📚 Common Misconceptions in IB and AQA Physics | IB与AQA物理常见误区
Physics is full of ideas that seem right at first glance but break down under closer scrutiny. Many students bring intuitive yet incorrect notions into their IB and AQA exams, leading to lost marks even when they have studied hard. Understanding where these common misconceptions arise can help you avoid them and build a stronger, more accurate grasp of the subject. In this article, we will examine ten of the most persistent myths in mechanics, electricity, waves, and thermal physics, clarifying the correct principles and showing how they are tested on your course.
物理学中充满了乍看之下正确、但稍加推敲就站不住脚的想法。许多学生带着直觉却错误的观念走进 IB 和 AQA 考场，即使努力学习仍会因此失分。了解这些常见误区的来源，可以帮助你避开陷阱，建立更扎实、更准确的理解。本文我们将剖析力学、电学、波动和热物理学中十个最顽固的迷思，阐述正确原理，并展示它们如何在你的课程中被考查。
1. Force and Motion: A Constant Force for Constant Motion? | 力与运动：恒力才能保持运动？
One of the oldest misconceptions is the belief that a continuous force is needed to keep an object moving at constant velocity. This idea, rooted in everyday experiences like pushing a shopping trolley, ignores the effect of friction. According to Newton’s first law, an object will continue moving at constant velocity unless a resultant force acts upon it. If you observe a moving object slowing down on a table, it is not because the ‘force has run out’, but because friction provides an unbalanced force opposing motion.
最古老的误区之一是认为要保持物体匀速运动，就必须持续施加一个力。这种源于推购物车等日常经验的观念，忽略了摩擦的作用。根据牛顿第一定律，除非受到合力作用，否则物体将保持匀速直线运动。如果你看到桌面上的物体越走越慢，那不是因为“力用完了”，而是摩擦提供了一个与运动方向相反的失衡力。
In the absence of friction (such as in deep space), no force is required to maintain constant velocity. The resultant force is linked not to velocity, but to acceleration: F = ma. So a constant force produces a constant acceleration, not a constant speed. This distinction is fundamental in IB and AQA mechanics questions involving free-body diagrams and calculations of net force.
在没有摩擦的环境中（如深空），物体保持匀速并不需要力。合力与速度无关，而与加速度相关：F = ma。因此恒力产生的是恒定的加速度，而不是恒定的速度。在 IB 和 AQA 力学的受力分析和合力计算题中，这一区分至关重要。
2. Gravity and Free Fall: Do Heavier Objects Fall Faster? | 重力与自由落体：重的物体下落更快吗？
A classic misconception, famously debunked by Galileo, is that heavier objects fall faster than lighter ones. In the absence of air resistance, all objects near the Earth’s surface experience the same gravitational acceleration g (about 9.81 m s⁻²), regardless of their mass. Thus a feather and a hammer dropped in a vacuum will hit the ground simultaneously.
一个经典误区——被伽利略著名地反驳过——是认为重的物体比轻的物体下落更快。在没有空气阻力的情况下，地球表面附近的所有物体都受到相同的重力加速度 g（约 9.81 m s⁻²），与质量无关。因此，在真空中同时释放的羽毛和锤子会同时落地。
The confusion arises because on Earth, air resistance affects lighter or less aerodynamically shaped objects more noticeably. In your exam, you must separate the idealised model (no air resistance) from real-world conditions. Many projectile motion problems in IB and AQA deliberately ignore air resistance, so never assume mass affects the acceleration due to gravity.
混淆的根源在于地球上空气阻力对较轻或流线型较差的物体影响更显著。在考试中，你必须区分理想化模型（无空气阻力）和现实条件。IB 和 AQA 中许多抛体运动问题都明确忽略空气阻力，因此决不要假设质量会影响重力加速度。
3. Newton’s Third Law: Action and Reaction Cancel Out? | 牛顿第三定律：作用力与反作用力会抵消吗？
Students often think that the two forces in an action–reaction pair cancel each other because they are equal in magnitude and opposite in direction. However, Newton’s third law forces act on different objects. For example, when you push a wall, you exert a force on the wall, and the wall exerts an equal and opposite force on you. These forces do not cancel because they are not acting on the same body.
学生常认为作用力与反作用力会相互抵消，因为它们大小相等、方向相反。但牛顿第三定律中的两个力作用在不同物体上。例如，你推墙时，你对墙施加一个力，墙同时对你施加一个等大反向的力。这两个力不抵消，因为它们不是作用在同一个物体上。
In motion problems, only forces acting on the same object should be combined to find the net force. The action force from you on the wall does not affect your motion directly; it is the reaction force from the wall on you that might cause you to accelerate backwards. Recognising this distinction is essential for correctly analysing force pairs in IB and AQA extended response questions.
在运动问题中，只有作用在同一物体上的力才能合成求出合力。你对墙的作用力并不直接影响你的运动；反而是墙对你的反作用力会使你向后加速。正确区分这一点，对于 IB 和 AQA 的拓展回答题中分析力对至关重要。
4. Friction: Always Opposing Motion? | 摩擦力：总是阻碍运动吗？
It is tempting to state that friction always opposes motion. In many cases, however, friction is what enables motion to occur. When you walk, static friction between your shoe and the ground pushes you forward — opposite to the direction your foot would otherwise slip. Similarly, the tyres of a car rely on static friction to accelerate the vehicle.
人们很容易说摩擦力总是阻碍运动。然而在许多情况下，摩擦力正是使运动发生的原因。行走时，鞋底与地面之间的静摩擦力将你向前推——与你脚本会打滑的方向相反。同样，汽车轮胎依靠静摩擦力使车辆加速。
Kinetic (dynamic) friction does oppose the relative motion between surfaces, but even here it is not simply ‘opposing the direction of travel’. For instance, a crate sliding on a lorry bed experiences friction that may act forwards or backwards relative to the ground, depending on the relative motion. Exam questions test this by asking you to identify the direction and type of friction. Always specify whether you are dealing with static or kinetic friction.
动摩擦力确实阻碍接触面间的相对运动，但即便如此，它也不是简单地“与运动方向相反”。例如，在卡车货厢上滑动的木箱，所受的摩擦力相对于地面可能向前也可能向后，具体取决于相对运动。考题会要求你判断摩擦力的方向和类型，一定要区分是静摩擦还是动摩擦。
5. Work and Energy: When Does Force Do Work? | 功与能量：力何时做功？
A widespread mistake is using the formula W = F s without considering the angle between force and displacement. Work is done only when a force has a component in the direction of displacement. The correct relationship is W = F s cosθ, where θ is the angle between the force and the direction of motion. If the force is perpendicular to the displacement (e.g. centripetal force in circular motion), no work is done.
一个普遍错误是使用公式 W = F s 却不考虑力与位移之间的角度。只有当力在位移方向上有分量时，该力才做功。正确的关系是 W = F s cosθ，其中 θ 是力与运动方向之间的夹角。如果力与位移垂直（如圆周运动中的向心力），则不做功。
Another related misconception is that ‘work done equals energy transferred’ means energy is something that gets used up. Energy is conserved; it merely changes form. In an IB or AQA question about efficiency or power, you must link work done to the transfer of energy, not to its consumption. Watch out for statements like ‘energy is lost’: energy is dissipated, often as internal (thermal) energy, but the total energy remains constant.
另一个相关误区是认为“做功等于能量转移”意味着能量被“用光”了。能量是守恒的，它只是转换了形式。在 IB 或 AQA 涉及效率或功率的题目中，你必须将做功与能量转移联系起来，而不是能量的消耗。留意“能量丢失”这样的说法：能量是被耗散了，通常转化为内能（热能），但总能量保持不变。
6. Electric Current: Does Current Get Used Up? | 电流：电流会被消耗吗？
One of the most common misconceptions in circuits is that current is ‘used up’ as it passes through a component like a bulb or a resistor. In reality, electric current is the rate of flow of charge, and in a series circuit the current is the same at all points. The charges do not disappear; they simply transfer energy to the components.
电路中最普遍的误区之一是认为电流在流经灯泡或电阻等元件时会被“消耗掉”。实际上，电流是电荷流动的速率，在串联电路中各处的电流处处相等。电荷并没有消失；它们只是将能量传递给了元件。
Students often confuse current with energy. The battery provides the energy that is transformed into light and heat in the bulb, but the charge carriers (electrons) return to the battery. When an ammeter registers a smaller reading after a resistor, it is almost always because the circuit is not a simple series loop, or the student has misconnected the meter. Remember: in a single-loop series circuit, current is conserved.
学生经常混淆电流和能量。电池提供能量，在灯泡中转化为光和热，但电荷载体（电子）会回到电池。如果电流表在电阻之后显示更小的读数，几乎总是因为电路不是简单的串联回路，或者学生接了错误的电表。记住：在单回路串联电路中，电流是守恒的。
7. Resistors in Parallel: Adding More Resistors Increases Total Resistance? | 并联电阻：增加支路总电阻会变大？
A very counterintuitive result for many learners is that adding a resistor in parallel actually decreases the total resistance of a circuit. The misconception arises from the idea that any extra resistance must make it harder for current to flow. However, each parallel branch provides an additional path for charge, increasing the total current for the same potential difference. Using the formula 1/Rₜ = 1/R₁ + 1/R₂, you can see that Rₜ is always less than the smallest individual resistance.
一个让许多学习者觉得反直觉的结果是：添加并联电阻实际上会减小电路的总电阻。误区源于认为任何额外的电阻都会让电流更难通过。然而，每一个并联支路都为电荷提供了额外的路径，在相同电势差下增加了总电流。根据公式 1/Rₜ = 1/R₁ + 1/R₂，可以看出总电阻 Rₜ 总是小于最小的单个电阻。
This principle is frequently tested in IB and AQA circuit analysis questions, where you need to calculate combined resistance or explain changes in ammeter readings when a switch is closed. Visualising each loop as a separate channel for electrons can help you remember that more parallel paths lower the overall resistance.
这一原理在 IB 和 AQA 的电路分析题中经常考查，需要你计算总电阻或解释开关闭合后电流表读数的变化。把每个回路想象成电子的独立通道，有助于记住：并联支路越多，总电阻越小。
8. Wave Speed: Does the Speed of a Wave Depend on Its Frequency? | 波速：波速取决于频率吗？
Many students incorrectly apply the wave equation v = f λ to conclude that if frequency increases, speed must also increase. In fact, wave speed is determined by the properties of the medium (such as tension and mass per unit length for a string, or depth for water waves). The frequency of a wave is set by the source. When a wave passes from one medium to another, its speed and wavelength change, but its frequency remains constant. For a given medium, increasing the frequency reduces the wavelength such that speed stays the same.
许多学生错误地运用波动方程 v = f λ 得出结论：频率增大，波速也一定增大。事实上，波速是由介质的性质决定的（如弦的张力与线密度、水波的深度等）。波的频率由波源决定。当波从一种介质进入另一种介质时，波速和波长都会改变，但频率保持不变。在给定介质中，增大频率会减小波长，从而使波速保持不变。
This misconception often surfaces in IB waves questions and AQA required practicals on waves in strings and ripples. Always consider what is fixed by the source and what is fixed by the medium. The speed is a property of the medium, not the source, unless the medium changes.
这个误区常在 IB 波动题和 AQA 关于弦波和涟漪的必做实验中浮现。务必考虑什么是波源决定的，什么是介质决定的。除非介质发生变化，波速是由介质决定的，不取决于波源。
9. Thermal Physics: Heat and Temperature Are the Same? | 热物理学：热量与温度是同一回事？
In everyday language we often use ‘heat’ and ‘temperature’ interchangeably, but in physics they have distinct meanings. Temperature is a measure of the average random kinetic energy of particles in a substance. Heat, on the other hand, is the energy transferred from a region of higher temperature to a region of lower temperature. An object does not ‘contain heat’; it has internal energy.
日常语言中，我们常混用“热量”和“温度”，但在物理学中它们有明确的区分。温度是物质内部分子无规则运动平均动能的量度。而热量是从高温区域传递到低温区域的能量。一个物体不“含有热量”；它具有内能。
A classic exam trap is to ask about the temperature change when a substance undergoes a phase change. While heat is being supplied, the temperature remains constant during melting or boiling because the energy goes into breaking intermolecular bonds, not raising kinetic energy. IB and AQA tests require you to interpret heating curves and specific latent heat calculations with this distinction in mind.
一个经典的考题陷阱是询问物质发生相变时的温度变化。在熔化或沸腾过程中，尽管在持续供热，温度却保持不变，因为输入的能量用于破坏分子间作用力，而不是增加动能。IB 和 AQA 的考试要求你根据这一区分来解读加热曲线和进行比潜热计算。
10. Collisions: Are Momentum and Kinetic Energy Both Conserved? | 碰撞：动量与动能都守恒吗？
Many students assume that if momentum is conserved in a collision, kinetic energy is also conserved. Total momentum is always conserved in all collisions within a closed system, but kinetic energy is only conserved in perfectly elastic collisions. In inelastic collisions, some kinetic energy is transformed into other forms such as thermal energy, sound, or potential energy of deformation.
许多学生想当然地认为，如果碰撞中动量守恒，那么动能也守恒。在封闭系统中，总动量在任何碰撞中都守恒，但动能仅在完全弹性碰撞中守恒。在非弹性碰撞中，部分动能转化为其他形式的能量，如内能、声能或形变势能。
IB and AQA questions frequently present data before and after a collision and ask you to determine whether it was elastic or inelastic. You must calculate the total kinetic energy before and after: if they are equal (or almost equal within uncertainties), the collision is elastic. Otherwise it is inelastic, even if momentum is conserved. Never confuse the conservation laws.
IB 和 AQA 的题目经常给出碰撞前后的数据，要求你判断是否为弹性碰撞。你必须计算碰撞前后的总动能：如果两者相等（或在不确定度范围内几乎相等），则是弹性碰撞；否则就是非弹性碰撞，即便动量守恒。切勿混淆这两个守恒定律。
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