📚 Forces and Motion: Key Concepts for Edexcel IGCSE Science | 力与运动：Edexcel IGCSE 科学核心概念
In Edexcel IGCSE Science (Physics), the study of forces and motion forms the basis of mechanics. You will be asked to interpret graphs, apply Newton’s laws, and use equations of motion to solve problems. This article reviews the essential topic in a concise and exam-friendly way.
在 Edexcel IGCSE 科学（物理部分）中，力与运动是力学的基础。考试会要求你解读图像、应用牛顿定律并运用运动学方程解决问题。本文以简洁、贴近考试的方式复习这一核心主题。
1. Scalars and Vectors | 标量与向量
A scalar quantity has magnitude only. Examples include distance, speed, mass, time and energy.
标量只有大小，没有方向。例如：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 and momentum.
向量既有大小又有方向。例如：位移、速度、加速度、力和动量。
- Distance is a scalar: the total path covered.
- Distance 距离是标量：物体经过的总路径长度。
- Displacement is a vector: the straight-line distance from start to finish in a specific direction.
- Displacement 位移是向量：从起点到终点的直线距离，且带有特定方向。
2. Speed, Velocity and Acceleration | 速率、速度与加速度
Speed is the distance travelled per unit time. Velocity is the displacement per unit time. Velocity has direction, so a car going around a bend at constant speed is changing velocity.
速率是单位时间内通过的距离；速度是单位时间内的位移。速度具有方向，因此汽车以恒定速率转弯时，它的速度正在改变。
Acceleration is the change in velocity per unit time. The unit is metre per second squared (m/s²).
加速度是单位时间内速度的变化量，单位是米每二次方秒（m/s²）。
acceleration = change in velocity ÷ time taken
加速度 = 速度变化量 ÷ 所用时间
| Quantity | Unit | Scalar/Vector |
| Speed | m/s | Scalar |
| Velocity | m/s | Vector |
| Acceleration | m/s² | Vector |
3. Distance-Time Graphs | 距离-时间图像
In a distance-time graph, the gradient (slope) represents speed. A horizontal line means the object is stationary. A steeper line means a higher speed.
在距离-时间图像中，斜率代表速率。水平线表示物体静止；直线越陡，表示速率越大。
- Flat line: object is not moving.
- Flat line 水平线：物体没有运动。
- Straight diagonal line: object moves at constant speed.
- Straight diagonal line 斜直线：物体以恒定速率运动。
- Curved line: speed is changing.
- Curved line 曲线：速率正在改变。
4. Velocity-Time Graphs | 速度-时间图像
In a velocity-time graph, the gradient represents acceleration. The area under the graph represents the distance travelled.
在速度-时间图像中，斜率代表加速度；图像与时间轴围成的面积代表通过的距离。
If the graph is a horizontal line, the object moves at constant velocity. If the graph goes upwards, the object accelerates; if it goes downwards, the object decelerates.
如果图像是水平线，说明物体做匀速运动。图像上升表示加速，图像下降表示减速。
5. Equations of Motion | 运动学方程
For uniform acceleration, you can use four equations that relate displacement (s), initial velocity (u), final velocity (v), acceleration (a) and time (t). These are often called the SUVAT equations.
对于匀加速直线运动，可以用四个方程联系位移 s、初速度 u、末速度 v、加速度 a 和时间 t。这些方程常被称为 SUVAT 方程。
v = u + at
s = ut + ½at²
v² = u² + 2as
s = ½(u + v)t
Always choose the equation that contains the three quantities you know and the one you need to find.
选择方程时，找出同时包含三个已知量和待求量的等式即可。
6. Newton’s First Law of Motion | 牛顿第一定律
Newton’s first law states that an object will remain at rest or continue to move at a constant velocity unless acted on by a resultant force.
牛顿第一定律指出：如果物体不受合外力作用，它将保持静止或继续做匀速直线运动。
This means if the resultant force is zero, the object’s velocity does not change. Friction is a common force that can slow objects down by opposing motion.
这意味着当合外力为零时，物体速度不变。摩擦力是一种常见的力，它会阻碍运动并使物体减速。
7. Newton’s Second Law of Motion | 牛顿第二定律
Newton’s second law links resultant force, mass and acceleration. The greater the force, the greater the acceleration; the greater the mass, the smaller the acceleration for the same force.
牛顿第二定律将合外力、质量和加速度联系起来。合力越大，加速度越大；质量越大，相同合力下加速度越小。
F = ma
合外力 = 质量 × 加速度
- F is the resultant force in newtons (N).
- F 是合外力，单位牛顿（N）。
- m is mass in kilograms (kg).
- m 是质量，单位千克（kg）。
- a is acceleration in metres per second squared (m/s²).
- a 是加速度，单位米每二次方秒（m/s²）。
8. Newton’s Third Law of Motion | 牛顿第三定律
Newton’s third law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
牛顿第三定律指出：当物体 A 对物体 B 施加一个力时，物体 B 同时对物体 A 施加一个大小相等、方向相反的力。
For example, when a rocket pushes hot gas downward, the gas pushes the rocket upward. The two forces are equal in magnitude and opposite in direction, but they act on different objects.
例如，火箭向下喷出高温燃气，燃气同时向上推动火箭。两个力大小相等、方向相反，但作用在不同物体上。
9. Weight, Mass and Gravitational Field Strength | 重力、质量与重力场强度
Mass is the amount of matter in an object and does not change with location. Weight is the gravitational force acting on an object and changes with gravitational field strength.
质量是物体所含物质的多少，不随位置改变。重力是作用在物体上的引力，会随重力场强度而变化。
W = mg
重力 = 质量 × 重力场强度
On Earth, g is approximately 9.8 N/kg. On the Moon, g is smaller, so an object weighs less despite having the same mass.
在地球上，g 约为 9.8 N/kg。在月球上，g 较小，因此同一物体质量不变但重力更小。
10. Terminal Velocity | 终端速度
When an object falls through a fluid (air), it experiences air resistance. The faster it falls, the greater the air resistance. Once air resistance equals the object’s weight, the resultant force becomes zero and the object falls at a constant speed called terminal velocity.
物体在空气等流体中下落时受到空气阻力。下落速度越快，空气阻力越大。当空气阻力等于物体重力时，合外力为零，物体开始以恒定速度下落，这个速度称为终端速度。
This is why a skydiver first accelerates, then feels a sudden slowing, and finally reaches a constant terminal velocity.
这就是为什么跳伞运动员先加速，然后感到速度减慢，最后达到恒定的终端速度。
11. Momentum and Collisions | 动量与碰撞
Momentum is a vector quantity defined as the product of mass and velocity. Its unit is kilogram metre per second (kg m/s).
动量是向量，定义为质量与速度的乘积，单位是千克米每秒（kg·m/s）。
momentum = mass × velocity
动量 = 质量 × 速度
In a closed system, the total momentum before a collision equals the total momentum after the collision. This is the principle of conservation of momentum.
在封闭系统中，碰撞前的总动量等于碰撞后的总动量。这就是动量守恒定律。
12. Stopping Distances | 刹车距离
The stopping distance of a vehicle is the sum of the thinking distance and the braking distance.
车辆的停车距离等于反应距离与制动距离之和。
stopping distance = thinking distance + braking distance
停车距离 = 反应距离 + 制动距离
- Thinking distance increases if the driver is tired, distracted, or has taken alcohol or drugs.
- 反应距离会因驾驶员疲劳、分心、饮酒或服药而增加。
- Braking distance increases with greater speed, worse road conditions, wet/icy roads, worn tyres or poor brakes.
- 制动距离会因速度更大、路面湿滑、轮胎磨损或刹车不良而增加。
- In particular, braking distance is proportional to the square of speed: if speed doubles, braking distance becomes about four times larger.
- 特别值得注意的是，制动距离与速度的平方成正比：速度加倍时，制动距离约变为原来的四倍。
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