUVAT equations. The examiner observed that students sometimes plug values into the wrong formula or fail to identify missing variables. List the five variables: s, u, v, a, t, and mark which three are known, then select the equation without the fourth variable.
应用题常需使用SUVAT方程。考官观察到，学生有时将数值代入错误的公式，或未能识别缺失的变量。列出五个变量：s、u、v、a、t，标出已知的三个，然后选择不含第四个变量的方程。
Common error: using v = u + at when acceleration is not uniform. Always check if the motion is uniformly accelerated. Equation reminders: v = u + at, s = ut + ½ at², v² = u² + 2as, s = ½ (u+v)t.
常见错误：当加速度不恒定时使用 v = u + at。始终检查运动是否是匀加速。方程提醒：v = u + at, s = ut + ½ at², v² = u² + 2as, s = ½ (u+v)t。
4. Unit Conversions and Standard Form | 单位转换与标准形式
The report highlighted that many candidates forgot to convert units to SI, especially grams to kilograms and centimetres to metres. This resulted in answers out by factors of 1000 or 100. When using Young modulus, all lengths must be in metres and force in newtons. Always write down converted values explicitly: 2.5 cm = 0.025 m.
报告强调，许多考生忘记将单位转换为国际单位，特别是克转为千克，厘米转为米。这导致答案差了1000倍或100倍。使用杨氏模量时，所有长度必须以米为单位，力以牛顿为单位。总是明确写下转换后的值：2.5 cm = 0.025 m。
Use standard form to handle very large or small numbers, e.g., 0.00045 m = 4.5 × 10⁻⁴ m. This reduces calculator errors. Also remember that 1 mm² = 1 × 10⁻⁶ m² when dealing with cross-sectional areas.
使用标准形式处理非常大或小的数字，如0.00045 m = 4.5 × 10⁻⁴ m。这减少计算器错误。还要记住，在处理横截面积时，1 mm² = 1 × 10⁻⁶ m²。
5. Energy Conservation Applications | 能量守恒应用
Problems involving work, kinetic energy, and potential energy require careful accounting of energy transfers. The examiner saw students omitting work done against friction or misidentifying the height in GPE = mgh.
涉及功、动能和势能的问题需要仔细核算能量转化。考官发现学生忽略了克服摩擦做的功，或错误识别了 GPE = mgh 中的高度。
Always define your system. If there is friction, include the work done against it: ΔEₖ + ΔEₚ = work done by external force – work against friction. Use the principle of conservation of energy efficiently: total initial energy = total final energy + energy dissipated.
始终定义你的系统。如果有摩擦，要包括克服摩擦做的功：ΔEₖ + ΔEₚ = 外力做功 – 克服摩擦的功。有效利用能量守恒原理：初始总能量 = 最终总能量 + 耗散的能量。
Kinetic energy is ½ mv², not mv². The examiner noted that many students halved the mass incorrectly when rearranging equations.
动能为 ½ mv²，而不是 mv²。考官指出，许多学生在变形方程时错误地将质量减半。
6. Momentum and Collisions | 动量与碰撞
Momentum questions were a source of sign errors. When objects move in opposite directions, assign positive and negative velocities. The examiner urged students to state the direction of momentum explicitly.
动量题是符号错误的来源。当物体向相反方向运动时，要赋予正负速度。考官敦促学生明确说明动量的方向。
In collisions, distinguish elastic (KE conserved) and inelastic (KE not conserved). Many candidates assumed conservation of kinetic energy when it was not stated. Write momentum conservation equation: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂, then solve.
在碰撞中，区分弹性（动能守恒）和非弹性（动能不守恒）。许多考生在未明确说明时假设动能守恒。写出动量守恒方程：m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂，然后求解。
In explosions or recoil problems, initial total momentum is zero; final momenta must sum to zero vectorially. The examiner recommended drawing a before-and-after sketch to assign velocity directions.
在爆炸或反冲问题中，初始总动量为零；最终动量必须矢量相加为零。考官建议画前后草图以分配速度方向。
7. Stress, Strain and Young Modulus | 应力、应变与杨氏模量
Students often confused stress with strain or used the wrong cross-sectional area. The examiner’s report pointed out that many forgot to calculate area from diameter: A =
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