Forces are everywhere | 力无处不在

📚 Forces are everywhere | 力无处不在

From a falling leaf to a rocket launch, forces are acting everywhere. In KS3 science, understanding forces helps explain how objects move, stop, change direction, and change shape. This article covers the essential ideas about forces for CIE KS3 learners, with clear definitions, examples, and simple equations.

从飘落的树叶到火箭发射，力无处不在地起作用。在 KS3 科学课程中，理解力有助于解释物体如何运动、停止、改变方向以及发生形变。本文涵盖 CIE KS3 学习者需要掌握的力的核心概念，包括清晰的定义、实例和简单公式。

1. What is a force? | 什么是力？

A force is a push or a pull acting on an object. Forces cannot be seen directly, but their effects can be observed when objects start moving, stop, speed up, slow down, change direction, or change shape.

力是作用在物体上的推或拉。力本身无法直接看到，但当物体开始运动、停止、加速、减速、改变方向或发生形变时，我们可以观察到力的作用效果。

Forces are measured in newtons (N), named after Sir Isaac Newton. A force has both size and direction, so the direction of a force is just as important as its magnitude.

力的单位是牛顿 (N)，以艾萨克·牛顿爵士的名字命名。力既有大小又有方向，因此力的方向与它的大小同样重要。

For example, pushing a door with the same force in the middle or near the hinge can produce different turning effects, showing that where a force acts is also important.

例如，用同样大小的力在门中部或靠近铰链处推门会产生不同的转动效果，这说明力的作用点也很重要。

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2. Types of forces | 力的类型

There are many types of forces in everyday life. They can be grouped into two main families: contact forces and non-contact forces.

日常生活中有很多类型的力。它们可以分为两大类：接触力和非接触力。

• Contact forces: friction, tension, normal reaction, air resistance, upthrust
• Non-contact forces: gravity, magnetic force, electrostatic force

接触力包括摩擦力、张力、法向反作用力、空气阻力、浮力；非接触力包括重力、磁力和静电力。

Knowing the type of force helps explain how objects interact with each other in different situations, from a book resting on a table to a magnet attracting a paper clip.

了解力的类型有助于解释物体在不同情况下如何相互作用，例如放在桌上的书或磁铁吸引回形针。

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3. Contact and non-contact forces | 接触力与非接触力

Contact forces only act when two objects are touching. For example, friction acts between your shoe and the ground, and tension acts in a rope when you pull it.

接触力只有在两个物体相互接触时才会产生。例如，摩擦力作用在你的鞋与地面之间，当你拉绳子时绳中会产生张力。

Non-contact forces act at a distance without any touching. Gravity pulls objects downward even when they are in the air, and magnets can attract or repel each other across a gap.

非接触力可以在不接触的情况下远距离作用。重力把物体向下拉即使物体在空中，磁铁可以隔着一段距离相互吸引或排斥。

Electrostatic force is another non-contact force. A charged balloon can attract small pieces of paper without touching them.

静电力是另一种非接触力。带电的气球可以在不接触的情况下吸引小纸片。

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4. Effects of forces | 力的作用效果

Forces can make a stationary object start moving. A footballer kicking a ball applies a force that moves the ball from rest.

力可以使静止的物体开始运动。足球运动员踢球时施加力使球从静止开始运动。

Forces can make a moving object stop or change direction. Braking a bicycle uses friction to slow down and stop, while a tennis racket changes the direction of a moving ball.

力可以使运动中的物体停下或改变方向。骑自行车刹车时利用摩擦力减速停下，而网球拍可以改变运动球的方向。

Forces can also change the shape of an object. Stretching a rubber band, squashing a can, or bending a plastic ruler are all examples of forces changing shape.

力还可以改变物体的形状。拉伸橡皮筋、压扁易拉罐或弯曲塑料尺都是力改变形状的例子。

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5. Measuring force | 测量力

A newton meter, also called a spring balance, is used to measure force. It contains a spring that stretches when a force is applied, and the scale shows the force in newtons (N).

牛顿计，也叫弹簧测力计，用来测量力。它内部有一个弹簧，当受到力时会拉伸，刻度显示以牛顿 (N) 为单位的力。

To measure weight, you hang the object from the newton meter and read the scale. The reading gives the gravitational force pulling the object down.

要测量重量，把物体挂在牛顿计上并读取刻度。读数表示把物体向下拉的重力。

For a rough estimation, a small apple weighs about 1 N on Earth. This helps you imagine how large a newton is in real life.

粗略估计，一个小的苹果在地球上重约 1 牛。这有助于想象 1 牛在现实生活中有多大。

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6. Balanced and unbalanced forces | 平衡力与不平衡力

When forces on an object are balanced, they cancel each other out. The object stays at rest or moves at a constant velocity in a straight line.

当作用在物体上的力平衡时，它们相互抵消。物体会保持静止或沿直线匀速运动。

When forces are unbalanced, there is a resultant force. The object accelerates, decelerates, or changes direction.

当力不平衡时，就存在合力。物体会加速、减速或改变方向。

A book resting on a table has balanced forces: its weight pulls downward, while the normal reaction from the table pushes upward with the same size force.

放在桌上的书受到平衡力：重力向下拉，桌子的法向反作用力以相同大小向上推。

The resultant force is the overall force left after combining all forces. If it is zero, the motion does not change.

合力是合并所有力后剩下的总力。如果合力为零，运动状态就不会改变。

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7. Friction | 摩擦力

Friction is a contact force that opposes motion between two surfaces. It acts in the opposite direction to the movement.

摩擦力是阻碍两个表面之间相对运动的接触力。它的方向与运动方向相反。

Friction can be useful. It helps us walk without slipping and allows car tyres to grip the road.

摩擦力很有用。它帮助我们走路不打滑，并让汽车轮胎抓地。

Friction can also be unwanted. It wears down engine parts and reduces efficiency by generating heat.

摩擦力也有不利的一面。它磨损发动机部件，并通过产生热量降低效率。

Lubricants such as oil reduce friction between moving parts, while rough surfaces increase friction. This is why winter shoes often have deeper treads.

润滑油等润滑剂可以减小运动部件之间的摩擦，而粗糙表面会增大摩擦。这就是为什么冬靴通常有更深的纹路。

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8. Gravity and weight | 重力与重量

Gravity is a non-contact force that pulls objects towards the centre of the Earth. It acts on all objects with mass.

重力是一种把物体拉向地球中心的非接触力。它作用于所有有质量的物体。

Weight is the force of gravity acting on an object’s mass. Weight is measured in newtons (N), while mass is measured in kilograms (kg).

重量是作用在物体质量上的重力。重量以牛顿 (N) 为单位，质量以千克 (kg) 为单位。

Weight can be calculated using the equation below, where W is weight in newtons (N), m is mass in kilograms (kg), and g is the gravitational field strength (about 10 N/kg on Earth):

W = m × g

重量可用以下公式计算：W = m × g，其中 W 是重量（N），m 是质量（kg），g 是重力场强度（地球上约为 10 N/kg）。

Mass does not change from place to place, but weight can change because gravity is different on the Moon or other planets.

质量不会随地点改变，但重量会改变，因为月球或其他行星上的重力不同。

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9. Air resistance and drag | 空气阻力与拖曳力

Air resistance is

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