IB Physics: Mastering Multiple-Choice Questions - Fast Elimination Techniques

📚 IB Physics: Mastering Multiple-Choice Questions – Fast Elimination Techniques | IB物理：选择题秒杀技巧

IB Physics Paper 1 demands speed and precision: you have to solve 30 or 40 multiple‑choice questions in a tight time window. Memorising every formula is not enough — you need a set of mental shortcuts that allow you to eliminate wrong answers instantly, even without a calculator. This guide collects proven “quick‑kill” techniques that target common question traps, helping you boost your score while saving precious minutes.

IB物理Paper 1要求又快又准：你必须在紧张的时间内完成30或40道选择题。光记住公式还不够——你需要一套能让你在不使用计算器的情况下瞬间排除错误选项的思维捷径。本文收集了行之有效的“秒杀”技巧，直击常见出题陷阱，帮你在提高得分的同时节省宝贵时间。

1. Dimensional Analysis | 量纲分析法

Before you reach for a formula, check the units of the four options. If the question asks for a force, any answer with units of energy (J) or pressure (Pa) can be thrown away at once. Dimensional analysis uses the fact that every physical quantity can be expressed in terms of base dimensions: mass (M), length (L), time (T), electric current (I), temperature (Θ), amount of substance (N) and luminous intensity (J).

在动手套公式之前，先检查四个选项的单位。如果题目问的是力，那么单位是能量(J)或压强(Pa)的答案可以立刻扔掉。量纲分析利用了一个事实：每一个物理量都可以用基本量纲表示——质量(M)、长度(L)、时间(T)、电流(I)、温度(Θ)、物质的量(N)和发光强度(J)。

For example, a question gives the formula T = 2π√(ℓ/g) for a pendulum. If an option reads T = 2π√(g/ℓ), check: √(g/ℓ) has dimensions √(LT⁻² / L) = √(T⁻²) = T⁻¹, which is not time. The incorrect option is eliminated without any calculation.

比如题目给出单摆公式 T = 2π√(ℓ/g)，某个选项却是 T = 2π√(g/ℓ)。检查量纲：√(g/ℓ) 的量纲为 √(LT⁻² / L) = √(T⁻²) = T⁻¹，这不是时间的量纲。无需任何计算，错误选项就被排除了。

Physical Quantity	Dimensions (MLT system)
Force	MLT⁻²
Energy / Work	ML²T⁻²
Pressure	ML⁻¹T⁻²
Power	ML²T⁻³
Frequency	T⁻¹
Electric potential	ML²T⁻³I⁻¹

Memorising the dimensions of common quantities lets you slash the pool of choices to one or two in seconds.

记住常见物理量的量纲能让你在几秒内把选项缩减至一到两个。

2. Limiting Cases and Extremes | 极限与极端情况

Plug extreme values into a given expression to see which option behaves correctly. Set a mass to zero, a distance to infinity, an angle to 0° or 90°, or a resistance to a short circuit. The option that survives these boundary tests is usually the right one.

把极端数值代入给定的表达式，看看哪个选项表现得合理。把质量设为零、距离设为无穷大、角度设为0°或90°、或者电阻设成短路。能通过这些边界检验的选项通常就是正确答案。

Suppose a problem offers four algebraic expressions for the acceleration of a block on an inclined plane. When the incline angle θ = 0°, the acceleration should be zero. Any option that does not vanish at θ = 0° can be crossed out. Similarly, when θ = 90°, the block is in free fall, so the acceleration must be g. Only the correct expression satisfies both extremes.

假设一道题给出了斜面上滑块加速度的四个代数表达式。当倾角 θ = 0° 时，加速度应为零。任何在 θ = 0° 时不等于零的选项都可以划掉。同样，当 θ = 90° 时，滑块处于自由落体，加速度应为 g。只有正确的表达式能同时满足两个极端情况。

This technique is especially powerful in questions involving springs, circuits, or fluid resistance, where you can let a spring constant approach zero (wire) or the speed of a fluid tend to zero.

这个技巧在涉及弹簧、电路或流体阻力的题目中特别有用，你可以让弹簧劲度系数趋近于零（变成绳子），或者让流体速度趋近于零。

3. Unit Consistency Check | 单位一致性检查

Transforming all given data into SI units before plugging into formulas is a habit that prevents silly mistakes. When time is short, simply scanning the units of each answer can reveal mismatched options. If the correct answer must be in metres per second, eliminate anything that yields m·s or m²·s⁻¹.

在代入公式之前把所有数据转化为国际单位制是一个能避免低级错误的习惯。时间紧迫时，仅仅快速浏览每个答案的单位就能暴露出不匹配的选项。如果正确答案必须是米每秒，那就排除所有算出来是 m·s 或 m²·s⁻¹ 的答案。

In IB Physics, conversion pitfalls are common: cm to m, mm² to m², km·h⁻¹ to m·s⁻¹, and grams to kilograms. A quick unit check on the intermediate steps can often tell you whether you forgot to square a length or to convert minutes to seconds.

在IB物理中，单位换算的陷阱很常见：厘米换成米、平方毫米换成平方米、千米每小时换成米每秒、克换成千克。对中间步骤快速做一次单位检查，经常能告诉你是否忘记了将长度平方，或者忘记将分钟换算成秒。

1 km·h⁻¹ = 1000 m / 3600 s = (1/3.6) m·s⁻¹

Keeping this conversion factor at your fingertips saves time on kinematics and momentum questions.

把这个换算因子牢记在心，能在运动学和动量的题目上节省时间。

4. Estimation and Order of Magnitude | 估算与数量级

Many Paper 1 questions require only the order of magnitude, not the exact value. Round numbers generously: π ≈ 3, g ≈ 10 m·s⁻², the speed of light as 3 × 10⁸ m·s⁻¹. By approximating, you can often identify the correct option because the wrong answers are off by factors of 10 or more.

很多Paper 1的题目只要求数量级，而不需要精确值。大方地取整：π ≈ 3，g ≈ 10 m·s⁻²，光速取 3 × 10⁸ m·s⁻¹。通过近似，你往往可以识别出正确选项，因为错误答案会相差10倍甚至更多。

For instance, a question might ask for the de Broglie wavelength of a walking person. Mass 70 kg, speed 1.5 m·s⁻¹, Planck’s constant h ≈ 6.6 × 10⁻³⁴ J·s. λ = h/(mv) ≈ 6.6×10⁻³⁴ / (70×1.5) ≈ 6.3×10⁻³⁶ m. The options will typically offer one value close to 10⁻³⁶ m and others around 10⁻³⁰ m or 10⁻⁴⁰ m; only one answer is plausible.

例如，一道题可能问一个行走的人的德布罗意波长。质量70 kg，速度1.5 m·s⁻¹，普朗克常数 h ≈ 6.6 × 10⁻³⁴ J·s。λ = h/(mv) ≈ 6.6×10⁻³⁴ / (70×1.5) ≈ 6.3×10⁻³⁶ m。选项通常会提供一个接近10⁻³⁶ m的数值，其他的则在10⁻³⁰ m或10⁻⁴⁰ m左右；只有一个答案是合理的。

Practise estimating the size of common objects: mass of a car, radius of the Earth, wavelength of visible light, energy of a photon. Having these benchmarks in your head makes order‑of‑magnitude questions a quick win.

练习估算常见物体的大小：汽车的质量、地球半径、可见光波长、光子能量。把这些基准值装进脑袋，能让数量级题目变成快速得分项。

5. Graph Interpretation Shortcuts | 图象快速解读技巧

Graph‑based multiple‑choice questions often test whether you recognise the gradient, area under the graph, or intercept. Instead of calculating every point, focus on the physical meaning: a velocity–time graph with constant positive slope means constant acceleration; its area gives displacement. If an option confuses gradient with area, discard it.

基于图象的选择题往往考查你是否能识别斜率、曲线下面积或截距。不要逐个点计算，而是关注物理意义：速度—时间图中正斜率不变表示匀加速；曲线下的面积给出位移。如果某个选项把斜率和面积搞混了，就排除它。

When the question provides a graph of force versus extension for a spring, the gradient is the spring constant k, but the area under the straight‑line part is the elastic potential energy. An option that states ‘the gradient represents the energy stored’ is wrong, and you can eliminate it instantly.

当题目给出弹簧的力—伸长量图象时，斜率是劲度系数 k，但直线段下面的面积是弹性势能。若某个选项说“斜率代表储存的能量”，它就是错的，你可以立刻排除。

For curved graphs, deciding whether to linearise (e.g. plotting v² against height to verify conservation of energy) is frequently tested. Spotting the linearising trick lets you jump straight to the answer without replotting data.

对于曲线图，是否要将其线性化（例如绘制 v² 对高度的图以验证能量守恒）是常考点。识别出线性化的技巧能让你无需重新描点就直接跳到答案。

6. Proportional Reasoning | 比例推理法

Many IB questions ask, “If the radius doubles, what happens to the centripetal force?” Instead of calculating with numbers, write the proportionality: F ∝ v²/r or F ∝ ω²r. By substituting the changes (×2, ×½, ×3 etc.), you obtain the factor without touching a calculator.

很多IB题目会问：“如果半径变为原来的两倍，向心力会怎样？”不要用数字计算，写出比例关系：F ∝ v²/r 或 F ∝ ω²r。代入变化的倍数（×2, ×½, ×3 等），你就能在不动计算器的情况下得出比例因子。

Take gravitational force: F = GmM/r², so F ∝ 1/r². If the separation triples, the force becomes (1/3)² = 1/9 of its original value. Wrong answers often mistake the inverse‑square law for a linear inverse law, giving 1/3 instead of 1/9. Proportional reasoning quickly isolates the correct option.

以万有引力为例：F = GmM/r²，所以 F ∝ 1/r²。如果间距变为原来的3倍，力就变成原来的 (1/3)² = 1/9。错误答案常把平方反比定律误认为线性反比，给出1/3而不是1/9。比例推理能快速锁定正确选项。

This technique also shines in thermodynamics (pressure–volume), electric fields (E ∝ 1/r²), and resistance (R ∝ L/A). Train yourself to spot the proportionality wording: “directly proportional to”, “inversely proportional to the square of”.

这个技巧在热力学（压强—体积）、电场（E ∝ 1/r²）和电阻（R ∝ L/A）中同样大放异彩。训练自己识别比例关系的措辞：“与……成正比”、“与……的平方成反比”。

7. Formula Rearrangement and Substitution Tricks | 公式变形与代入技巧

Rather than solving an equation step‑by‑step, many multiple‑choice questions can be answered by plugging the option back into the formula. For “which expression gives the time of flight?” type questions, take one option, substitute known quantities with symbols, and see if it yields the expected physical outcome.

很多选择题不需要一步步解方程，可以把选项回代到公式中求解。对于“下列哪个表达式给出了飞行时间？”这类题目，选一个选项，把已知量用符号代入，看看是否得到预期的物理结果。

For instance, a projectile problem offers four expressions for maximum height. Take the one with correct dimensions and test it against a known special case (e.g. launch angle 90°). The expression must reduce to v²/(2g). Any option that does not match in this limit can be eliminated.

比如，一道抛体题给出了最大高度的四个表达式。选取那个量纲正确的，再用已知的特殊情况（如抛射角 90°）检验。表达式必须能简化为 v²/(2g)。在该极限下不匹配的选项就可以排除。

Memorise the standard rearrangements of key formulas: for example, in single‑slit diffraction, the half‑angular width θ ≈ λ/b. A question might present four options for b; quickly solving b = λ/θ and rounding numbers can give you the answer without detailed algebra.

记住关键公式的常见变形：例如，在单缝衍射中，半角宽度 θ ≈ λ/b。题目可能给出 b 的四个选项；快速解出 b = λ/θ 并取整就能得到答案，无需繁琐的代数运算。

8. Symmetry and Conservation Laws | 对称性与守恒定律

Conservation of momentum, energy, charge, baryon number, and lepton number are tested repeatedly. When you see a particle interaction or collision, first check whether the options conserve the relevant quantities. Incorrect options often violate charge conservation or suggest an energy that exceeds the initial total energy.

动量守恒、能量守恒、电荷守恒、重子数守恒和轻子数守恒是反复考查的内容。当你看到粒子相互作用或碰撞时，首先检查选项是否守恒了相关物理量。错误选项常常违反电荷守恒，或者给出的能量超过了初始总能量。

In collision problems, if option B gives a final kinetic energy larger than the initial, it violates the law of conservation of energy. Option C may show the final momentum vector opposite to the initial direction without an external impulse — also impossible. Such violations let you rule out answers in a glance.

在碰撞问题中，如果选项B给出的末动能大于初始动能，它就违反了能量守恒定律。选项C可能显示末动量矢量与初始方向相反却没有外力冲量——这也是不可能的。这样的违规让你可以一眼排除答案。

In circuit questions, symmetry can simplify mesh analysis. A balanced Wheatstone bridge has no current through the galvanometer. Recognising such symmetries avoids solving simultaneous equations.

在电路题中，对称性可以简化网格分析。平衡的惠斯通电桥中没有电流流过检流计。识别出这样的对称性能避免求解联立方程。

9. Numerical Estimation and Rounding | 数值估算与四舍五入

When the answer choices differ significantly, approximate each number to one or two significant figures and perform mental arithmetic. 287 × 0.034 can be rounded to 300 × 0.03 = 9; the exact answer 9.758 is near enough to pick the correct option among 0.98, 9.8, 98, 980.

当选项之间相差很大时，把每个数字近似到一或两位有效数字，然后心算。287 × 0.034 可以取整为 300 × 0.03 = 9；精确答案 9.758 足够靠近9.8，从而能在 0.98、9.8、98、980 中选出正确选项。

Know your calculator‑free squares and square roots: √2 ≈ 1.4, √3 ≈ 1.7, √5 ≈ 2.2, √10 ≈ 3.16. If a question asks for the refractive index using Snell’s law and you get sin 30° = 0.5, sin 19° ≈ 0.33, then n ≈ 1.5 — you can identify the answer without writing down heavy trig tables.

记住不用计算器的常用平方根：√2 ≈ 1.4, √3 ≈ 1.7, √5 ≈ 2.2, √10 ≈ 3.16。如果一道题要求用斯涅尔定律求折射率，你得到 sin 30° = 0.5，sin 19° ≈ 0.33，那么 n ≈ 1.5——你无需写下繁琐的三角函数表就能找出答案。

Rounding also aids in radioactive decay questions. If the half‑life is 3.5 days and 10 days have passed, roughly three half‑lives have gone by, so the fraction remaining is about (1/2)³ = 1/8. The options are likely to include 1/8, 1/4, 1/2, and 1/16; only 1/8 is correct.

四舍五入也有助于放射性衰变问题。如果半衰期是3.5天，经过10天，大概已经过了3个半衰期，因此剩余的比例大致为 (1/2)³ = 1/8。选项很可能会包含1/8、1/4、1/2和1/16；只有1/8是正确的。

10. Process of Elimination with Absurd Options | 荒谬选项排除法

Exam writers deliberately place one or two absurd options: a wavelength longer than the observable universe, a force larger than the weight of a mountain, a speed exceeding the speed of light. Train your eye to catch these impossibilities. Once you flag an option as physically impossible, cross it out.

出题人会故意放入一两个荒谬选项：比可观测宇宙还大的波长、比一座山还重的力、超过光速的速度。训练你的眼睛去捕捉这些不可能的情况。一旦你标记某个选项在物理上不可能，就划掉它。

For example, a question on photoelectric effect might offer a maximum kinetic energy of 10¹⁵ J for an electron ejected by visible light. A quick mental check using the photon energy E = hf shows this is impossibly large; the other three options are in the eV range and are physically sound.

例如，一道光电效应的题目可能会给出可见光逸出电子的最大动能为 10¹⁵ J。用心算快速检查光子能量 E = hf，就会知道这个大得离谱；另外三个选项在电子伏特量级，物理上是合理的。

Similarly, in astronomical distances, a figure like “10² m to the nearest star” is comically wrong. Knowing the order of magnitude of the light‑year (≈10¹⁶ m) helps you discard such answers instantly.

同样，在天文距离中，“到最近恒星的距离为 10² m”这样的数字可笑得离谱。了解光年的数量级（≈10¹⁶ m）能帮你瞬间抛弃这类答案。

11. Pattern Recognition in Answer Choices | 答案模式识别

Over years of IB papers, certain patterns repeat. In “which of the following statements is correct?” questions, the longest or most qualified statement is often the correct one because the author ensures it is true under all conditions. Treat this as a weak heuristic, but useful when guessing.

在多年的IB试卷中，某些模式反复出现。在“下列哪项陈述是正确的？”这类题目中，最长或限定条件最多的陈述往往是正确的，因为出题人要确保它在所有条件下都成立。把这当作一个弱启发式，但在猜测时很有用。

When four graphs are given, identical curves with slight distortions may appear. The correct graph usually has the exact asymptotic behaviour or the correct sign of curvature. For example, an I–V characteristic of a filament lamp must curve upwards from the origin, not downwards. Scanning for the right curvature eliminates the others.

当给出四幅曲线图时，可能出现形状相近但有轻微扭曲的曲线。正确的图象通常具备准确的渐近行为或正确的曲率符号。例如，灯丝的 I–V 特性曲线必须从原点向上弯曲，而不是向下。扫一眼正确的曲率就能排除其他选项。

For “select the correct unit” questions, the IB often lists J, W, N, Pa, and the answer is likely the one that defies the simple pattern — but only if you have checked dimensionally. Never rely on patterns alone; use them when you are stuck.

对于“选出正确单位”的题目，IB常把 J、W、N、Pa 列出来，正确的那个往往违反简单模式——但这只在你经过量纲检查之后才有意义。绝不要只依赖模式；只有在你卡住的时候才用。

12. Time Management and Guessing Strategy | 时间管理与猜测策略

IB Physics Paper 1 has no penalty for wrong answers, so never leave a bubble blank. If you cannot eliminate any option after 40 seconds, pick the same letter (e.g. “C”) for all guess‑questions to maintain a consistent probability. Spend your time on questions you know you can solve quickly.

IB物理Paper 1答错不扣分，所以绝不要留空。如果40秒内你无法排除任何选项，就给所有猜测题都选同一个字母（如“C”），以保持稳定的概率。把时间花在你确定能快速解出的题目上。

During the reading time, scan through the paper and mark questions as “quick,” “medium,” and “time‑consuming.” Solve all the quick ones first to bank marks, then tackle medium ones, and leave the most stubborn ones for the last few minutes when educated guessing becomes necessary.

在阅卷时间里，浏览整份试卷，把题目标记为“快”、“中等”和“耗时”。先解决所有快题以积攒分数，然后处理中等难度题，把最顽固的题留到最后几分钟，那时候有根据的猜测就变得必要了。

If you have two nearly identical options differing only by a sign, the correct one is often among them. In that case, use limiting cases or dimensional analysis to choose the right sign. And always keep an eye on the clock: if you have spent more than 90 seconds on a single question, move on and return later if time allows.

如果有两个几乎相同、仅符号不同的选项，正确的往往就是其中之一。此时，用极限情况或量纲分析来选出正确的符号。始终留意时钟：如果在一道题上花的时间超过90秒，就跳过去，若时间允许再回来。

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IB Physics: Mastering Multiple-Choice Questions – Fast Elimination Techniques | IB物理：选择题秒杀技巧