📚 Force and Motion | 力与运动
In this article, we explore the fundamental concepts of force and motion, which form the core of the physics component in the Cambridge Primary Science Stage 9 curriculum. You will learn how forces affect the movement of objects, how to measure force, and how to interpret motion using graphs and equations. This knowledge is essential for tackling exam questions in the CPS_S9_WB_2P workbook and beyond.
在本文中，我们将深入探索力与运动的基本概念，这是剑桥小学科学阶段9课程中物理部分的核心内容。你将学习力如何影响物体的运动、如何测量力，以及如何运用图像和方程来解读运动。这些知识对于解答 CPS_S9_WB_2P 练习册中的考题至关重要，也是进一步学习物理的基础。
1. What is a Force? | 什么是力？
A force is a push or a pull that acts on an object. Forces can cause an object to speed up, slow down, change direction, or change shape. Force is a vector quantity, which means it has both magnitude (size) and direction. In the International System of Units, force is measured in newtons (N), named after the famous physicist Sir Isaac Newton.
力是对物体的推或拉。力可以使物体加速、减速、改变方向或改变形状。力是矢量，意味着它既有大小又有方向。在国际单位制中，力的单位是牛顿（N），以著名物理学家艾萨克·牛顿爵士的名字命名。
- Pushing a door open requires a force in the direction of the push. 推开门需要沿推力方向施加力。
- Pulling a cart requires a force in the direction of the pull. 拉车需要沿拉力方向施加力。
- Gravity pulls all objects toward the center of the Earth. 重力将一切物体拉向地球中心。
When multiple forces act on an object, they combine to produce a resultant force. If the forces act in the same direction, they add together; if they act in opposite directions, they subtract. When the resultant force is zero, the object is said to be in equilibrium and remains stationary or moves at a constant speed.
当多个力作用于同一物体时，它们会合成为一个合力。如果力的方向相同，则相加；如果方向相反，则相减。当合力为零时，物体处于平衡状态，保持静止或匀速直线运动。
Resultant Force = Force₁ + Force₂ (same direction)
合力 = 力₁ + 力₂（同方向）
Resultant Force = Force₁ − Force₂ (opposite direction)
合力 = 力₁ − 力₂（反方向）
2. Types of Forces | 力的类型
Forces can be classified into two main categories: contact forces and non-contact forces. Contact forces require physical contact between two objects, while non-contact forces act at a distance without any direct contact. Understanding these categories helps us explain a wide range of physical phenomena in daily life and in the laboratory.
力可以分为两大类：接触力和非接触力。接触力需要两个物体之间发生物理接触，而非接触力则可以在没有直接接触的情况下作用于远处。理解这些分类有助于我们解释日常生活和实验室中各种物理现象。
Contact forces | 接触力：
- Friction — opposes the sliding of one surface over another. 摩擦力 — 阻碍一个表面在另一个表面上滑动。
- Tension — the pulling force transmitted through a rope or string. 张力 — 通过绳子或细线传递的拉力。
- Air resistance — a type of friction that acts on objects moving through air. 空气阻力 — 作用于空气中运动物体的摩擦阻力。
- Normal reaction force — the support force exerted by a surface on an object resting on it. 法向反作用力 — 表面对置于其上物体的支撑力。
Non-contact forces | 非接触力：
- Gravity — the attractive force between all objects with mass. 重力 — 所有有质量物体之间的吸引力。
- Magnetic force — the attraction or repulsion between magnets or magnetic materials. 磁力 — 磁体或磁性材料之间的吸引或排斥力。
- Electrostatic force — the force between electrically charged objects. 静电力 — 带电物体之间的力。
| Type of Force | 力的类型 | Contact or Non-contact? | 接触力还是非接触力？ | Example | 示例 |
|---|---|---|
| Friction | 摩擦力 | Contact | 接触力 | Brakes on a bicycle | 自行车刹车 |
| Gravity | 重力 | Non-contact | 非接触力 | Apple falling from a tree | 苹果从树上落下 |
| Magnetic force | 磁力 | Non-contact | 非接触力 | Magnets attracting each other | 磁铁相互吸引 |
| Tension | 张力 | Contact | 接触力 | Pulling a rope in a tug-of-war | 拔河时拉绳子 |
3. Measuring Forces | 力的测量
Forces are measured using a device called a spring balance, also known as a force meter or newton meter. This instrument works on the principle that a spring stretches by an amount proportional to the force applied to it. The greater the force, the greater the extension of the spring.
力使用弹簧测力计进行测量，该仪器也被称为弹簧秤或牛顿计。它的工作原理是：弹簧的伸长量与施加在其上的力成正比。力越大，弹簧的伸长量就越大。
The unit of force is the newton (N). One newton is approximately the force required to lift a small apple (about 100 grams) against gravity. A force of 1 N can be defined more precisely as the force needed to give a 1-kilogram mass an acceleration of 1 metre per second squared.
力的单位是牛顿（N）。大约需要1牛顿的力才能将一个约100克的小苹果抵抗重力举起。更精确地说，1牛顿的力被定义为使1千克质量的物体产生每秒平方1米加速度所需的力。
1 N = 1 kg × 1 m/s²
When using a spring balance to measure force, it is important to hold the balance vertically and ensure the object is hanging freely. The reading should be taken when the spring has stopped moving and the pointer is steady. This ensures an accurate measurement of the gravitational force acting on the object.
使用弹簧测力计测量力时，需要垂直握持测力计，确保物体自由悬挂。读数应在弹簧停止移动、指针稳定时读取，这样可以确保准确测量作用在物体上的重力。
4. Friction | 摩擦力
Friction is a contact force that opposes the relative motion between two surfaces in contact. It always acts in the opposite direction to the movement of the object. Friction arises because even smooth-looking surfaces have tiny irregularities that catch on each other when they slide.
摩擦力是阻碍两个接触表面相对运动的接触力。它总是沿着物体运动的反方向作用。摩擦力产生的原因是即使看起来光滑的表面也有微小的不规则凹凸，当它们滑动时会相互卡住。
Friction has both advantages and disadvantages in everyday life. On one hand, friction allows us to walk without slipping, enables vehicles to brake, and lets us write with a pen or pencil. On the other hand, friction causes wear and tear on machinery, produces unwanted heat, and makes moving objects slow down. Engineers often reduce friction by using lubricants such as oil or grease, or by using ball bearings in wheels.
摩擦力在日常生活中既有优点也有缺点。一方面，摩擦力使我们能够行走而不滑倒、使车辆能够刹车、使我们能用笔或铅笔书写。另一方面，摩擦力导致机器磨损、产生不必要的热量，并使运动物体减速。工程师通常使用润滑油或润滑脂等润滑剂来减少摩擦，或者在轮子中使用滚珠轴承。
The amount of friction depends on two main factors: the roughness of the surfaces in contact and the force pressing the surfaces together. Rougher surfaces produce greater friction, and increasing the pressing force also increases friction.
摩擦力的大小取决于两个主要因素：接触面的粗糙程度和将表面压在一起的力量。表面越粗糙，摩擦力越大；增加压力也会增大摩擦力。
- Walking on icy ground is difficult because friction is small. 在结冰的地面上行走很困难，因为摩擦力很小。
- Car tires have tread patterns that increase friction on the road. 汽车轮胎有花纹，可以增加与路面的摩擦力。
- Applying sand to slippery floors increases friction and prevents accidents. 在光滑的地面上撒沙子可以增加摩擦力并防止事故。
5. Speed and Velocity | 速率与速度
Speed is a measure of how fast an object is moving. It is defined as the distance travelled per unit of time. Speed is a scalar quantity, meaning it has only magnitude and no direction. The average speed of an object can be calculated using the following equation:
速率是衡量物体运动快慢的物理量，定义为单位时间内行驶的距离。速率是标量，只有大小而没有方向。物体的平均速率可以用以下方程计算：
Speed = Distance ÷ Time
速率 = 距离 ÷ 时间
For example, if a car travels 120 kilometres in 2 hours, its average speed is 60 km/h. In scientific work, speed is often expressed in metres per second (m/s). To convert from km/h to m/s, divide by 3.6.
例如，如果一辆汽车在2小时内行驶了120千米，其平均速率为60千米/小时。在科学研究中，速率通常以米/秒（m/s）表示。要将千米/小时转换为米/秒，需要除以3.6。
Velocity is a similar concept to speed, but it is a vector quantity. Velocity includes both speed and direction. For instance, a car moving north at 60 km/h has a different velocity from a car moving south at 60 km/h, even though their speeds are the same. When an object changes direction, its velocity changes even if its speed remains constant.
速度与速率概念相似，但速度是矢量。速度同时包含速率和方向。例如，以60千米/小时向北行驶的汽车与以60千米/小时向南行驶的汽车速度不同，尽管它们的速率相同。当物体改变方向时，即使速率不变，其速度也发生了变化。
In calculations, the triangle method (also known as the formula triangle) is a helpful way to rearrange the speed equation. By covering the quantity you want to find, you can easily determine whether to multiply or divide the other two quantities.
在计算中，三角记忆法（也称为公式三角形）是重新排列速率方程的有用方法。通过遮住你想求的量，可以轻松确定另外两个量是相乘还是相除。
Average Speed = Total Distance ÷ Total Time
平均速率 = 总距离 ÷ 总时间
6. Distance-Time Graphs | 距离-时间图像
A distance-time graph is a powerful visual tool used to describe the motion of an object. In this type of graph, the horizontal axis (x-axis) represents time, and the vertical axis (y-axis) represents distance from a starting point. The shape of the line tells us how the object is moving.
距离-时间图像是描述物体运动的有力视觉工具。在这类图像中，横轴（x轴）表示时间，纵轴（y轴）表示距起点的距离。线的形状告诉我们物体的运动方式。
A horizontal line on a distance-time graph indicates that the object is stationary, because the distance does not change with time. A straight line sloping upwards indicates that the object is moving at a constant speed; the steeper the line, the faster the speed. A curved line indicates that the object is accelerating or decelerating.
距离-时间图像上的水平线表示物体静止，因为距离不随时间改变。向上倾斜的直线表示物体以恒定速率运动；线越陡，速率越快。曲线表示物体在加速或减速。
Speed on a distance-time graph = Gradient (slope) of the line
距离-时间图像上的速率 = 直线的斜率（坡度）
To calculate the speed from a distance-time graph, choose two points on the straight line and divide the change in distance by the change in time. The gradient can be calculated using the following method:
要从距离-时间图像中计算速率，在直线上选择两个点，将距离的变化量除以时间的变化量。可以使用以下方法计算斜率：
Gradient = (Distance₂ − Distance₁) ÷ (Time₂ − Time₁)
斜率 = （距离₂ − 距离₁）÷（时间₂ − 时间₁）
- Horizontal line: object is not moving, speed = 0. 水平线：物体不动，速率 = 0。
- Steep slope: high speed. 陡峭的斜率：高速。
- Gentle slope: low speed. 平缓的斜率：低速。
- Curving upward: acceleration. 向上弯曲：加速。
- Curving downward: deceleration. 向下弯曲：减速。
7. Newton’s Laws of Motion | 牛顿运动定律
Sir Isaac Newton published his three laws of motion in 1687 in his landmark work the Principia. These laws describe the relationship between forces and the motion of objects and remain the foundation of classical mechanics to this day.
艾萨克·牛顿爵士于1687年在他的里程碑著作《自然哲学的数学原理》中发表了三大运动定律。这些定律描述了力与物体运动之间的关系，至今仍是经典力学的基础。
First Law — Law of Inertia | 第一定律 — 惯性定律： An object at rest will remain at rest, and an object in motion will continue moving at a constant velocity, unless acted upon by an unbalanced force. In other words, objects resist changes to their state of motion. For example, when a bus suddenly brakes, passengers lurch forward because their bodies tend to keep moving.
第一定律 — 惯性定律： 静止的物体将保持静止，运动的物体将保持匀速运动，除非受到不平衡力的作用。换句话说，物体抵抗其运动状态的改变。例如，当公交车突然刹车时，乘客会向前倾倒，因为他们的身体倾向于保持运动状态。
Second Law — Force, Mass and Acceleration | 第二定律 — 力、质量与加速度： The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass. This law is expressed by the famous equation F = m × a, where F is the resultant force in newtons, m is the mass in kilograms, and a is the acceleration in metres per second squared.
第二定律 — 力、质量与加速度： 物体的加速度与作用于其上的合力成正比，与其质量成反比。这一定律由著名方程 F = m × a 表达，其中 F 是以牛顿为单位的合力，m 是以千克为单位的质量，a 是以米/秒²为单位的加速度。
F = m × a | 力 = 质量 × 加速度
Third Law — Action and Reaction | 第三定律 — 作用力与反作用力： For every action force, there is an equal and opposite reaction force. When a rocket pushes hot gas downward, the gas pushes the rocket upward with an equal force. When a book rests on a table, the book exerts a downward force on the table, and the table exerts an equal upward force on the book.
第三定律 — 作用力与反作用力： 每一个作用力都有一个大小相等、方向相反的反作用力。当火箭向下喷射高温气体时，气体以相等的力将火箭向上推。当书本放在桌上时，书对桌子施加向下的力，而桌子对书施加相等的向上力。
8. Weight and Mass | 重量与质量
Mass and weight are two different concepts that are often confused. Mass is the amount of matter contained in an object and is measured in kilograms (kg) or grams (g). Mass remains constant regardless of where the object is located in the universe. A 10 kg mass on Earth is still a 10 kg mass on the Moon or in outer space.
质量与重量是两个经常被混淆的不同概念。质量是物体所含物质的量，以千克（kg）或克（g）为单位。无论物体位于宇宙的哪个位置，质量都保持不变。地球上10千克的质量在月球上或太空中仍然是10千克。
Weight, on the other hand, is the force of gravity acting on an object’s mass. Weight is a force, so it is measured in newtons (N). Weight depends on the gravitational field strength, which varies from place to place. The equation for weight is:
另一方面，重量是作用在物体质量上的重力。重量是力，因此以牛顿（N）为单位。重量取决于重力场强度，而重力场强度因地而异。重量的方程是：
Weight = Mass × Gravitational Field Strength
重量 = 质量 × 重力场强度
W = m × g | 重量 = 质量 × g
On the surface of the Earth, the gravitational field strength is approximately 10 N/kg (or 9.8 N/kg for more precise calculations). Therefore, a person with a mass of 50 kg has a weight of 500 N on Earth. On the Moon, the gravitational field strength is only about 1.6 N/kg, so the same person would weigh only 80 N, while their mass would remain 50 kg.
在地球表面，重力场强度约为10 N/kg（更精确的计算为9.8 N/kg）。因此，质量50千克的人在地球上的重量为500牛顿。在月球上，重力场强度仅为约1.6 N/kg，因此同样的人只重80牛顿，而质量仍然是50千克。
| Quantity | 量 | Mass | 质量 | Weight | 重量 |
|---|---|---|
| Is it a force? | 是力吗？ | No | 不是 | 更多咨询请联系16621398022（同微信）
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