Gravity: IGCSE AQA Physics Revision Notes | 万有引力：IGCSE AQA物理考点精讲

📚 Gravity: IGCSE AQA Physics Revision Notes | 万有引力：IGCSE AQA物理考点精讲

Gravity is one of the fundamental forces that shapes our universe. In IGCSE AQA Physics, you will explore how gravity gives objects weight, keeps planets in orbit, and governs the motion of satellites. This revision guide breaks down every key concept, from W = mg to orbital mechanics, with clear explanations and exam‑ready details.

引力是塑造我们宇宙的基本力之一。在 IGCSE AQA 物理课程中，你将探索引力如何赋予物体重量、使行星保持在轨道上，并支配卫星的运动。这本复习指南详细拆解了从 W = mg 到轨道力学的每一个核心概念，提供清晰的解释和备考细节。

1. Weight, Mass and Gravitational Field Strength | 重量、质量与引力场强度

Weight is the force of gravity acting on an object’s mass. It is measured in newtons (N) and must not be confused with mass, which is the amount of matter in an object and is measured in kilograms (kg).

重量是作用在物体质量上的引力。它以牛顿 (N) 为单位，切勿与质量混淆——质量是物体所含物质的多少，以千克 (kg) 为单位。

Gravitational field strength, represented by the symbol g, tells us how strong gravity is at a particular location. On the surface of the Earth, g is approximately 9.8 N/kg. This means every kilogram of mass experiences a downward pull of 9.8 newtons.

引力场强度，用符号 g 表示，告诉我们某一地点引力有多强。在地球表面，g 大约为 9.8 N/kg。这意味着每千克质量会受到 9.8 牛顿的向下拉力。

The same object would weigh less on the Moon because the Moon’s gravitational field strength is smaller – about 1.6 N/kg. Weight depends on both mass and the local g, while mass stays the same everywhere.

同一物体在月球上会更轻，因为月球的引力场强度较小——大约为 1.6 N/kg。重量取决于质量和当地的 g，而质量在任何地方都保持不变。

2. The Equation W = mg | 公式 W = mg

The relationship between weight, mass and gravitational field strength is given by the simple formula:

重量、质量和引力场强度之间的关系由以下简单公式给出：

W = m g

Where W is weight in newtons (N), m is mass in kilograms (kg), and g is gravitational field strength in newtons per kilogram (N/kg). This equation is central to IGCSE exam questions and is often used to calculate the weight of an object on different planets.

其中 W 是重量，单位为牛顿 (N)；m 是质量，单位为千克 (kg)；g 是引力场强度，单位为牛顿每千克 (N/kg)。这个公式是 IGCSE 考试问题的核心，常用来计算物体在不同行星上的重量。

To find weight on Earth, you simply multiply the mass by 9.8. For example, a 2.0 kg textbook has a weight of approximately 2.0 × 9.8 = 19.6 N. If that textbook were taken to Mars, where g ≈ 3.7 N/kg, its weight would drop to about 7.4 N.

要计算地球上的重量，只需将质量乘以 9.8。例如，一本 2.0 kg 的教科书重量约为 2.0 × 9.8 = 19.6 N。如果这本教科书被带到火星，那里 g ≈ 3.7 N/kg，它的重量将降至约 7.4 N。

3. Measuring g on Earth | 测量地球表面的 g 值

In the laboratory, you can determine the value of g by timing a falling object or by using a pendulum. A common method involves dropping a steel ball from a known height and measuring the time it takes to fall. Using s = ½ a t² with u = 0, you can calculate the acceleration due to gravity, which equals g.

在实验室中，你可以通过计时自由落体或使用摆锤来测定 g 值。一种常见方法是从已知高度释放钢球并测量其下落时间。利用 s = ½ a t² (初速 u = 0)，可以计算出重力加速度，即 g。

Another method uses a mass on a spring or a simple pendulum, where the period T is related to gravitational acceleration. In IGCSE practical assessments, you may be asked to describe the procedure, identify sources of error such as air resistance or reaction time, and suggest improvements.

另一种方法使用弹簧上的重物或单摆，其周期 T 与重力加速度有关。在 IGCSE 实践评估中，你可能会被要求描述实验步骤，指出误差来源（如空气阻力或反应时间），并提出改进建议。

4. The Nature of Gravity: A Universal Force | 引力的本质：普遍存在的力

Gravity is a force of attraction that acts between any two objects with mass. This force is universal – it operates everywhere in the universe. Even you exert a gravitational pull on the person sitting next to you, although it is far too small to notice.

引力是作用在任何两个有质量物体之间的吸引力。这个力是普遍存在的——它在宇宙中无处不在。即使是你，也会对坐在你旁边的人施加引力，只是它太小了，根本察觉不到。

Because gravity acts between all masses, it is sometimes called ‘universal gravitation’. The larger the masses involved, the stronger the gravitational attraction. This is why we feel a noticeable pull from the Earth, which has a huge mass of about 6.0 × 10²⁴ kg.

由于引力作用在所有质量之间，有时被称为“万有引力”。涉及的质量越大，引力就越强。这就是为什么我们能感觉到来自地球的明显拉力，其质量约为 6.0 × 10²⁴ 千克。

5. Gravitational Force Depends on Mass and Distance | 引力取决于质量和距离

Newton’s law of universal gravitation describes how gravitational force changes with mass and separation. Qualitatively, the force is directly proportional to the product of the two masses and inversely proportional to the square of the distance between their centres.

牛顿万有引力定律描述了引力如何随质量和距离变化。定性地说，力与两个质量的乘积成正比，与它们中心之间距离的平方成反比。

F = G m₁ m₂ / r²

Here G is the gravitational constant, but IGCSE examiners rarely require calculations with this full formula. What matters is recognising that if you double the distance between two objects, the gravitational force between them becomes one‑quarter (1/2²). If you triple the distance, the force drops to one‑ninth (1/3²).

这里 G 是引力常数，但 IGCSE 考官很少要求用这个完整公式计算。重要的是要认识到：如果将两个物体间的距离增加一倍，它们之间的引力将变为原来的四分之一 (1/2²)；如果距离增加两倍，力将降至九分之一 (1/3²)。

Similarly, doubling the mass of one object doubles the force. This inverse‑square relationship explains why gravity gets dramatically weaker as you move away from Earth.

同理，将其中一个物体的质量加倍，力也会加倍。这种平方反比关系解释了为什么离开地球越远，引力会急剧变弱。

6. Gravity and Orbits: Planets Around the Sun | 引力与轨道：行星绕太阳运动

The planets in our Solar System follow nearly circular orbits around the Sun. This orbital motion is caused by the Sun’s gravitational pull. Without gravity, the planets would simply fly off into space in a straight line. Instead, gravity provides the centripetal force needed to keep each planet in its path.

我们太阳系中的行星沿着近乎圆形的轨道围绕太阳运行。这种轨道运动是由太阳的引力作用引起的。如果没有引力，行星将沿直线飞离太空。相反，引力提供了使每颗行星保持在轨道上所需的向心力。

You do not need to calculate centripetal force in detail for IGCSE, but you should understand that a steady inward force is required for circular motion. Here, that inward force is gravity, constantly pulling the planet toward the Sun.

在 IGCSE 中，你不需要详细计算向心力，但应理解圆周运动需要一个稳定的向内力。在这里，这个向内力就是引力，它持续将行星拉向太阳。

The orbital speed of a planet depends on its distance from the Sun. Planets closer to the Sun, like Mercury, experience a stronger gravitational pull and must move faster to stay in orbit. Planets farther away, like Neptune, orbit more slowly.

行星的轨道速度取决于它与太阳的距离。距离太阳较近的行星，如水星，受到的引力更强，必须以更快的速度运行才能保持在轨道上。距离较远的行星，如海王星，轨道速度较慢。

7. Satellites and Orbital Motion | 卫星与轨道运动

A satellite is any object that orbits a planet. The Moon is a natural satellite of Earth, while thousands of artificial satellites have been launched for communication, weather monitoring, navigation and scientific research. All satellites stay in orbit because of the planet’s gravity acting as the centripetal force.

卫星是任何绕行星运行的天体。月球是地球的天然卫星，而人类已发射了数千颗人造卫星，用于通信、气象监测、导航和科学研究。所有卫星之所以能留在轨道上，是因为行星的引力充当了向心力。

To launch a satellite into a stable orbit, it must be given a high enough horizontal speed. If the speed is too low, the satellite will fall back to Earth. The minimum speed required for a stable low Earth orbit is roughly 8 km/s. At this speed, the curvature of the satellite’s descent matches the curve of the Earth, so it keeps ‘falling around’ the planet without hitting the surface.

要将卫星送入稳定轨道，必须赋予它足够高的水平速度。如果速度太低，卫星将坠回地球。稳定近地轨道所需的最低速度约为 8 km/s。在这个速度下，卫星下落的弧度与地球的曲率匹配，因此它一直绕着地球“下落”，而不会撞击地面。

Geostationary satellites orbit at a special altitude of about 36 000 km above the equator, taking exactly 24 hours to complete one orbit. Because they appear stationary relative to the ground, they are ideal for TV broadcasting and global communications.

地球静止轨道卫星运行在赤道上方约 36 000 km 的特定高度，恰好用 24 小时完成一周运行。由于它们相对于地面看起来静止不动，因此是电视广播和全球通信的理想选择。

8. Gravitational Field Strength Beyond Earth | 地球以外的引力场强度

The value of g is not constant, even on Earth. It decreases slightly with altitude. If you climb to the top of a tall mountain, you are farther from the Earth’s centre, so g becomes a tiny bit smaller. In most IGCSE calculations, however, 9.8 N/kg is used as an average near the surface.

g 的值并非恒定，即使在地球上也是如此。它会随着高度略有下降。如果你爬到一座高山的山顶，你离地心更远了，因此 g 会稍微变小一点。不过在大多数 IGCSE 计算中，9.8 N/kg 被用作接近地表处的平均值。

On other planets and moons, g varies considerably. The table below shows approximate values of g for some celestial bodies. These values help compare the weight of the same object on different worlds.

在其他行星和卫星上，g 值差异很大。下表列出了一些天体的近似 g 值。这些数值有助于比较同一物体在不同世界上的重量。

Celestial Body	g (N/kg)
Earth	9.8
Moon	1.6
Mars	3.7
Jupiter	24.7

Notice how a person with a mass of 60 kg would weigh only 96 N on the Moon, whereas on Jupiter they would weigh a staggering 1482 N. Gravity shapes each planet’s atmosphere, surface features and potential for life.

注意，一个质量为 60 kg 的人在月球上仅重 96 N，而在木星上将重达惊人的 1482 N。引力塑造了每颗行星的大气层、表面特征以及生命存在的可能性。

9. Weightlessness and Free Fall | 失重与自由落体

Astronauts in the International Space Station appear to float because they are in a state of continuous free fall. The gravity in low Earth orbit is still nearly 90% of that on the surface, but the astronaut and the spacecraft are falling together at the same rate, creating the sensation of weightlessness.

国际空间站中的宇航员看似漂浮，是因为他们处于持续自由落体状态。近地轨道的引力仍有地表引力的近 90%，但宇航员和航天器以相同的速率一起下落，造成了失重感。

When you stand on a weighing scale, the scale measures the normal contact force pushing up on you, which equals your weight when you are in equilibrium. During free fall, there is no contact force, so a scale would read zero. This is a key point to distinguish weight (the gravitational pull) from apparent weight.

当你站在体重秤上时，秤测量的是向上推你的法向接触力，在平衡状态下等于你的重量。在自由落体过程中，没有接触力，因此体重秤读数将为零。这是区分重量（引力）和视重的关键点。

10. Applications of Understanding Gravity | 理解引力的应用

Gravity not only governs the motion of planets and satellites but also has everyday applications. For example, understanding weight and g helps engineers design safe buildings that can support their own weight and resist gravitational forces. Parachute design relies on balancing weight against air resistance.

引力不仅支配着行星和卫星的运动，还具有日常应用。例如，理解重量和 g 有助于工程师设计安全的建筑，使其能够支撑自身重量并抵抗引力。降落伞的设计则依赖于平衡重量和空气阻力。

Space missions use gravity assists, a technique where a spacecraft flies past a planet to gain speed using the planet’s gravity, saving fuel. Our knowledge of gravity also enables accurate GPS systems, which depend on satellites and careful calculations of gravitational effects on time.

太空任务使用引力助推技术，即航天器飞越行星，利用行星引力获得速度，从而节省燃料。我们对引力的认识还实现了精确的 GPS 系统，这依赖于卫星以及引力对时间影响的精细计算。

On a smaller scale, gravity ensures that hot air rises and cool air sinks, driving weather patterns and ocean currents that are vital for life on Earth.

在较小的尺度上，引力确保热空气上升、冷空气下沉，从而驱动对地球生命至关重要的天气模式和洋流。

Published by TutorHao | Physics Revision Series | aleveler.com

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Gravity: IGCSE AQA Physics Revision Notes | 万有引力：IGCSE AQA物理考点精讲