📚 GCSE Edexcel Physics – Gravity: Essential Points Explained | 万有引力考点精讲
Gravity is one of the fundamental forces that shape our universe. In GCSE Edexcel Physics, understanding gravity means mastering the concepts of weight, mass, gravitational field strength, and their applications in motion, stability, and space. This article breaks down every key point you need for the exam, with clear explanations, core equations, and common pitfalls.
引力是塑造我们宇宙的基本力之一。在 GCSE Edexcel 物理中，理解万有引力意味着掌握重量、质量、引力场强度等概念，以及它们在运动、稳定性和太空中的应用。本文拆解了考试所需的每一个关键知识点，配有清晰的解释、核心方程和常见误区。
1. What is Gravity? | 什么是重力？
Gravity is a non-contact force of attraction that exists between any two objects with mass. The strength of this gravitational pull depends on the masses of the objects and the distance between them. On Earth, gravity is what gives us weight and keeps us on the ground.
重力是一种非接触吸引力，存在于任何两个有质量的物体之间。引力的大小取决于物体的质量和它们之间的距离。在地球上，重力赋予我们重量并让我们留在地面。
In physics, we often treat the gravitational force exerted by the Earth on relatively small objects as acting from a single point – the centre of the Earth. This force causes freely falling objects to accelerate downwards at a constant rate.
在物理学中，我们通常将地球对较小物体的引力看作从地心这一单点发出。这个力使自由下落的物体以恒定的加速度向下运动。
2. Mass vs. Weight | 质量与重量的区别
Mass is a measure of the amount of matter in an object and is a scalar quantity. It does not change with location – a 1 kg bag of flour has the same mass on the Moon as on Earth. Weight, on the other hand, is the force due to gravity acting on that mass and is a vector quantity measured in newtons (N).
质量是衡量物体所含物质的多少的物理量，是一个标量，不随地点改变——一袋 1 千克的面粉在月球上和在地球上质量相同。而重量是由于重力作用在该质量上的力，是一个矢量，单位为牛顿 (N)。
- Mass: measured in kilograms (kg), scalar, does not depend on gravity. / 质量：单位千克 (kg)，标量，不依赖重力。
- Weight: measured in newtons (N), vector, depends on the gravitational field strength. / 重量：单位牛顿 (N)，矢量，依赖引力场强度。
3. The Weight Equation: W = m g | 重量公式：W = m g
The relationship between mass and weight is given by the equation W = m × g, where W is weight in newtons, m is mass in kilograms, and g is the gravitational field strength. This equation is central to many exam questions.
质量与重量之间的关系由公式 W = m × g 给出，其中 W 为重量（牛顿），m 为质量（千克），g 为引力场强度。该公式是许多考题的核心。
W = m × g
When an object is placed on a surface, its weight acts downwards. We can measure weight using a force meter (spring balance) calibrated in newtons.
当物体放在支撑面上时，其重量向下作用。我们可以用牛顿标定的测力计（弹簧秤）测量重量。
4. Gravitational Field Strength (g) Explained | 引力场强度 (g) 详解
Gravitational field strength (g) is the force per unit mass exerted on a small mass placed at a point in the field. It defines how strong gravity is at a particular location and is measured in N/kg (which is equivalent to m/s²).
引力场强度 (g) 是作用在场中某点处小质量上的力与质量之比。它定义了某特定地点引力有多强，单位是 N/kg（等效于 m/s²）。
The value of g on the Earth’s surface is approximately 9.8 N/kg, but Edexcel often uses g = 10 N/kg in calculations for simplicity. This means that a 1 kg mass experiences a gravitational force of about 10 N.
地球表面的 g 值约为 9.8 N/kg，但 Edexcel 为简化常使用 g = 10 N/kg 进行计算。这意味着 1 千克的质量会受到约 10 牛的重力。
5. Value of g on Earth and Common Values | 地球上 g 的值与常见数值
On Earth, the standard value for equations is 9.8 m/s² or 9.8 N/kg. You must be comfortable converting between these units – they are numerically identical. In exam problems, you may be told to use 10 N/kg for quick working.
在地球上，方程的标准值为 9.8 m/s² 或 9.8 N/kg。你必须熟练掌握这两种单位的换算——它们在数值上完全相同。试题中可能要求使用 10 N/kg 以简化运算。
It is vital to remember that g is not a universal constant – it varies slightly with altitude and latitude. But for GCSE calculations, we treat it as constant near the Earth’s surface.
要记住 g 并非一个普适常量——它会随海拔和纬度略有变化。但在 GCSE 计算中，我们将其看作是在地表附近恒定的。
6. Centre of Mass and Stability | 质心与稳定性
The weight of an object acts as if it is concentrated at a single point called the centre of mass. For symmetrical objects with uniform density, the centre of mass is at the geometric centre. The position of the centre of mass determines an object’s stability.
物体的重量似乎集中在一个称为质心的单点上。对于密度均匀的对称物体，质心在几何中心。质心的位置决定了物体的稳定性。
An object is stable if its centre of mass is low and its base is wide. If a vertical line through the centre of mass falls outside the base, the object will topple. This is crucial for understanding balance and design.
如果物体的质心低且底座宽，它就很稳定。若一条穿过质心的竖直线落在了底座之外，物体便会倾倒。这对于理解平衡和设计至关重要。
7. Gravity and Free Fall Motion | 重力与自由落体运动
When the only force acting on an object is its weight (e.g. in a vacuum), it undergoes free fall and accelerates downwards at g. This acceleration is independent of the object’s mass – a hammer and a feather fall at the same rate without air resistance.
当作用在物体上的唯一力是它的重量时（如在真空中），物体便做自由落体运动，并以加速度 g 向下运动。该加速度与物体的质量无关——在没有空气阻力的情况下，锤子和羽毛以同样的速度下落。
This can be shown using the equations of uniformly accelerated motion, with a = g. The relationship v = u + g t (with downward positive) helps solve many free fall problems. Air resistance is often ignored at GCSE but can be mentioned to explain terminal velocity.
这可以用匀加速运动方程来证明，其中 a = g。关系式 v = u + g t（设向下为正）帮助解决许多自由落体问题。在 GCSE 中空气阻力常被忽略，但可以提及以解释终速。
8. Measuring Weight and Determining g | 测量重量和测定 g
You can measure weight directly with a calibrated newton meter. Alternatively, you can find weight by measuring the mass with a balance and multiplying by a known g. In the lab, g can be determined from the gradient of a weight–mass graph.
你可以用已校准的牛顿计直接测量重量。或者，可以用天平测出质量再乘以已知的 g。在实验室里，可以通过重量–质量图的斜率来测定 g。
weight = gradient × mass → g = weight / mass
Plotting weight on the y‑axis and mass on the x‑axis gives a straight line through the origin; the slope is g. This is a classic practical task and a common exam data‑analysis question.
将重量画在 y 轴、质量画在 x 轴上，得到一条通过原点的直线；其斜率就是 g。这是一个经典的实验任务，也是常见的数据分析考题。
9. Weight on Different Planets | 不同行星上的重量
An object’s mass remains constant throughout the universe, but its weight changes depending on the gravitational field strength of the celestial body. On the Moon, g ≈ 1.6 N/kg, so a 60 kg person would weigh only about 96 N compared with 600 N on Earth.
一个物体的质量在宇宙中处处相同，但其重量会随着天体引力场强度的不同而变化。在月球上 g ≈ 1.6 N/kg，因此一个 60 kg 的人只重约 96 N，而在地球上则重约 600 N。
In exam questions, you may be asked to calculate weight on Mars or the Moon using the equation W = m g with the given g value. Remember always to state the unit (N) and show the substitution clearly.
考试中可能会要求使用给定 g 值通过 W = m g 计算在火星或月球上的重量。切记要写明单位 (N) 并清晰地展示代入过程。
10. How g Varies with Distance | g 随距离如何变化
Gravitational field strength decreases as you move away from a planet. The further from the centre of the Earth, the weaker the pull. This variation is roughly described by an inverse-square relationship – doubling the distance reduces g to about a quarter.
随着远离行星，引力场强度会减小。离地心越远，引力越弱。这种变化大致遵循平方反比关系——距离加倍，g 约减为原来的四分之一。
g ∝ 1 / distance² (qualitative understanding only)
At GCSE, you do not need to perform calculations with this relationship, but you should know that the gravitational field from an object spreads out and gets weaker with distance. This helps explain why astronauts experience weightlessness in orbit.
在 GCSE 中，你无需用该关系进行计算，但应该知道物体产生的引力场会向外扩散并随距离变弱。这有助于解释为什么宇航员在轨道上会体验到失重状态。
11. Common Misconceptions & Exam Traps | 常见误区与考试陷阱
Misconception 1: ‘Mass and weight are the same.’ In everyday language we often confuse them, but in physics they are completely different quantities. Always check that you are using kg for mass and N for weight.
误区一：“质量和重量是一样的。”日常用语中我们常混淆它们，但在物理中它们是截然不同的量。一定要检查质量用 kg 而重量用 N。
Misconception 2: ‘g is always 10 m/s².’ This is an approximation used for convenience. The actual value is about 9.8 m/s², and it varies on different planets and at different altitudes. Your answer must be consistent with the value given in the question.
误区二：“g 总是 10 m/s²。”这只是为了方便而使用的近似值。实际值约为 9.8 m/s²，且在不同行星和不同高度处会变化。你的答案必须与题目所给数据一致。
Exam trap: Forgetting to convert units before substituting into W=m g. If mass is given in grams, you must convert it to kilograms by dividing by 1000.
考试陷阱：代入 W=m g 前忘记单位转换。如果质量以克给出，必须除以 1000 将其转换为千克。
12. Key Definitions Recap | 关键定义回顾
Before the exam, make sure you can define the following accurately:
考试前，请确保你能够准确定义下列术语：
- Gravitational force – a non-contact force of attraction between masses. / 万有引力——质量之间的非接触吸引力。
- Weight – the force acting on an object due to gravity; W = m g. / 重量——因重力作用在物体上的力；W = m g。
- Mass – the quantity of matter in an object, measured in kg. / 质量——物体所含物质的量，单位为 kg。
- Gravitational field strength (g) – force per unit mass on a small object; unit N/kg. / 引力场强度 (g)——作用在单位质量小物体上的力；单位 N/kg。
- Centre of mass – the point where the entire weight of an object appears to act. / 质心——物体的全部重量看似汇聚的那一点。
- Free fall – motion under gravity alone, with acceleration g. / 自由落体——仅在重力作用下的运动，加速度为 g。
Keep these definitions sharp, and you will avoid losing marks on core knowledge questions.
牢记这些定义，你就能避免在基础知识题上丢分。
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