📚 Understanding Gravity: Key Questions for KS3 CIE Science | 理解重力：KS3 CIE 科学核心问题
Gravity is one of the most important forces in the universe. For KS3 CIE Science, you need to explain what gravity is, how it affects mass and weight, and how to answer common questions about falling objects, orbits and gravitational fields. This revision guide covers the key ideas and exam-style questions you are likely to meet.
重力是宇宙中最重要的力之一。在 KS3 CIE 科学课程中，你需要解释什么是重力、它如何影响质量与重量，以及如何回答关于落体、轨道和引力场的常见问题。这份复习指南涵盖了你可能遇到的核心概念和考题类型。
1. What is gravity? | 什么是重力？
Gravity is a non-contact force that pulls objects with mass towards each other. You do not need to touch an object for gravity to act. The more mass an object has, the stronger its gravitational pull. On Earth, we feel gravity as the force that pulls us down towards the ground.
重力是一种非接触力，它将有质量的物体相互拉近。重力不需要接触物体就能产生作用。物体的质量越大，它的引力就越强。在地球上，我们感受到的重力就是把我们拉向地面的力。
Gravity always attracts; it never repels. This is different from magnetic and electric forces, which can both attract and repel. Gravity is also very weak compared with other forces, but it acts over very large distances and holds planets, stars and galaxies together.
重力总是吸引，从不排斥。这与磁力和电力不同，磁力和电力既能吸引也能排斥。与其他力相比，重力非常弱，但它可以在非常大的距离上起作用，并把行星、恒星和星系维系在一起。
2. Why do objects fall towards the Earth? | 为什么物体会落向地球？
Objects fall towards the Earth because the Earth's mass is extremely large. The Earth creates a gravitational field around it, and any object inside that field experiences a force pulling it towards the centre of the Earth. We call this force the object's weight.
物体落向地球是因为地球的质量非常大。地球在它周围产生一个引力场，处在该场中的任何物体都会受到一个拉向地球中心的力。我们把这个力称为物体的重量。
When you drop a ball, gravity pulls it downwards. If there were no air resistance, all objects would fall with the same acceleration, no matter how heavy they are. This idea was famously demonstrated by Galileo and later tested on the Moon during the Apollo 15 mission.
当你松开一个球时，重力会把它向下拉。如果没有空气阻力，所有物体都会以相同的加速度下落，无论它们有多重。这一观点由伽利略著名地提出，后来在阿波罗 15 号任务中在月球上得到了验证。
3. What is the difference between mass and weight? | 质量与重量有什么区别？
Mass is the amount of matter in an object. It is measured in kilograms (kg) and does not change wherever you go in the universe. You could be on Earth, the Moon or floating in space and your mass would stay the same.
质量是物体所含物质的多少。它的单位是千克（kg），无论你在宇宙中的哪个地方，质量都不会改变。无论你在地球、月球还是漂浮在太空中，你的质量都保持不变。
Weight is the force of gravity acting on an object's mass. Because weight is a force, it is measured in newtons (N). Your weight can change if you move to a place where gravity is stronger or weaker. You weigh less on the Moon because the Moon's gravity is weaker than Earth's.
重量是作用在物体质量上的重力。因为重量是一种力，所以它的单位是牛顿（N）。如果你去一个重力更强或更弱的地方，你的重量会改变。你在月球上体重更轻，因为月球的重力比地球弱。
4. How do we calculate weight on Earth? | 如何计算地球上的重量？
You can calculate weight using the equation:
你可以用以下公式计算重量：
W = m × g
In this equation, W is weight in newtons (N), m is mass in kilograms (kg), and g is the gravitational field strength. On Earth, g is about 10 N/kg at KS3 level (more precisely 9.8 N/kg). This means every 1 kg of mass experiences a gravitational force of about 10 N.
在这个公式中，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。在 KS3 阶段，地球上 g 约为 10 N/kg（更精确为 9.8 N/kg）。这意味着每 1 kg 的质量受到大约 10 N 的重力。
weight on Earth is W = 45 × 10 = 450 N. Always show the unit N, because weight is a force.
例题：一名学生的质量为 45 kg。他在地球上的重量为 W = 45 × 10 = 450 N。一定要写出单位 N，因为重量是力。
5. Why is your weight different on the Moon? | 为什么在月球上你的重量不同？
The Moon has a smaller mass than the Earth, so its gravitational field strength is weaker. On the Moon, g is about 1.6 N/kg. This is roughly one sixth of Earth's gravity. Your mass stays the same, but your weight is much smaller.
月球的质量比地球小，所以它的引力场强度较弱。在月球上，g 约为 1.6 N/kg。这大约是地球重力的六分之一。你的质量保持不变，但你的重量会小得多。
hat is why astronauts can jump much higher on the Moon even when wearing heavy spacesuits.
同样是 45 kg 的学生，在月球上的重量为 W = 45 × 1.6 = 72 N。这就是为什么宇航员即使穿着沉重的宇航服，在月球上也能跳得更高的原因。
6. What is a gravitational field? | 什么是引力场？
A gravitational field is the region around a mass where another mass experiences a gravitational force. The Earth's gravitational field pulls objects towards the centre of the Earth. We cannot see the field, but we can observe its effects, such as falling objects and orbiting satellites.
引力场是质量周围使另一个质量受到引力作用的区域。地球的引力场将物体拉向地心。我们看不见这个场，但可以观察到它的效果，比如下落的物体和绕轨道运行的卫星。
Field strength is measured in newtons per kilogram (N/kg). The closer you are to a planet, the stronger its gravitational field. As you move further away from Earth, gravity becomes weaker, but it never completely disappears.
场强的单位是牛顿每千克（N/kg）。你离行星越近，它的引力场就越强。当你离地球越来越远时，重力会变弱，但永远不会完全消失。
7. How does gravity keep the Moon in orbit? | 重力如何让月球保持在轨道上？
The Moon is constantly falling towards the Earth because of gravity. However, the Moon also has a sideways motion. The combination of this forward motion and the pull of gravity makes the Moon follow a curved path around the Earth. We call this an orbit.
由于重力，月球始终在向地球“下落”。然而，月球同时也有侧向运动。这种向前运动与重力拉力的结合，使月球沿着一条绕地球的弯曲路径运行。我们称之为轨道。
If gravity suddenly disappeared, the Moon would fly off in a straight line into space. Gravity provides the centripetal force needed to keep objects in circular motion.
如果重力突然消失，月球将沿直线飞入太空。重力提供了物体做圆周运动所需的向心力。
8. What is free fall and air resistance? | 什么是自由落体和空气阻力？
Free fall happens when gravity is the only force acting on an object. In a vacuum, a feather and a hammer fall at the same rate because there is no air resistance. This was demonstrated on the Moon by Apollo 15 astronaut David Scott.
当重力是作用在物体上的唯一力时，就会发生自由落体。在真空中，羽毛和锤子以相同的速率下落，因为没有空气阻力。阿波罗 15 号宇航员大卫·斯科特在月球上证明了这一点。
On Earth, air resistance acts against falling objects. It increases as the object moves faster. When air resistance becomes equal to the object's weight, the object stops accelerating and falls at a constant speed called terminal velocity.
在地球上，空气阻力会阻碍下落物体。物体运动得越快，空气阻力越大。当空气阻力等于物体的重量时，物体不再加速，并以恒定速度下落，这个速度称为终端速度。
9. Why do astronauts feel weightless? | 为什么宇航员会感到失重？
Astronauts on the International Space Station are not beyond Earth's gravity. In fact, Earth's gravity at the ISS is still about 90% of that on the ground. They feel weightless because they are in free fall around the Earth.
国际空间站上的宇航员并不是不受地球重力影响。事实上，在地球空间站高度，地球重力仍约为地面的 90%。他们感到失重是因为他们正在绕地球自由落体。
Both the astronauts and the space station are falling towards Earth at the same rate, so they do not push against the floor. This creates the sensation of weightlessness, just like when a lift suddenly drops.
宇航员和空间站以相同的速率向地球下落，因此他们不会压在地板上。这就产生了失重的感觉，就像电梯突然下降时一样。
10. Common exam-style questions on gravity | 重力常见考题类型
predict what happens to weight on different planets. You should also be able to describe how gravity affects falling objects and orbits.
KS3 CIE 考试经常要求你解释质量与重量的区别、使用 W = m × g 计算重量，或预测在不同行星上重量会如何变化。你还应该能够描述重力如何影响下落物体和轨道。
Typical questions include: 'State the unit of weight', 'Why does a hammer and a feather fall at the same rate on the Moon?', and 'Calculate the weight of a 5 kg object on Earth'. Always use the correct units and show your working.
常见题目包括：“写出重量的单位”、“为什么锤子和羽毛在月球上以相同速率下落？”以及“计算地球上 5 kg 物体的重量”。一定要使用正确的单位并写出计算过程。
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