📚 KS3 CIE Forces: Year 8 Scheme of Work | KS3 CIE 八年级力教学方案
This Year 8 Forces topic introduces the key ideas of force, motion, friction, gravity and pressure. You will learn how to measure forces, how to combine them as vectors, and how they affect everyday objects from falling balls to stretching springs.
本八年级力专题介绍力、运动、摩擦力、重力和压力的核心概念。你将学习如何测量力、如何将它们作为矢量合成，以及它们如何影响从下落的球到被拉伸的弹簧等日常物体。
1. What is a Force? | 什么是力？
A force is a push or a pull acting on an object. Forces can change the shape of an object, change its speed, or change its direction of motion. The SI unit of force is the newton, written as N.
力是作用在物体上的推或拉。力可以改变物体的形状、改变其速度或改变其运动方向。力的国际单位是牛顿，写作 N。
Force is a vector quantity, meaning it has both size and direction. In force diagrams, arrows are drawn to represent forces; the length of the arrow shows the magnitude and the arrowhead shows the direction.
力是矢量，也就是说它既有大小又有方向。在力的示意图中，用箭头表示力；箭头的长度表示大小，箭头指向表示方向。
Examples of forces in action include kicking a ball, stretching a spring, and dropping a stone. In each case the force changes motion or shape.
力作用的例子包括踢球、拉长弹簧和丢下石头。每种情况下力都会改变运动或形状。
2. Types of Forces | 力的类型
Forces can be divided into contact forces and non-contact forces. Contact forces act when two objects are touching, such as friction, tension, normal reaction and air resistance.
力可分为接触力和非接触力。接触力在两个物体接触时作用，例如摩擦力、拉力、法向反作用力和空气阻力。
Non-contact forces act at a distance without touching, such as gravitational force, magnetic force and electrostatic force. Gravity pulls objects towards the centre of the Earth.
非接触力无需接触即可远距离作用，例如万有引力、磁力和静电力。重力把物体拉向地球中心。
When drawing force arrows, start from the object and point in the direction of the force. A contact force such as friction acts along the surface, while weight always acts downwards.
画力的箭头时，从物体出发并指向力的方向。摩擦力等接触力沿表面作用，而重力始终向下作用。
3. Measuring Forces | 测量力
A newton meter, also called a spring balance, is used to measure forces. You hang the object on the hook and read the scale in newtons. The spring inside stretches in proportion to the force applied.
牛顿计，也叫弹簧秤，用来测量力。把物体挂在挂钩上，读取以牛顿为单位的刻度。内部的弹簧按照所施加的力成比例地伸长。
fractions of a newton, while larger forces may use kilonewtons (kN), where 1 kN = 1000 N.
测量前，检查牛顿计在不受力时读数是否为零。较小的力可用零点几牛顿测量，较大的力可能用千牛（kN），1 kN = 1000 N。
In a practical, you might use a newton meter to measure the weight of different objects. The reading on the meter is the force needed to support the object against gravity.
在实验中，你可以用牛顿计测量不同物体的重量。秤上的读数就是支撑物体抵抗重力所需的力。
4. Balanced and Unbalanced Forces | 平衡力与不平衡力
When two forces acting on an object are equal in size and opposite in direction, they are balanced. The resultant force is zero, and the object remains at rest or continues moving at a constant speed in a straight line.
当作用在物体上的两个力大小相等、方向相反时，它们是平衡的。合力为零，物体保持静止或沿直线匀速运动。
If forces are unbalanced, there is a resultant force. This causes the object to accelerate, decelerate or change direction according to Newton's first law.
如果力不平衡，就存在合力。根据牛顿第一定律，这会使物体加速、减速或改变方向。
Think of a tug of war: if both teams pull with the same force, the rope does not move. If one team pulls harder, the forces become unbalanced and the rope moves towards that team.
想一想拔河：如果两队拉力相同，绳子不动。如果一队拉得更用力，力就不平衡，绳子会向那一队移动。
5. Resultant Force and Motion | 合力与运动
The resultant force is the single force that has the same effect as all the forces acting together. When forces act in the same direction, add them; when they act in opposite directions, subtract the smaller from the larger.
合力是单独一个力，其效果与所有力共同作用相同。当力沿同一方向作用时，将它们相加；当它们沿相反方向作用时，用较大力减去较小力。
N to the right. The direction of the resultant force is the direction of the larger force.
例如，如果 10 N 的力把箱子向右推，4 N 的力向左推，则合力为 10 N − 4 N = 6 N 向右。合力的方向与较大力的方向相同。
Free-body diagrams show only the forces acting on one object. They help you calculate the resultant force and predict whether the object will start moving, speed up, slow down or stay still.
受力图只显示作用在一个物体上的力。它们帮助你计算合力，并预测物体是开始运动、加速、减速还是保持静止。
6. Friction | 摩擦力
Friction is a contact force that opposes motion between two surfaces in contact. It arises because surfaces are not perfectly smooth at the microscopic level, with tiny bumps that interlock.
摩擦力是阻碍两个接触表面相对运动的接触力。它产生的原因是表面在微观尺度上并不完全光滑，存在相互咬合的微小凸起。
Friction can be useful: it helps shoes grip the ground, allows car tyres to stop safely, and enables a match to light when rubbed. Without friction, walking and driving would be almost impossible.
摩擦力可能是有用的：它帮助鞋子抓地，使汽车轮胎能安全停下，并使火柴摩擦点燃。没有摩擦力，行走和驾驶几乎不可能。
However, friction can also be harmful because it wears out moving parts and wastes energy as heat. Lubricants such as oil and grease reduce friction, while rough surfaces and greater contact force increase friction.
然而，摩擦力也可能有害，因为它会磨损运动部件并以热的形式浪费能量。油和润滑脂等润滑剂能减小摩擦，而粗糙表面和更大的接触力会增大摩擦。
7. Air Resistance and Terminal Velocity | 空气阻力与终极速度
Air resistance, also called drag, is a frictional force that acts against an object moving through air. It increases with the speed of the object and with the surface area facing the air.
空气阻力，也叫阻力，是一种阻碍物体在空气中运动的摩擦力。它随物体速度和迎风面积的增大而增大。
When an object falls, its weight pulls it downwards while air resistance pushes upwards. As speed increases, air resistance grows until it equals the weight. The forces become balanced, so the object stops accelerating and falls at a constant speed called terminal velocity.
当物体下落时，重力把它向下拉，而空气阻力
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