📚 Common Misconceptions in IB & Edexcel Physics | IB与爱德思物理常见误区
Physics students often develop intuitive but incorrect ideas that can hinder their understanding of core concepts. In both the IB Diploma Programme and the Pearson Edexcel International A Level syllabi, addressing these misconceptions early is crucial for success in exams and practical applications. This article highlights some of the most common pitfalls, explains why they feel convincing, and provides clear, corrected explanations that map directly onto the assessment objectives of both specifications.
物理学生常常形成一些直观但错误的观念，这会阻碍他们对核心概念的理解。在IB文凭课程和培生爱德思国际A Level课程体系中，尽早纠正这些误区对于考试成功和实际应用至关重要。本文将指出一些最常见的陷阱，解释它们为何听起来很有说服力，并提供清晰、正确的解释，这些解释直接对应两个考纲的评估目标。
1. Force and Motion – Constant Force for Constant Velocity? | 力与运动——恒力才能保持匀速？
One of the oldest misconceptions is that a constant force is needed to maintain a constant velocity. Students often think that if you stop pushing an object, it will eventually stop because the force 'runs out'.
最古老的误区之一是认为需要恒定的力来维持匀速运动。学生们常认为如果停止推一个物体，它最终会停下来，因为力'耗尽'了。
In reality, Newton's First Law states that an object will remain at rest or move with constant velocity unless a net external force acts on it. Constant velocity means zero net force. The misconception persists because in everyday life we experience friction. When you push a box across the floor, it stops when you stop pushing because friction (an unbalanced force) decelerates it. In the absence of friction, such as in deep space, an object maintains its velocity indefinitely without any applied force. Note carefully that zero net force does not mean zero velocity, and it does not mean zero force; it means the forces cancel vectorially.
实际上，牛顿第一定律指出，除非受到净外力作用，物体将保持静止或匀速直线运动。匀速运动意味着净力为零。这一误区之所以存在，是因为日常生活中我们受摩擦力影响。当你在地板上推箱子，停止推时它会停下，这是因为摩擦力（不平衡力）使其减速。在没有摩擦的情况下，如深空，物体会无限期保持其速度，无需任何施加的力。要特别注意：净力为零并不意味着速度为零，也不意味着没有力；它意味着所有力在矢量上相互抵消。
Fₙₑₜ = 0 ⇔ a = 0 ⇔ constant velocity
Thus, the driver does not maintain the motion; rather, unbalanced forces change it.
因此，推动者并非维持运动；相反，不平衡力才会改变运动。
Exam tip: both IB and Edexcel frequently ask you to identify the forces acting on a body in a free-body diagram. Always draw only the forces acting on the chosen body, not the forces it exerts on other objects, and resolve them into components before concluding anything about acceleration.
考试提示：IB和爱德思都经常要求你在受力图中标出作用在物体上的力。始终只画出作用在你所选物体上的力，而不是它施加在其他物体上的力，并在得出关于加速度的结论前先把它们分解成分量。
2. Action-Reaction Pairs Balance Each Other? | 作用力与反作用力相互平衡？
Many students assume that Newton's third law pairs cancel out, so that a book resting on a table is 'held up' by the equal and opposite force exerted by the table. This leads to a confused explanation in which the weight of the book and the contact force from the table are described as an action-reaction pair.
许多学生认为牛顿第三定律中的一对力会相互抵消，因此放在桌上的书是被桌子施加的等大反向的力'托住'的。这导致一种混乱的解释：把书本的重力和桌子的接触力说成是一对作用力与反作用力。
In fact, Newton's third law pairs act on two different bodies, so they can never cancel. Cancellation (equilibrium) requires two forces acting on the same body. For the book on the table, the weight of the book and the normal contact force from the table both act on the book, and because the book is in equilibrium they are equal in magnitude and opposite in direction. The third law partner of the book's weight is the gravitational pull that the book exerts on the Earth, and the third law partner of the table's push on the book is the book's push on the table.
事实上，牛顿第三定律的一对力作用在两个不同的物体上，所以它们永远不可能抵消。抵消（平衡）要求两个力作用在同一个物体上。对于桌上的书，书的重力和桌子给的支持力都作用在书上，由于书处于平衡状态，它们大小相等、方向相反。书本重力的第三定律配对力，是书对地球的吸引力；而桌子推书的第三定律配对力，是书对桌子的压力。
F₁₂ = −F₂₁
The key checklist for a genuine Newton's third law pair is: equal magnitude, opposite direction, same line of action, same type of force, acting on two different bodies, and arising at the same instant.
判断真正的一对牛顿第三定律力的关键清单是：大小相等、方向相反、作用在同一直线上、属于同一种力、作用在两个不同物体上，并且同时产生。
3. Heavier Objects Fall Faster | 更重的物体下落更快？
A very common intuition is that a heavy stone falls faster than a light feather, so 'heavier objects fall faster'. This is only true on Earth in the presence of air resistance, not as a statement about gravity itself.
一个非常普遍的直觉是重石头比轻羽毛下落得快，所以'更重的物体下落更快'。这只在有空气阻力存在的地球上成立，而不能作为关于引力本身的陈述。
e mass. Because weight equals mg, the mass cancels, leaving a = g regardless of the object' ground together. This was demonstrated famously on the Moon during the Apollo 15 mission.
对于自由落体，唯一的力是重力，所以加速度等于净力除以质量。由于重力等于mg，质量被约掉，剩下 a = g，与物体质量无关。在真空中，羽毛和铁锤会同时落地。这一点在阿波罗15号任务中于月球上得到了著名演示。
a = Fₙₑₜ ⁄ m = mg ⁄ m = g ≈ 9.81 m s⁻²
Air resistance explains the everyday observation: it depends on shape and speed rather than mass, so a feather decelerates much more than a stone. Eventually an object reaches terminal velocity when drag equals weight and the net force becomes zero. Students should also avoid confusing mass (a scalar measure of inertia, in kg) with weight (a force, in N), since IB and Edexcel both test this distinction explicitly.
空气阻力解释了日常观察：它取决于形状和速度而非质量，所以羽毛受到的减速远大于石头。最终物体会达到终速度，此时阻力等于重力，净力变为零。学生还应避免混淆质量（惯性的标量度量，单位kg）与重量（一个力，单位N），因为IB和爱德思都明确考查这一区分。
4. Centrifugal Force Pushes Objects Outwards | 离心力把物体向外推？
When you sit in a car going round a corner, you feel pushed outwards, so it seems obvious that a 'centrifugal force' throws you outward. In an inertial (non-rotating) frame of reference, no such outward force exists.
当你坐在拐弯的车里时，你会感觉被向外推，所以看起来显然存在一个'离心力'把你甩出去。在惯性（不旋转）参考系中，这种向外的力并不存在。
What actually happens is that your body continues in a straight line because of inertia, while the car turns. The car door or seat then pushes you inwards, and this inward force provides the centripetal acceleration. The sensation of being 'thrown out' is simply your inertia resisting the direction change. IB and Edexcel both require you to identify the real inward force: tension in a string, friction from the road, gravity on a satellite, or the normal contact force on a banked track.
实际情况是：由于惯性，你的身体继续沿直线运动，而车在转弯。车门或座椅随后把你向内侧推，这个向内的力提供向心加速度。被'甩出去'的感觉，只是你的惯性在抵抗方向的改变。IB和爱德思都要求你指出真正向内的力：绳中的张力、路面的摩擦力、作用在卫星上的引力，或倾斜赛道上的支持力。
F = mv² ⁄ r = mω²r, a = v² ⁄ r
A centrifugal term may be introduced only when working in a rotating reference frame, where it is a fictitious or pseudo force. In exam answers, write about the inward resultant force whenever the frame is inertial.
只有当你选择旋转参考系时，才可以引入离心项，此时它是一种虚拟力或伪力。在考试作答中，当参考系是惯性系时，应始终讨论向内的合力。
5. Work and Energy – Energy Is 'Used Up' | 功与能量——能量被'用完'了？
Students frequently say that energy is 'used up' or 'lost' when a machine runs. This phrasing suggests energy can disappear, which contradicts the principle of conservation of energy.
学生常说机器运转时能量被'用完'或'丢失'了。这种说法暗示能量可以消失，这与能量守恒原理相矛盾。
Energy is conserved: it is transferred from one store to another, or dissipated to the thermal store of the surroundings. When a ball bounces and eventually stops, the mechanical energy has not vanished; it has been transferred to the internal energy of the ball and the floor through inelastic deformation and sound. Edexcel uses the term 'dissipated', while IB refers to 'degraded' energy, but the physics is identical.
能量是守恒的：它从一个能量库转移到另一个能量库，或者耗散到周围环境的热能库中。当球弹跳并最终停下时，机械能并没有消失；它通过非弹性形变和声音转化为球和地面的内能。爱德思使用'耗散'一词，而IB称为'退化'能量，但物理本质完全相同。
W = Fs cos θ, P = W ⁄ t = Fv
Another related slip is forgetting the cosine factor. Work done is the product of the force and the displacement in the direction of that force, so a force perpendicular to the motion does zero work. This is why the centripetal force never changes the kinetic energy of a body in uniform circular motion, and why carrying a suitcase horizontally does no work against gravity.
另一个相关的疏忽是忘记余弦因子。功是力与该力方向上位移的乘积，因此垂直于运动方向的力做功为零。这就是为什么向心力永远不改变匀速圆周运动物体的动能，也是为什么水平提着一个行李箱并不克服重力做功。
6. Current Is 'Used Up' in a Circuit | 电流在电路中被'消耗'了？
Many students describe current as being 'used up' as it passes through a lamp or resistor, imagining that the current leaving a component is smaller than the current entering it.
许多学生把电流描述为在通过灯泡或电阻时被'消耗'，以为离开元件的电流小于进入元件的电流。
Electric charge is conserved, so the current in a series circuit is the same at every point. What decreases across each component is the potential difference, that is, the energy transferred per unit charge. A lamp does not swallow electrons; it converts electrical energy into light and heat, and the same number of electrons per second flows out as flows in.
电荷守恒，所以串联电路中每一点的电流都相同。在每个元件上减小的是电位差，也就是每单位电荷转移的能量。灯泡并不会吞掉电子；它把电能转化为光和热，流出的电子数每秒与流入的完全相同。
I = ΔQ ⁄ Δt, Σ I(in) = Σ I(out)
This is Kirchhoff's junction rule, and it follows directly from conservation of charge. When a question asks where the energy goes, answer in terms of energy transfer per unit charge (voltage) rather than in terms of current being consumed.
这就是基尔霍夫节点定律，它直接源自电荷守恒。当题目问到能量去了哪里时，请从每单位电荷的能量转移（电压）角度回答，而不是说电流被消耗了。
7. Ohm's Law Always Applies and
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