📚 IB & OCR Physics: Multiple-Choice Question Hacks | IB OCR 物理：选择题秒杀技巧
Multiple-choice questions in IB Physics Paper 1 and OCR A Level Physics Papers 1 & 2 demand rapid, accurate reasoning. Instead of fully solving every problem, you can apply a set of ‘hacks’ — dimensional analysis, extreme cases, symmetry, estimation, graph shortcuts, and strategic elimination — to slash through distractors. This guide compiles classroom-tested tactics that work across Mechanics, Fields, Waves, and Thermal Physics, helping you secure marks with confidence.
在 IB 物理试卷 1 和 OCR A Level 物理试卷 1 与 2 中，选择题要求快速准确的推理。与其逐题完整计算，不如运用一套“秒杀”技巧——量纲分析、极限假设、对称性、估算、图像捷径和策略性排除——来横扫干扰项。本指南汇集了经过课堂检验的战术，适用于力学、场、波和热物理，帮助你从容拿分。
1. Dimensional Analysis to the Rescue | 量纲分析秒杀
Every physical quantity has a unique combination of base units (kg, m, s, A, K, mol). If a multiple-choice option does not yield the correct base-unit combination for the target quantity, it must be wrong. For instance, force has units kg m s⁻²; any option giving kg m² s⁻² is energy, not force.
每个物理量都有唯一的基本单位组合（kg, m, s, A, K, mol）。如果某个选项不能得出目标量的正确基本单位组合，它就必然是错的。例如，力的单位是 kg m s⁻²；任何得出 kg m² s⁻² 的选项都是能量，不是力。
Write the units of each variable in a formula and multiply. In a problem asking for kinetic energy, the expression must produce kg m² s⁻². The classic Eₖ = ½mv² passes: kg ×(m s⁻¹)² = kg m² s⁻²; mv³ fails immediately.
写出公式中每个变量的单位然后相乘。在求动能的问题中，表达式必须导出 kg m² s⁻²。经典的 Eₖ = ½mv² 通过检验：kg ×(m s⁻¹)² = kg m² s⁻²；而 mv³ 立刻不合格。
2. Extreme Case Assumptions | 极限假设
Set one variable to zero or to a very large value and test the behaviour of each candidate answer. In a circuit, if resistance R → 0, current should increase dramatically; if an option predicts zero current, discard it. In a gravitational field inside a spherical shell, the field is zero — any formula that does not tend to zero as r → 0 (inside) must be eliminated.
将某一个变量设为零或极大值，检验每个候选项的变化趋势。在电路中，若电阻 R → 0，电流应当剧增；如果某选项预测电流为零，立即舍弃。在球壳内的引力场中，场强为零——任何在 r → 0（内部）时不趋近于零的公式都应排除。
For a projectile launched vertically, let air resistance become negligible. The time of flight should approach 2u/g. Any option that does not simplify to this limiting form is a distractor.
对于竖直发射的抛体，令空气阻力可忽略。飞行时间应趋近于 2u/g。任何不能简化成这一极限形式的选项都是干扰项。
3. Symmetry and Conservation | 对称性与守恒
Use symmetry to deduce correct answers instantly. In an elastic head-on collision between two identical masses where one is initially at rest, the incoming mass stops and the target moves off with the incoming speed. An option showing both masses moving after the collision with different speeds is physically impossible under this symmetry.
利用对称性可立即推断正确答案。在两个相同质量的弹性正碰中，若一球初始静止，入射球会停下，靶球以入射速度离开。任何显示碰后两球以不同速度运动的选项在对称情况下都是不可能的。
Conservation laws operate like embedded filters. Momentum is a vector: if the net external force is zero, the total momentum vector must be unchanged. In a multi-choice question on an explosion, check whether the vector sum of the momenta in each option equals the initial total momentum (zero, if originally stationary).
守恒定律如同内置过滤器。动量是矢量：如果合外力为零，总动量矢量必须保持不变。在关于爆炸的选择题中，检查各个选项中动量的矢量和是否等于初始总动量（若原静止则为零）。
4. Estimation and Orders of Magnitude | 估算与数量级
Many OCR and IB questions ask for approximate values: the wavelength of red light (~7×10⁻⁷ m), the mass of an electron (~9×10⁻³¹ kg), or the typical speed of sound in air (~340 m s⁻¹). If three options are orders of magnitude away, the physically plausible one stands out. Practise linking everyday numbers to physical constants.
许多 OCR 与 IB 题目要求近似值：红光的波长（~7×10⁻⁷ m）、电子质量（~9×10⁻³¹ kg）或空气中声速的典型值（~340 m s⁻¹）。如果三个选项差了几个数量级，物理上合理的那个会凸显出来。练习将日常数据与物理常数联系起来。
Use Fermi-style estimation: round every number to one significant figure and powers of ten. For the number of air molecules in a typical classroom, approximate volume ≈ 10² m³, number density ≈ 3×10²⁵ m⁻³, giving ≈ 3×10²⁷ molecules — enough to spot a distractor claiming 10¹⁵.
使用费米式估算：将所有数字四舍五入到一位有效数字并写成十的次幂。对于典型教室中的空气分子数，估算体积 ≈ 10² m³，数密度 ≈ 3×10²⁵ m⁻³，得出约 3×10²⁷ 个分子——足够识别声称 10¹⁵ 的干扰项。
5. Graph Interpretation Shortcuts | 图像速读
Before calculating, glance at axis labels, units, and the shape. In a velocity–time graph, displacement is the area under the curve; acceleration is the gradient. An option that confuses area with slope is a common trap. For an I–V characteristic of a diode, the graph is not a straight line through origin — any option suggesting a constant ratio ΔV/ΔI for all V is incorrect.
动笔计算前，先扫一眼坐标轴标签、单位及形状。在速度–时间图中，位移是曲线下的面积；加速度是斜率。混淆面积与斜率的选项是常见陷阱。对于二极管的 I–V 特性曲线，图像不是一根经过原点的直线——任何暗示 ΔV/ΔI 对所有 V 均恒定的选项都是错的。
Use the ‘halfway’ test: at the midpoint of a linear spring extension, the work done is half the product of final force and extension — a straight-line graph of force versus extension gives a triangular area. Options offering the full rectangle area are distractors.
运用“半程”测试：在弹簧线性伸长一半处，做功为最终力与伸长量乘积的一半——力-伸长直的线图下是三角形面积。给出完整矩形面积的选项是干扰项。
6. Unit Traps and Conversions | 单位陷阱与换算
Examiners love inserting prefixes like n, μ, m, k, M in options to test conversions. Always convert all quantities to SI base units immediately. A capacitance calculation in μF must be turned into farads: 1 μF = 10⁻⁶ F. A common trick is to place the correct numerical value alongside an unshielded microfarad, misleading candidates who forget the 10⁻⁶ factor.
出题者喜欢在选项中插入 n、μ、m、k、M 等词头来考查换算。务必立即将所有物理量转换为 SI 基本单位。含有 μF 的电容计算必须转换为法拉：1 μF = 10⁻⁶ F。常见的陷阱是在正确数字旁边放上未转换的微法，误导忘记乘以 10⁻⁶ 的考生。
| Prefix | Symbol | Factor |
|---|---|---|
| pico | p | 10⁻¹² |
| nano | n | 10⁻⁹ |
| micro | μ | 10⁻⁶ |
| milli | m | 10⁻³ |
| kilo | k | 10³ |
| mega | M | 10⁶ |
When an option presents a speed in km s⁻¹ while the stem uses m s⁻¹, convert immediately: 3 km s⁻¹ = 3000 m s⁻¹. Two otherwise similar options can be distinguished purely by this conversion.
当选项以 km s⁻¹ 标出速度而题干用的是 m s⁻¹ 时，立刻转换：3 km s⁻¹ = 3000 m s⁻¹。两个看似相同的选项可仅通过此换算被区分开。
7. Elimination of Obvious Outliers | 排除明显错误项
Scan all four options rapidly. Often two options will have the opposite sign or lack the required vector direction. If a question asks for the magnitude of a force, any negative value can be struck off. In refraction, the angle of refraction cannot exceed 90° — eliminate any option showing θ₂ > 90°.
快速扫读全部四个选项。常常有两个选项符号相反或缺少必要的矢量方向。若题目询问力的大小，任何负值都可划掉。在折射中，折射角不能超过 90°——排除任何显示 θ₂ > 90° 的选项。
Check for dimensional inconsistency as a first filter. If two options carry units of force and the other two carry units of energy, and the question asks for work done, throw away the force-unit pair instantly. This often reduces a 1-in-4 guess to a 50/50 choice.
用单位不一致性作为第一道筛选。如果两个选项带有力的单位，另两个带有能量单位，而题目求的是做功，则立即丢弃那对力单位的选项。这常常将四选一的猜测变为二选一。
8. Substitute Special Values | 代入特殊值
When a question provides a general algebraic expression, substitute a simple numeric value to test each option. For example, if the problem asks for the equivalent resistance of two resistors R₁ and R₂ in parallel, try R₁ = R₂ = 2 Ω. The correct formula 1/R_eq = 1/R₁ + 1/R₂ gives R_eq = 1 Ω. An option yielding 4 Ω is wrong.
当题目给出一般代数表达式时，代入一个简单的数值来检验各个选项。例如，若题目要求两并联电阻 R₁ 和 R₂ 的等效电阻，尝试 R₁ = R₂ = 2 Ω。正确公式 1/R_eq = 1/R₁ + 1/R₂ 得出 R_eq = 1 Ω。若某个选项给出 4 Ω，便是错的。
Choose values that create zeros or ones where possible: set initial velocity u = 0, height h = 0, or charge q = 0. The physical outcome should simplify dramatically, exposing flawed options. In kinematic equations, setting u = 0 collapses the terms, making it easy to see which option matches s = ½at².
尽量选择会形成 0 或 1 的值：令初速度 u = 0、高度 h = 0 或电荷 q = 0。物理结果应该大幅简化，从而暴露有缺陷的选项。在运动学方程中，令 u = 0 可消去多项，轻易看出哪个选项符合 s = ½at²。
9. Concept First, Calculation Second | 概念优先，计算在后
Many MCQs are designed to test understanding, not arithmetic. If a question shows two bulbs in series and asks which is brighter, recall that brightness depends on power, P = I²R, and current is the same. The bulb with higher resistance dissipates more power. No need to compute exact wattage — the concept alone picks the right answer.
许多选择题旨在考查理解，而非运算。若题目给出两个串联灯泡，问哪个更亮，回忆亮度取决于功率 P = I²R，且串联电流相同。电阻更高的灯泡耗散功率更大。根本不需要计算具体功率——概念本身就能选出正确答案。
Newton’s third law is a classic concept trap. If a 2 kg mass collides with a 5 kg mass, the force on each is equal in magnitude. Options that show a larger force on the lighter mass violate this principle. Spotting such an immediate violation saves precious time.
牛顿第三定律是一个经典的概念陷阱。如果 2 kg 的物体撞击 5 kg 的物体，二者所受的力大小相等。显示较轻物体受力更大的选项违背该原理。立刻识别这种违规能节省宝贵时间。
10. Beware Absolute Wording | 警惕绝对化措辞
Words like ‘always’, ‘never’, ‘must’, and ‘definitely’ are red flags in physics. Almost every statement with an absolute qualifier can be falsified by a counterexample. For instance, “The acceleration of an object is always in the direction of its motion” is false because deceleration opposes motion. An option containing such an absolute is very likely a distractor.
“总是”、“绝不”、“必定”和“绝对”之类的词汇在物理中是危险信号。几乎所有带有绝对限定词的陈述都能被反例证伪。比如，“物体的加速度总与运动方向一致”错误，因为减速时加速度与运动方向相反。包含此类绝对词项的选项极可能是干扰项。
Exceptions abound in physics. Light always travels at 3×10⁸ m s⁻¹? Only in a vacuum; it slows in glass. A moving charged particle in a magnetic field always experiences a force? Not if its velocity is parallel to the field. Train yourself to instantaneously question absolute claims.
物理中例外比比皆是。光总是以 3×10⁸ m s⁻¹ 行进？仅在真空中；在玻璃中会变慢。磁场中运动的带电粒子一定受力？若其速度平行于磁场则不受力。训练自己本能地质疑绝对化陈述。
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导