📚 KS3 CIE Science: Understanding Gravity | KS3 CIE 科学：理解重力
Gravity is one of the most familiar forces in the universe. It pulls objects toward the ground, keeps the Moon orbiting Earth, and holds the planets in orbit around the Sun. In KS3 CIE Science, you need to understand what gravity is, how it relates to mass and weight, and how it affects falling objects and motion in space.
重力是宇宙中最常见的力之一。它把物体拉向地面，让月球绕地球运行，也使行星绕太阳运转。在 KS3 CIE 科学课程中，你需要理解什么是重力、它与质量和重量有什么关系，以及它如何影响下落物体和太空中的运动。
1. What Is Gravity? | 什么是重力？
Gravity is a force of attraction that pulls objects with mass toward each other. On Earth, gravity pulls every object toward the centre of the planet. This force is why a dropped pencil falls to the floor and why your body stays on the ground instead of floating away.
重力是一种吸引力，把有质量的物体彼此拉近。在地球上，重力把每个物体拉向地球的中心。这就是为什么一支掉落的铅笔会落到地板上，也是为什么你的身体能留在地面而不会飘走。
Gravity acts everywhere in the universe, not just on Earth. Any object that has mass produces a gravitational pull. However, you only notice the pull from very large objects such as Earth, the Moon, or the Sun because their masses are enormous.
重力在宇宙中无处不在，并不仅限于地球。任何有质量的物体都会产生引力。然而，你只能注意到地球、月球或太阳等非常大质量物体的引力，因为它们的质量巨大。
On Earth, the direction of gravity is always downwards, toward the centre of the planet. When you stand on the ground, the ground pushes back on your feet, balancing the downward pull of gravity.
在地球上，重力的方向总是向下，指向地球中心。当你站在地面上时，地面对你的脚施加一个向上的支持力，与向下的重力相平衡。
2. Gravity as a Non-Contact Force | 作为非接触力的重力
Gravity is a non-contact force. This means it can act between objects even when they are not touching. A contact force, such as friction or air resistance, requires surfaces to touch. Gravity can pull an apple from a tree across empty space without any direct contact.
重力是一种非接触力。这意味着它可以在物体不接触的情况下产生作用。像摩擦力或空气阻力这样的接触力需要物体表面相互接触。重力则可以在隔着空无一物的空间的情况下，把苹果从树上拉下来，而无需直接接触。
Non-contact forces also include magnetism and electrostatic force. In the same way, gravity acts through a gravitational field. Around Earth, there is a region called the gravitational field where Earth's gravitational pull can be felt.
非接触力还包括磁力和静电力。同样，重力通过引力场产生作用。在地球周围存在一个叫做引力场的区域，在这个区域内可以感受到地球的引力。
The arrows used to represent gravity point toward the centre of the attracting object. On a force diagram for a falling apple, the weight arrow is drawn straight downwards.
用来表示重力的箭头指向吸引物体的中心。在下落的苹果的受力图中，重量的箭头竖直向下。
3. Mass and Weight: Two Different Ideas | 质量与重量：两个不同的概念
In everyday language, people often use 'mass' and 'weight' as if they are the same. In science they are different. Mass is the amount of matter in an object. It is measured in kilograms (kg) and does not change with location. Weight is the force of gravity acting on that mass. It is measured in newtons (N) and can change if the gravitational field changes.
在日常生活中，人们常把 "质量" 和 "重量" 当作一回事。但在科学中它们是不同的。质量是物体所含物质的多少，单位是千克（kg），不随位置改变。重量是作用在质量上的重力，单位是牛顿（N），如果重力场变化，重量也会变化。
A 1 kg bag of sugar has the same mass on Earth, on the Moon, and in space. However, its weight is smaller on the Moon and almost zero far away from any planet because the gravitational pull is weaker.
一袋 1 千克的糖在地球、月球和太空中都有相同的质量。但在月球上它的重量更小，在远离任何行星的地方重量几乎为零，因为那里的引力更弱。
Mass is a scalar quantity: it only has size. Weight is a vector quantity: it has both size and direction, and its direction is always the direction of the gravitational pull.
质量是标量：它只有大小。重量是矢量：它既有大小又有方向，方向始终与重力的方向一致。
4. The Weight Equation | 重量公式
The relationship between mass and weight is given by the equation:
质量与重量之间的关系由以下公式给出：
W = m × g
Here, W stands for weight in newtons (N), m stands for mass in kilograms (kg), and g stands for the gravitational field strength in newtons per kilogram (N/kg).
其中，W 代表重量，单位为牛顿（N）；m 代表质量，单位为千克（kg）；g 代表重力场强度，单位为牛顿每千克（N/kg）。
On Earth, the gravitational field strength g is approximately 9.8 N/kg. For KS3 calculations, your teacher may ask you to use 10 N/kg to make the numbers easier.
在地球上，重力场强度 g 约为 9.8 N/kg。在 KS3 的计算中，老师可能会要求你使用 10 N/kg，以便简化计算。
For example, the weight of a 2 kg object on Earth is:
例如，一个质量为 2 kg 的物体在地球上的重量为：
W = 2 kg × 10 N/kg = 20 N
Always check that your mass is in kilograms and your final answer is in newtons.
一定要检查质量是否以千克为单位，最终答案是否以牛顿为单位。
5. Measuring Weight and Mass | 测量重量与质量
Weight and mass are measured with different instruments. Mass is measured with a balance, such as a top-pan balance, and is recorded in kilograms or grams.
重量和质量用不同的仪器测量。质量用天平（如托盘天平）测量，单位记录为千克或克。
Weight is measured with a force meter, also called a newton meter or spring balance. The object is hung from the hook, and the scale reads the force in newtons.
重量用测力计来测量，也叫牛顿计或弹簧秤。把物体挂在挂钩上，刻度盘就会以牛顿为单位显示力的大小。
- Balance measures mass (kg). | 天平测量质量（kg）。
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