📚 Common Misconceptions in AS Physics | AS 物理常见误区
Studying AS Physics introduces many fundamental concepts that can easily be misunderstood. Clarifying these common misconceptions early on is crucial for building a solid foundation. Below we explore ten widespread errors that students often make, with clear explanations to set the record straight.
学习 AS 物理时会接触到许多基本概念，这些概念很容易被误解。尽早澄清这些常见误区对于打下坚实基础至关重要。下面我们探讨学生经常犯的十个普遍错误，并给出清晰的解释，以正本清源。
1. Speed vs Velocity | 速率与速度
A common mistake is using ‘speed’ and ‘velocity’ interchangeably. Speed is a scalar quantity that only measures how fast something is moving, whereas velocity is a vector that also specifies direction. For instance, a car moving at 60 km/h has a speed of 60 km/h, but its velocity might be 60 km/h due north.
常见的错误是将“速率”和“速度”混为一谈。速率是标量，只衡量物体运动的快慢；而速度是矢量，同时指明方向。例如，一辆以 60 km/h 行驶的汽车，其速率为 60 km/h，但速度可能是向北 60 km/h。
Misconception: If an object returns to its starting point, its average velocity is zero but the average speed is not. Many students mistakenly think average speed must also be zero. Correct understanding: Average speed = total distance / total time, while average velocity = displacement / time. Displacement can be zero even when distance is non-zero.
误区：物体回到起点时，平均速度为零，但平均速率不为零。许多学生误以为平均速率也必然为零。正确理解：平均速率 = 总路程/总时间，而平均速度 = 位移/时间。即使路程非零，位移也可以为零。
2. Mass vs Weight | 质量与重量
Many students believe that mass and weight are the same thing. Mass is the amount of matter in an object, measured in kilograms, and it is constant regardless of location. Weight is the gravitational force acting on that mass, measured in newtons, and it changes depending on the gravitational field strength.
许多学生认为质量和重量是一回事。质量是物体所含物质的多少，单位是千克，无论位置如何都保持不变。重量是作用在该质量上的重力，单位是牛顿，会随引力场强度的变化而改变。
Misconception: An astronaut in space is ‘weightless’ so has zero mass. In reality, mass remains unchanged; weightlessness occurs because the astronaut is in free fall, experiencing no support force, but weight (mg) still exists though not felt.
误区：太空中的宇航员“失重”，因而质量为零。实际上质量不变；失重是因为宇航员处于自由落体状态，没有支撑力，但重力（mg）仍然存在，只是感受不到。
Using W = mg makes it clear that weight depends on g. On the Moon, g is about 1.6 N/kg, so an object’s weight is only about one-sixth of its Earth weight, but its mass stays the same.
利用公式 W = mg 可以清楚看出重量依赖于 g。在月球上，g 约为 1.6 N/kg，因此物体的重量大约只有地球重量的六分之一，但质量不变。
3. Balanced Forces Always Mean the Object is at Rest | 平衡力一定意味着物体静止？
Students often think that if the resultant force on an object is zero, the object must be stationary. According to Newton’s First Law, an object with zero resultant force will continue in its state of rest or uniform motion in a straight line. Therefore, an object moving at constant velocity also has balanced forces.
学生们常常认为，如果物体所受合力为零，那么物体必定静止。根据牛顿第一定律，合力为零的物体将保持静止或匀速直线运动状态。因此，匀速运动的物体同样处于平衡状态。
For example, a car cruising at a constant speed on a straight road has the driving force balanced by resistive forces. It is not stationary but forces are balanced.
例如，一辆在笔直道路上匀速行驶的汽车，驱动力与阻力平衡。它并非静止，但力是平衡的。
4. The Direction of Current vs Electron Flow | 电流方向与电子流动方向混淆
Conventional current flows from positive to negative, while electrons flow from negative to positive. Students often confuse the two, especially when applying Fleming’s left-hand rule or in electrolysis. Remember: conventional current is the direction positive charges would move; in metal wires, it is opposite to the electron drift.
常规电流方向是从正极到负极，而电子流动方向是从负极到正极。学生们经常混淆这两者，尤其是在应用弗莱明左手定则或电解的时候。记住：常规电流是正电荷移动的方向；在金属导线中，它与电子漂移的方向相反。
Misconception: In a diode, current flows easily when connected ‘forward biased’ because electrons flow from p-type to n-type? Actually, forward bias allows conventional current from p to n, which corresponds to electrons moving from n to p. Understanding this prevents errors in circuit analysis.
误区：在二极管中，正向偏置时电流容易流通是因为电子从 p 型流向 n 型？实际上，正向偏置允许常规电流从 p 流向 n，这对应于电子从 n 移向 p。理解这一点可以避免电路分析中的错误。
5. Voltage is ‘Used Up’ in a Circuit | 电路中的电压被“消耗”了？
A common misunderstanding is that voltage (potential difference) gets used up as current passes through components. In reality, energy is transferred, not voltage. The sum of the potential differences across components in a series circuit equals the supply e.m.f., but voltage itself is not consumed; it’s a measure of energy per unit charge transferred.
一个常见的误解是：电流通过元件时电压会被“用完”。实际上，被转移的是能量，而不是电压。串联电路中各元件两端的电压之和等于电源电动势，但电压本身并不被消耗；它是每单位电荷转移的能量量度。
Think of it like a lift: the height (potential) changes from top to bottom, but height is not ‘used up’ — it is the change in height that allows work to be done. Similarly, charges gain electrical potential energy from the battery and lose it in components.
可以将其想象成电梯：高度（势）从顶部到底部发生变化，但高度并没有被“用完”——正是高度的变化使做功成为可能。同样，电荷从电池获得电势能，并在元件中消耗掉。
6. Horizontal and Vertical Motions in Projectiles Affect Each Other | 抛体运动中水平与竖直运动相互影响？
A fundamental error is believing that the horizontal motion of a projectile influences its vertical motion, or vice versa. In fact, under constant gravity, the horizontal and vertical components of motion are independent. A bullet fired horizontally and a bullet dropped from the same height will hit the ground simultaneously if we ignore air resistance.
一个基本错误是认为抛体的水平运动会影响竖直运动，反之亦然。实际上，在恒定重力作用下，运动的水平和竖直分量是相互独立的。如果忽略空气阻力，水平射出的子弹与从同一高度释放的子弹会同时落地。
Misconception: A heavier object falls faster, so it affects projectile range. Correct: In the absence of air resistance, all objects accelerate at g regardless of mass, so mass does not affect the time of flight. Range depends on horizontal velocity and time of flight.
误区：重物下落得更快，因此影响抛体的射程。正确：在没有空气阻力的情况下，所有物体的重力加速度均为 g，与质量无关，所以质量不影响飞行时间。射程取决于水平速度和飞行时间。
Vertical displacement is given by y = u_y t + ½gt², with the sign convention accounting for direction. Horizontal displacement is simply x = u_x t. No gravitational term appears in the horizontal equation, confirming independence.
竖直位移可由 y = u_y t + ½gt² 给出，其中符号规定考虑了方向。水平位移仅为 x = u_x t。水平方程中没有出现重力项，证明了运动的独立性。
7. Action and Reaction Forces Cancel Each Other Out | 作用力与反作用力相互抵消？
Students often think that Newton’s third law pair of forces cancel each other because they are equal and opposite. However, action and reaction act on different objects, so they never cancel in terms of the motion of a single object. A book on a table: the weight of the book and the normal force from the table are not an action-reaction pair; they act on the same object (the book) and can cancel. The reaction to the book’s weight is the book pulling on the Earth.
学生们常常认为牛顿第三定律中的一对力会相互抵消，因为它们大小相等、方向相反。但是，作用力与反作用力作用在不同的物体上，因此它们永远不会影响同一个物体的运动而抵消。一本书放在桌子上：书的重力和桌子对书的支持力不是一对作用力与反作用力；它们作用在同一物体（书）上，可以抵消。书的重力的反作用力是书对地球的引力。
Misconception: A horse pulling a cart moves forward because the cart pulls back with an equal force, so the forward pull wins? Actually, the horse-cart interaction is an action-reaction pair, but the horse’s feet push against the ground; the ground pushes the horse forward, allowing the system to accelerate. It’s the external friction force that causes motion.
误区：马拉着车前进，车以同样大小的力拉马，所以马的前进拉力获胜？实际上，马与车的相互作用是一对作用力与反作用力，但马的蹄子向后蹬地，地面对马施加向前的摩擦力，从而使系统加速。是外部的摩擦力导致了运动。
8. Particles of a Medium Travel with the Wave | 波传播时介质质点随波迁移？
A classic misconception is that when a wave passes, the particles of the medium travel along with the wave. In transverse and longitudinal waves, particles oscillate around a fixed point and do not move with the wave. The wave transfers energy, not matter. A cork on water bobs up and down but does not move horizontally with the ripples.
一个经典的误解是：当波通过时，介质的质点会随着波一起迁移。在横波和纵波中，质点围绕固定点振动，并不随波前进。波传递的是能量，而不是物质。水面上的软木塞上下起伏，但不会随水波水平移动。
This confusion often arises in sound waves: students think air molecules travel from the source to the ear. In reality, air molecules vibrate back and forth, creating compressions and rarefactions that propagate, but the molecules themselves only have small oscillatory displacements.
这种混淆常出现在声波上：学生认为空气分子从声源旅行到了耳朵。实际上，空气分子前后振动，形成传播的疏密波，而分子本身只有很小的振荡位移。
9. Ohm’s Law is Universal for All Conductors | 欧姆定律适用于所有导体？
Many students assume that V = IR means resistance is constant for any component. Ohm’s law states that the current through a conductor is directly proportional to the voltage across it,
Published by TutorHao | Physics Revision Series | aleveler.com
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