GCSE Edexcel Physics: Common Mistake Questions Explained | GCSE Edexcel 物理：易错题精讲

📚 GCSE Edexcel Physics: Common Mistake Questions Explained | GCSE Edexcel 物理：易错题精讲

In GCSE Edexcel Physics, many students lose marks not because they do not understand the concepts, but because they fall into classic traps set by examiners. This article explains the most common mistake questions, clarifies misconceptions, and provides clear strategies to avoid these errors. Read through each section and check your understanding with the worked examples.

在 GCSE Edexcel 物理考试中，许多学生丢分并非因为不理解概念，而是因为掉进了考官设下的经典陷阱。本文讲解了最常见的易错题，澄清常见误区，并提供清晰的解题策略以避免这些错误。通读每个部分并用示例检验你的理解。

1. Average Speed vs Instantaneous Speed | 平均速度与瞬时速度

One typical mistake is calculating average speed using the simple average of initial and final speeds: (u + v)/2. This formula only applies when acceleration is constant (uniform acceleration). For a journey with varying speeds or stops, you must use the definition: average speed = total distance ÷ total time.

一个典型错误是用初速度和末速度的简单平均值 (u+v)/2 来计算平均速度。该公式仅适用于加速度恒定的（匀加速）情况。对于速度变化或中途停止的旅程，必须使用定义：平均速度 = 总距离 ÷ 总时间。

Example: A car travels at 5 m/s for 10 s, then at 15 m/s for 20 s. Total distance = (5 × 10) + (15 × 20) = 50 + 300 = 350 m. Total time = 30 s. Average speed = 350 ÷ 30 ≈ 11.67 m/s. The incorrect method (5+15)/2 = 10 m/s is wrong.

示例：一辆汽车以 5 m/s 行驶 10 s，然后以 15 m/s 行驶 20 s。总距离 = (5×10)+(15×20) = 50+300 = 350 m。总时间 = 30 s。平均速度 = 350 ÷ 30 ≈ 11.67 m/s。错误方法 (5+15)/2=10 m/s 是错误的。

Remember: on a distance–time graph, the gradient at a point gives the instantaneous speed, not the average speed over the entire graph.

记住：在距离–时间图上，某一点的斜率给出的是瞬时速度，而非整个图形的平均速度。

2. Resultant Force and Motion | 合力与运动

Students often think that a force is needed to keep an object moving. According to Newton’s first law, a resultant force of zero means the object will remain at rest or move with constant velocity in a straight line. No forward force is required to maintain constant speed if no opposing forces act.

学生常误以为需要力来维持物体运动。根据牛顿第一定律，合力为零意味着物体将保持静止或沿直线匀速运动。如果没有阻力作用，维持恒定速度并不需要向前的力。

Misconception: At the highest point of a vertical throw, the velocity is zero, so students might think the resultant force is zero. In fact, gravity still acts, so the resultant force is mg downwards, and the acceleration is g. The object is momentarily at rest but accelerating.

误区：在竖直上抛的最高点，速度为零，学生可能认为合力为零。事实上，重力仍然作用，因此合力为向下的 mg，加速度为 g。物体只是瞬间静止，但有加速度。

Check: A spaceship in deep space moving at constant speed with engines off. There is no resultant force; it continues at that speed.

检验：在外太空关闭引擎、以恒定速度飞行的飞船。它不受合力作用，因此保持那个速度继续运动。

3. Energy Conservation and Efficiency | 能量守恒与效率计算

A common error is writing ‘energy is used up’. Energy is never destroyed; it is transferred or dissipated into less useful forms (usually thermal energy). In efficiency calculations, efficiency = (useful output energy ÷ total input energy) × 100%.

常见错误是写“能量用完了”。能量永远不会被消灭；它被转移或耗散为不太有用的形式（通常是热能）。在效率计算中，效率 = （有用的输出能量 ÷ 总输入能量）× 100%。

Tricky question: a motor lifts a 2 kg mass through 3 m. The work done on the load (useful) = mgh = 2 × 10 × 3 =

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