📚 IGCSE Science: Forces and Motion Key Points | IGCSE 科学：力与运动 考点精讲
Understanding forces and motion is fundamental to IGCSE Science. This article covers essential concepts including scalars and vectors, speed, acceleration, Newton’s laws, momentum, terminal velocity, Hooke’s law and circular motion, presented in clearly paired English and Chinese explanations to help you master the exam points.
理解力与运动是 IGCSE 科学的基础。本文涵盖标量和矢量、速率、加速度、牛顿定律、动量、终端速度、胡克定律和圆周运动等重要概念，以清晰的英中对照解释帮助你掌握考点。
1. Scalar and Vector Quantities | 标量与矢量
A scalar quantity has only magnitude (size). Typical scalars include distance, speed, mass, energy, time and temperature.
标量只有大小（量值）。典型的标量包括距离、速率、质量、能量、时间和温度。
A vector quantity has both magnitude and direction. Examples are displacement, velocity, acceleration, force, weight and momentum. Vectors are often represented by arrows; the length shows magnitude and the arrowhead indicates direction.
矢量既有大小也有方向。例如位移、速度、加速度、力、重量和动量。矢量通常用箭头表示；箭头的长度表示大小，箭头指向表示方向。
2. Speed and Velocity | 速率与速度
Speed is a scalar that tells how fast an object is moving. Average speed is given by:
average speed = total distance / total time
ch which can be written as v = s / t, where s is the distance travelled and t is the time taken.
速率是标量，表示物体运动的快慢。平均速率由下式给出：
平均速率 = 总距离 / 总时间
可写作 v = s / t，其中 s 是行程距离，t 是所用时间。
Velocity is a vector; it is speed in a given direction. An object moving in a circle at constant speed has a constantly changing velocity because its direction changes.
速度是矢量，是给定方向上的速率。一个物体以恒定速率做圆周运动时，速度不断变化，因为其方向在变。
3. Acceleration | 加速度
Acceleration is the rate of change of velocity. It is a vector and can be positive (speeding up) or negative (slowing down, also called deceleration).
加速度是速度的变化率。它是一个矢量，可以是正的（加速）或负的（减速，也称为减速度）。
The average acceleration is calculated by:
a = (v – u) / t
where u is initial velocity, v is final velocity and t is the time taken. Uniform acceleration means the velocity changes by equal amounts in equal time intervals.
平均加速度的计算公式为：
a = (v – u) / t
其中 u 为初速度，v 为末速度，t 为所用时间。匀加速运动意味着在相等的时间间隔内速度的变化量相等。
4. Motion Graphs (Distance-Time and Speed-Time) | 运动图像（距离-时间图与速度-时间图）
In a distance-time graph, the gradient represents speed. A straight line means constant speed, while a horizontal line means the object is stationary. A curved line indicates changing speed (acceleration).
在距离-时间图中，斜率表示速率。直线代表匀速运动，水平线代表物体静止。曲线表示速率在变化（加速）。
In a speed-time graph, the gradient gives acceleration, and the area under the line gives the distance travelled. A horizontal line in a speed-time graph represents constant speed, and a straight sloping line indicates uniform acceleration.
在速度-时间图中，斜率表示加速度，图线下的面积表示所经过的距离。速度-时间图中的水平线代表匀速运动，倾斜直线代表匀变速运动。
5. Newton’s Laws of Motion | 牛顿运动定律
First Law (Law of Inertia): An object remains at rest or in uniform motion in a straight line unless acted upon by a resultant force. This means objects resist changes in their state of motion.
第一定律（惯性定律）： 除非受到合外力作用，物体会保持静止或匀速直线运动状态。这意味着物体会抵抗其运动状态的改变。
Second Law: The acceleration of an object is directly proportional to the resultant force and inversely proportional to its mass. It is summarised by the equation:
F = m a
where F is the resultant force in newtons (N), m is mass in kg, and a is acceleration in m/s².
第二定律： 物体的加速度与合外力成正比，与其质量成反比。总结为公式：
F = m a
其中 F 是合外力，单位为牛顿（N），m 是质量（kg），a 是加速度（m/s²）。
Third Law: Whenever two objects interact, they exert equal and opposite forces on each other. These forces are of the same type and act on different objects; they do not cancel out.
第三定律： 两个物体相互作用时，彼此施加大小相等、方向相反的力。这两个力属于同一种类型，作用在不同物体上；它们不会相互抵消。
6. Resultant Force and Equilibrium | 合力与平衡
The resultant force (net force) is the single force that has the same effect as all
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