📚 Mastering Forces and Motion for IGCSE Edexcel Science | 掌握IGCSE Edexcel 科学：力与运动
Forces and motion form the backbone of physics in the IGCSE Edexcel Science syllabus. Understanding how objects move, what causes them to change speed or direction, and how energy is transferred during these processes is essential for tackling both paper-based questions and practical investigations. This article will guide you through the key concepts step by step, with clear definitions, graphs, equations and worked examples.
力与运动是 IGCSE Edexcel 科学课程中物理部分的基石。理解物体如何运动、是什么导致它们改变速度或方向，以及在这些过程中能量如何转移，对于应对书面题目和实验探究都至关重要。本文将逐步带你梳理核心概念，配合清晰的定义、图像、方程和计算实例。
1. Scalars and Vectors | 标量与矢量
In physics, quantities are divided into scalars and vectors. A scalar has magnitude (size) only, such as distance, speed, mass and energy. A vector has both magnitude and direction, such as displacement, velocity, acceleration and force. When adding vectors, you must consider their direction – for example, if two forces of 5 N act in opposite directions, the resultant force is zero.
在物理中，量分为标量和矢量。标量只有大小（没有方向），比如距离、速率、质量和能量。矢量既有大小又有方向，比如位移、速度、加速度和力。在矢量合成时，必须考虑方向——例如，两个5 N的力沿相反方向作用，合力为零。
2. Speed and Velocity | 速度与速率
Speed is a scalar quantity defined as the distance travelled per unit time. It is calculated using the formula:
speed = distance ÷ time
Velocity is a vector and measures the rate of change of displacement. If an object moves in a straight line without turning, its speed and velocity have the same magnitude. The standard unit for both is metres per second (m/s).
速率是一个标量，定义为单位时间内通过的距离。计算公式如下：
速率 = 距离 ÷ 时间
速度是矢量，衡量位移变化的快慢。如果物体沿直线运动且不改变方向，速率和速度的大小相等。两者的标准单位都是米每秒（m/s）。
3. Acceleration | 加速度
Acceleration is the rate at which velocity changes. It is a vector quantity and can be positive (speeding up) or negative (slowing down, often called deceleration). The average acceleration is given by:
a = (v − u) ÷ t
where v is final velocity, u is initial velocity and t is time. The unit of acceleration is metres per second squared (m/s²). Near the Earth’s surface, the acceleration due to gravity, g, is approximately 9.8 m/s².
加速度描述速度变化的快慢，它是一个矢量，有正（加速）和负（减速，常称减速度）之分。平均加速度的计算公式为：
a = (v − u) ÷ t
其中 v 为末速度，u 为初速度，t 为时间。加速度的单位是米每二次方秒（m/s²）。在地球表面附近，重力加速度 g 约为 9.8 m/s²。
4. Distance-Time Graphs | 路程-时间图
A distance-time graph shows how the distance of an object from a starting point changes over time. The gradient (slope) of the line represents speed. A horizontal line indicates the object is stationary; a straight sloping line means constant speed; a curve means changing speed (acceleration). To find the speed at a particular instant, draw a tangent and calculate its gradient.
路程-时间图展示物体距起点的距离如何随时间变化。图中直线的斜率代表速率。水平线表示物体静止；倾斜的直线表示匀速；曲线表示速度变化（加速度）。要找出某一瞬间的速率，可作切线并计算切线斜率。
5. Velocity-Time Graphs | 速度-时间图
Velocity-time graphs are powerful tools for analysing motion. The gradient gives acceleration, and the area under the graph gives displacement (or distance travelled if motion is in one direction). A horizontal line means constant velocity, a sloping line indicates uniform acceleration, and the area of a trapezium or triangle under the line can be used to calculate displacement.
速度-时间图是分析运动的利器。图中斜率表示加速度，图线下的面积表示位移（如果单向运动就是路程）。水平线代表匀速，倾斜线代表匀加速；利用图线下的梯形或三角形面积可以计算位移。
6. Newton’s First Law of Motion | 牛顿第一运动定律
Newton’s First Law states that an object will remain at rest or continue to move at constant velocity unless acted upon by a resultant (unbalanced) force. This property is called inertia. It explains why passengers lurch forward when a car brakes suddenly: their bodies tend to keep moving forward due to inertia.
牛顿第一定律指出，物体将保持静止或匀速直线运动状态，除非受到非零的合力（外力不平衡）作用。这种性质称为惯性。它解释了为什么急刹车时乘客会前倾：由于惯性，身体倾向于继续向前运动。
7. Newton’s Second Law and Force | 牛顿第二定律与力
Newton’s Second Law relates resultant force, mass and acceleration:
F = m × a
where F is the resultant force in newtons (N), m is mass in kilograms (kg), and a is acceleration in m/s². This equation shows that for a given mass, acceleration is directly proportional to the resultant force. It also underpins the definition of the newton: 1 N is the force needed to accelerate 1 kg by 1 m/s².
牛顿第二定律将合力、质量和加速度联系起来：
F = m × a
其中 F 为合力，单位牛顿（N）；m 为质量，单位千克（kg）；a 为加速度，单位 m/s²。这个方程表明，对于一定质量，加速度与合力成正比。它也是牛顿单位的定义基础：1 N 是使 1 kg 物体产生 1 m/s² 加速度所需的力。
8. Newton’s Third Law | 牛顿第三定律
Newton’s Third Law states that for every action force there is an equal and opposite reaction force. These forces act on two different bodies and are of the same type. For example, when you push against a wall, the wall pushes back on you with an equal force. This law explains how rockets move: exhaust gases are pushed backwards, and the rocket is pushed forwards.
牛顿第三定律指出，每一个作用力都有一个大小相等、方向相反的反作用力。这两个力作用在两个不同的物体上，并且属于同种类型的力。例如，你推墙时，墙会以同样大小的力推你。这一定律解释了火箭的运动原理：向后喷射废气时，火箭受到向前的推力。
9. Momentum | 动量
Momentum (p) is a vector quantity calculated as the product of an object’s mass and velocity:
p = m × v
The unit is kg m/s. In a closed system, the total momentum before a collision or explosion equals the total momentum afterwards – this is the principle of conservation of momentum. This principle allows us to predict the resulting velocities in collisions, such as car crashes or snooker balls striking each other.
动量（p）是一个矢量，等于物体质量与速度的乘积：
p = m × v
单位是 kg m/s。在一个封闭系统中，碰撞或爆炸前的总动量等于之后的总动量——这就是动量守恒定律。利用这一定律，我们可以预测碰撞后物体的速度，例如汽车碰撞或斯诺克球的撞击。
10. Work, Energy and Power | 功、能量与功率
Work is done when a force moves an object. The amount of work (W) is given by:
W = F × d
where F is the force in the direction of motion and d is the distance moved. Work is measured in joules (J). Energy is the capacity to do work, and power is the rate at which work is done: P = W / t, measured in watts (W). One watt is one joule per second. Kinetic energy (KE = ½mv²) and gravitational potential energy (GPE = mgh) are frequently tested, and energy transfers between these forms can be analysed using conservation of energy.
力使物体移动时做了功。功（W）的计算公式为：
W = F × d
其中 F 是沿运动方向的力，d 是移动的距离。功的单位是焦耳（J）。能量是做功的能力，功率是做功的快慢：P = W / t，单位为瓦特（W）。1 瓦特等于每秒 1 焦耳。动能（KE = ½mv²）和重力势能（GPE = mgh）是常见考点，利用能量守恒可以分析二者之间的转换。
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