📚 IGCSE Edexcel Science: Forces and Motion – Key Points | IGCSE Edexcel 科学：力与运动 考点精讲
Welcome to this comprehensive revision guide on Forces and Motion for the IGCSE Edexcel Science syllabus. This article walks you through the fundamental concepts you will encounter in your exam, from scalar and vector quantities to Newton’s laws, graphs, and momentum. Each section pairs key explanations in English and Chinese, ensuring you build both subject knowledge and bilingual confidence.
欢迎阅读这篇针对 IGCSE Edexcel 科学大纲的力与运动综合复习指南。本文将带你梳理考试中会遇到的核心概念，从标量与矢量、牛顿定律到图线分析和动量。每个知识要点均提供中英双语解释，帮助你扎实掌握物理原理并提升双语应试能力。
1. Scalar and Vector Quantities | 标量与矢量
A scalar quantity has only magnitude (size), no direction. Common examples are distance, speed, mass, time and energy.
标量只有大小，没有方向。常见的标量有距离、速率、质量、时间和能量。
A vector quantity has both magnitude and direction. Examples include displacement, velocity, acceleration, force, weight and momentum.
矢量既有大小又有方向。典型的矢量包括位移、速度、加速度、力、重力和动量。
When adding vectors that lie along the same line, you simply add or subtract magnitudes depending on direction. For non-parallel vectors, you must use tip-to-tail diagrams or Pythagoras with trigonometry to find the resultant.
同一直线上的矢量直接按方向加减大小。不平行的矢量则必须使用首尾相接作图法，或勾股定理配合三角函数求出合矢量。
2. Speed and Velocity | 速率与速度
Speed is the rate at which an object covers distance. It is a scalar. Average speed is calculated as total distance divided by total time.
速率是物体运动快慢的量度，是标量。平均速率等于总距离除以总时间。
Velocity is speed in a specified direction, making it a vector. The magnitude of velocity is speed, but velocity can be positive or negative depending on the chosen direction.
速度是给定方向上的速率，因此是矢量。速度的大小就是速率，但根据选定的方向，速度可以是正值或负值。
For uniform motion, the relationship is simple: speed = distance / time. When direction changes, the velocity changes even if speed remains constant.
对于匀速运动，关系很简单：速率 = 距离 / 时间。如果方向改变，即使速率不变，速度也发生了变化。
3. Acceleration | 加速度
Acceleration is the rate of change of velocity. It is a vector and can be positive (speeding up in the forward direction) or negative (slowing down).
加速度是速度的变化率，是矢量。加速度可以为正（沿正方向加速）或为负（减速）。
a = (v – u) / t
Where v is final velocity, u is initial velocity and t is the time taken. The unit is metres per second squared (m/s²).
其中 v 为末速度，u 为初速度，t 为所用时间。单位是米每二次方秒 (m/s²)。
Deceleration (negative acceleration) occurs when an object slows down, meaning the change in velocity is in the opposite direction to the motion.
当物体减速时出现负加速度（减速度），意味着速度变化的方向与运动方向相反。
4. Distance-Time Graphs | 距离–时间图
A distance-time graph shows how an object’s distance from a starting point changes over time. The gradient (slope) of the line represents the object’s speed.
距离–时间图展示物体离起点的距离如何随时间变化。图线的斜率（坡度）表示物体的速率。
A horizontal line indicates the object is stationary. A straight sloping line represents constant speed, and a steeper slope means a higher speed.
水平线表示物体静止。倾斜直线表示匀速运动，斜率越大则速率越大。
A curved line shows acceleration or deceleration. If the curve becomes steeper, the object is accelerating; if it flattens, the object is slowing down.
曲线表示加速度或减速度。若曲线越来越陡，物体在加速；若曲线趋于平缓，物体在减速。
5. Velocity-Time Graphs | 速度–时间图
A velocity-time graph has velocity on the vertical axis and time on the horizontal axis. The gradient equals the acceleration of the object.
速度–时间图的纵轴为速度，横轴为时间。斜率等于物体的加速度。
A horizontal line means constant velocity (zero acceleration). A straight line sloping upwards indicates constant positive acceleration; sloping downwards shows constant negative acceleration.
水平线段表示速度恒定（加速度为零）。向上倾斜的直线表示恒定的正加速度；向下倾斜表示恒定的负加速度。
The area under the graph between the line and the time axis represents the displacement (distance travelled in a given direction) during that time interval.
图线与时间轴之间所围的面积代表该时间段内的位移（特定方向上的移动距离）。
6. Forces and Resultant Force | 力与合力
A force is a push or pull that can change an object’s shape, speed or direction. Force is a vector measured in newtons (N).
力是能够改变物体形状、速率或方向的推或拉。力是矢量，单位是牛顿 (N)。
The resultant (or net) force is the single force that has the same effect as all the original forces acting together. It is found by vector addition.
合力（净力）是与所有实际作用力总效果相同的单一力，可通过矢量加法求得。
If the resultant force on an object is zero, the forces are balanced and the object remains in equilibrium (no change in motion). If the resultant force is not zero, the object accelerates in the direction of the resultant force.
如果物体所受合力为零，诸力平衡，物体处于平衡状态（运动状态不变）。如果合力不为零，物体将沿合力方向加速。
7. Newton’s First Law | 牛顿第一定律
Newton’s First Law (the law of inertia) states that an object at rest stays at rest, and an object in motion continues to move with constant velocity, unless acted upon by a resultant external force.
牛顿第一定律（惯性定律）指出：静止物体将保持静止，运动物体将保持匀速直线运动状态，除非受到外界的合外力作用。
Inertia is the tendency of an object to resist changes in its motion. The greater the mass of an object, the greater its inertia.
惯性是物体抵抗运动状态改变的性质。物体的质量越大，惯性越大。
This law explains why passengers lurch forward when a car brakes suddenly: their bodies try to continue moving forward due to inertia.
这一定律解释了为什么汽车急刹车时乘客会向前冲：由于惯性，他们的身体试图继续保持向前运动。
8. Newton’s Second Law (F = m a) | 牛顿第二定律 (F = ma)
The acceleration of an object depends on the resultant force acting on it and the object’s mass. The relationship is one of the most important equations in physics:
物体的加速度取决于作用在其上的合力以及物体的质量。这一关系是物理学中最重要的方程之一：
F = m × a
Here, F is the resultant force in newtons (N), m is the mass in kilograms (kg), and a is the acceleration in metres per second squared (m/s²).
其中 F 为合力，单位为牛顿 (N)；m 为质量，单位为千克 (kg)；a 为加速度，单位为米每二次方秒 (m/s²)。
The acceleration is always in the same direction as the resultant force. For a given mass, a larger force produces a proportionally larger acceleration.
加速度的方向始终与合力方向相同。对于一定质量，力越大，产生的加速度越大，且成正比。
9. 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 two forces act on different objects and are of the same type.
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。这两个力作用在不同物体上，且属于同种类型的力。
For example, when you push on a wall, the wall pushes back on you with an equal force. When a swimmer pushes water backwards, the water pushes the swimmer forwards.
例如，你推墙时，墙也以等大的力反推你。游泳者向后推水，水便向前推游泳者。
It is crucial to recognise that the action and reaction forces do not cancel each other out in the motion of a single object, because they act on different bodies.
必须认识到，作用力和反作用力并不会在单个物体的运动分析中相互抵消，因为它们分别作用在不同的物体上。
10. Momentum and Conservation | 动量与守恒
Momentum is a vector quantity defined as the product of an object’s mass and its velocity.
动量是矢量，定义为物体质量与速度的乘积。
p = m × v
Momentum is measured in kilogram metres per second (kg m/s). The direction of momentum is the same as the direction of velocity.
动量的单位是千克米每秒 (kg m/s)。动量的方向与速度方向一致。
In a closed system (no external resultant force), total momentum before a collision or explosion equals total momentum after the event. This is the principle of conservation of momentum.
在封闭系统（无外合力）中，碰撞或爆炸前的总动量等于事件后的总动量，这就是动量守恒定律。
For a collision between two objects: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂, where u are initial velocities and v are final velocities. In explosions, the total momentum remains zero if the objects were initially at rest.
对于两物体碰撞：m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂，其中 u 为初速度，v 为末速度。在爆炸中，若初始静止，总动量仍为零。
11. Stopping Distances and Safety | 刹车距离与安全
The total stopping distance of a vehicle is the sum of the thinking distance and the braking distance. Thinking distance is the distance travelled during the driver’s reaction time; braking distance is the distance travelled under the braking force until the vehicle stops.
车辆的总停车距离是思考距离与制动距离之和。思考距离是驾驶员反应时间内行驶的距离；制动距离是在制动力作用下车至完全停止所驶过的距离。
Factors increasing thinking distance include tiredness, alcohol, drugs, distractions, and high speed. Factors increasing braking distance include wet or icy roads, worn tyres, poor brakes, and high speed.
增大思考距离的因素有疲劳、酒精、药物、注意力分散和高速。增大制动距离的因素包括湿滑结冰路面、轮胎磨损、刹车不良以及高速。
Speed affects both distances dramatically. Doubling the speed roughly quadruples the braking distance because the kinetic energy increases with the square of the speed.
速度对两者影响显著。速度加倍时，制动距离大致增加为原来的四倍，因为动能与速度的平方成正比。
Safety features such as seat belts, airbags and crumple zones extend the time over which momentum changes during a collision, reducing the force on the occupants.
安全带、安全气囊和吸能区等安全装置延长了碰撞时动量变化的时间，从而减小了乘客所受的力。
12. Terminal Velocity | 终端速度
When an object falls through a fluid (such as air), two main forces act: weight downwards and air resistance (drag) upwards. Initially, weight is greater and the object accelerates downwards.
物体在流体（如空气）中下落时，主要受两个力作用：向下的重力和向上的空气阻力。最初重力较大，物体向下加速。
As speed increases, air resistance grows. Eventually the air resistance equals the weight, so the resultant force becomes zero and the object stops accelerating. It then falls at a constant speed called terminal velocity.
随着速度增加，空气阻力增大。最终空气阻力等于重力，合力为零，物体不再加速，以恒定速度下落，这就是终端速度。
A velocity-time graph for a skydiver shows an initial steep slope (high acceleration), then a curve that flattens to a horizontal line at terminal velocity. After the parachute opens, the upward force increases suddenly, deceleration occurs, and a new lower terminal velocity is reached.
跳伞者的速度–时间图会显示初始较陡的斜线（高加速度），然后曲线趋于水平达到终端速度。打开降落伞后，向上的力骤增，出现减速，并达到一个新的、更低的终端速度。
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