📚 Forces are Everywhere | 力无处不在
Forces are pushes or pulls that can change the motion, shape or direction of an object. In KS3 CIE science, the topic 'Forces are Everywhere' tests your ability to identify forces, draw force diagrams, calculate resultant forces and explain everyday effects such as friction and gravity. This article follows common CIE mark scheme points so you know exactly what examiners expect.
力是推或拉，能改变物体的运动、形状或方向。在 KS3 CIE 科学中，“力无处不在”这一主题考查你识别力、画力的示意图、计算合力以及解释摩擦和重力等日常现象的能力。本文按照 CIE 常见评分方案要点编写，让你明确考官期望什么。
1. What is a Force? | 什么是力？
A force is a push or a pull acting on an object. It is a vector quantity, meaning it has both magnitude (size) and direction. The SI unit of force is the newton (N), named after Sir Isaac Newton. Forces can make objects start moving, stop moving, speed up, slow down, change direction, or change shape.
力是作用在物体上的推或拉。它是矢量，既有大小又有方向。力的国际单位是牛顿 (N)，以艾萨克·牛顿爵士命名。力可以使物体开始运动、停止、加速、减速、改变方向或改变形状。
- State clearly: 'A force is a push or a pull' — 清楚表述：“力是推或拉” (1分)
- Give the unit as newton (N) — 单位写为牛顿 (N) (1分)
- Mention that forces have both size and direction — 提到力既有大小又有方向 (1分)
2. Contact and Non-contact Forces | 接触力与非接触力
Forces can be divided into contact forces and non-contact forces. Contact forces require the objects to touch, such as friction, tension, air resistance and normal contact force. Non-contact forces act over a distance without touching, such as gravitational force, magnetic force and electrostatic force.
力可分为接触力和非接触力。接触力需要物体相互接触，例如摩擦力、张力、空气阻力和法向接触力。非接触力不接触也能作用，例如重力、磁力和静电力。
| Contact forces 接触力 | Non-contact forces 非接触力 |
|---|---|
| Friction 摩擦力 | Gravity 重力 |
| Air resistance 空气阻力 | Magnetic force 磁力 |
| Tension 张力 | Electrostatic force 静电力 |
| Normal contact force 法向接触力 | Nuclear force 核力 |
3. Representing Forces | 表示力
A force is represented by an arrow. The length of the arrow shows the magnitude (size) of the force, and the direction of the arrow shows the direction in which the force acts. The arrow should start from the object or the point of contact. Forces are usually labelled with their name or symbol, for example 'F' for force, 'W' for weight, or 'f' for friction.
力用箭号表示。箭号长度表示力的大小，箭号方向表示力的作用方向。箭号应从物体或接触点画出。力通常用名称或符号标注，例如 “F” 表示力，“W” 表示重量，“f” 表示摩擦力。
In CIE mark schemes, drawing a correct force diagram often earns marks for: arrow drawn in the correct direction, arrow length proportional to force size, and correct labelling of each force.
在 CIE 评分方案中，正确画力的示意图通常得分点包括：箭号方向正确、箭号长度与力的大小成比例、以及每个力的标注正确。
4. Measuring Forces | 测量力
A newton meter (also called a spring balance) is used to measure forces. It contains a spring that stretches when a force is applied. The scale is marked in newtons. Before measuring, check that the reading is zero when no force is applied. In CIE practical questions, you may be asked to read the scale accurately or describe how to reduce errors.
牛顿计（也叫弹簧秤）用于测量力。它内部有一个弹簧，施加力时会伸长。刻度以牛顿为单位。测量前要检查没有施加力时读数是否为零。在 CIE 实验题中，可能要求你准确读取刻度或描述如何减少误差。
- Read the scale at eye level to avoid parallax error — 视线与刻度水平以避免视差 (1分)
- Ensure the meter reads zero before use — 使用前确保读数为零 (1分)
- Record readings in newtons (N) — 记录读数单位为牛顿 (N) (1分)
5. Balanced and Unbalanced Forces | 平衡力与非平衡力
When forces on an object are balanced, they cancel each other out. The resultant force is zero. A stationary object stays still, and a moving object continues at constant speed in a straight line. When forces are unbalanced, there is a resultant force in one direction. This causes the object to accelerate, decelerate or change direction.
当物体上的力平衡时，它们相互抵消，合力为零。静止的物体保持静止，运动的物体保持匀速直线运动。当力不平衡时，某一方向存在合力，这会使物体加速、减速或改变方向。
Resultant force = larger force − smaller force (when forces act in opposite directions)
合力 = 较大的力 − 较小的力（当力方向相反时）
rward, while friction is 300 N backward. The resultant force is 500 − 300 = 200 N forward.
例子：汽车发动机提供向前 500 N 的力，而摩擦力为向后 300 N。合力为 500 − 300 = 200 N 向前。
6. Friction | 摩擦力
Friction is a contact force that opposes motion between two surfaces in contact. It always acts in the opposite direction to the motion (or attempted motion) of the object. Friction can be useful or unwanted. Useful friction allows us to walk, hold objects and stop cars. Unwanted friction wastes energy and causes wear.
摩擦力是一种阻碍相互接触的两个表面之间运动的接触力。它总是与物体运动（或试图运动）的方向相反。摩擦可以是有益的，也可以是有害的。有益的摩擦使我们能走路、握住物品和停住汽车。有害的摩擦浪费能量并造成磨损。
- Definition: 'friction opposes motion' — 定义：“摩擦阻碍运动” (1分)
- Direction: opposite to motion — 方向：与运动方向相反 (1分)
- Reduce friction: lubrication, streamlining, smoother surfaces — 减小摩擦：润滑、流线型、更光滑的表面 (1分)
- Increase friction: rough surfaces, more contact force — 增大摩擦：粗糙表面、更大的接触力 (1分)
7. Gravity and Weight | 重力与重量
entre of the Earth (or any other mass). Weight is the force of gravity acting on an object' weight = mass × gravitational field strength. On Earth, g is about 10 N/kg (or 9.8 N/kg).
重力是一种非接触力，将物体拉向地球（或任何其他质量）的中心。重量是作用在物体质量上的重力。重量以牛顿为单位，而质量是物质的多少，以千克为单位。关系为：重量 = 质量 × 重力场强度。在地球上，g 约为 10 N/kg（或 9.8 N/kg）。
W = m × g
rk schemes often award one mark for the correct formula and one mark for the correct unit.
例子：一个 2 kg 的物体重量 W = 2 × 10 = 20 N。CIE 评分方案通常公式正确得 1 分，单位正确得 1 分。
8. Air Resistance and Terminal Velocity | 空气阻力与终速
Air resistance (drag) is a contact force that opposes the motion of an object moving through air. The faster the object moves, the greater the air resistance. When a falling object's weight equals the air resistance, the forces balance and the object falls at constant speed called terminal velocity.
空气阻力（阻力）是一种阻碍物体在空气中运动的接触力。物体运动越快，空气阻力越大。当下落物体的重量等于空气阻力时，力达到平衡，物体以恒定速度下落，这个速度称为终速。
In parachute questions, examiners expect you to state that opening the parachute increases air resistance, so the upward force becomes larger than the weight, causing the parachutist to decelerate until forces balance again.
在降落伞题目中，考官希望你说明打开降落伞会增大空气阻力，因此向上的力大于重量，使跳伞者减速直到力再次平衡。
9. Forces in Everyday Life | 日常生活中的力
Forces explain many everyday situations. Walking requires friction between shoes and the ground. Car brakes use friction to slow down. A stretched spring exerts an elastic force. A floating object experiences upthrust that balances its weight. Magnets attract or repel without touching.
力可以解释许多日常现象。走路需要鞋子与地面之间的摩擦。汽车刹车利用摩擦减速。拉伸的弹簧产生弹力。漂浮的物体受到与重量平衡的浮力。磁铁不接触就能吸引或排斥。
- Walking: friction between shoes and ground — 走路：鞋与地面之间的摩擦
- Braking: friction between brake pads and wheel — 刹车：刹车片与车轮之间的摩擦
- Floating: upthrust balanced by weight — 漂浮：浮力与重量平衡
- Stretched spring: elastic force — 拉伸的弹簧：弹力
10. Common CIE Mark Scheme Keywords | CIE 常见评分关键词
Using the right scientific words is essential for gaining marks. Below are keywords that frequently appear in CIE mark schemes for the 'Forces are Everywhere' topic. Always spell them correctly and use them in context.
使用正确的科学术语是得分的关键。以下是 CIE “力无处不在” 评分方案中经常出现的关键词。务必拼写正确并在上下文中使用。
| Keyword 关键词 | Meaning 含义 |
|---|---|
| Force 力 | A push or pull 推或拉 |
| Newton 牛顿 | Unit of force 力的单位 |
| Resultant force 合力 | The overall force acting on an object 作用在物体上的总力 |
| Balanced forces 平衡力 | Forces that cancel; zero resultant 相互抵消的力；合力为零 |
| Unbalanced forces 非平衡力 | Forces that cause acceleration 导致加速的力 |
| Friction 摩擦力 | Force opposing motion 阻碍运动的力 |
| Weight 重量 | Force of gravity on a mass 质量受到的重力 |
| Terminal velocity 终速 | Constant falling speed when forces balance 力平衡时的恒定下落速度 |
11. Exam Technique for Forces Questions | 力题的答题技巧
CIE forces questions often include diagrams, calculations and short explanations. Read the question carefully and underline command words such as 'state', 'describe', 'explain' and 'calculate'. Show all working for calculations and always include units in your final answer.
CIE 力类题目通常包括示意图、计算和简答。仔细读题，并在指令词如 “state”、“describe”、“explain”、“calculate” 下标出。计算题要展示所有步骤，最终答案务必包含单位。
- State: give a short answer without explanation — State：给出简短答案，无需解释
- Describe: say what happens, step by step — Describe：逐步说明发生了什么
- Explain: give a scientific reason using force concepts — Explain：使用力的概念给出科学原因
- Calculate: show formula, substitution and unit — Calculate：展示公式、代入和单位
12. Quick Revision Checklist | 快速复习清单
Before your test, check that you can complete every item in the table below. This checklist summarises the essential knowledge and skills from the 'Forces are Everywhere' topic.
考试前，检查你是否能完成下表中的每一项。这份清单总结了 “力无处不在” 主题的核心知识和技能。
| Check item 检查项目 | Can you do it? 你能做到吗？ |
|---|---|
| Define force and give its unit 定义力并写出单位 | Yes / No 是 / 否 |
| Name contact and non-contact forces 说出接触力和非接触力 | Yes / No 是 / 否 |
| Draw and label force arrows 画并标注力的箭号 | Yes / No 是 / 否 |
| Calculate resultant force 计算合力 | Yes / No 是 / 否 |
| Use W = m × g 使用 W = m × g | Yes / No 是 / 否 |
| Explain friction in everyday life 解释日常生活中的摩擦 | Yes / No 是 / 否 |
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