📚 Forces and Motion Key Points for GCSE WJEC Science | GCSE WJEC 科学：力与运动 考点精讲
Understanding forces and motion is fundamental to physics. In the WJEC GCSE Science specification, you need to describe and explain how objects move, what causes changes in motion, and how forces interact. This article covers all the key concepts you must master for the exam, from scalars and vectors to momentum and stopping distances.
理解力与运动是物理学的基础。在 WJEC GCSE 科学考试大纲中，你需要描述和解释物体如何运动、什么导致运动变化以及力如何相互作用。本文涵盖了你必须掌握的所有关键概念，从标量与矢量到动量与停车距离。
1. Scalars and Vectors | 标量与矢量
Scalar quantities have magnitude (size) only, such as speed, distance, mass, and temperature.
标量只有大小（数量），如速率、距离、质量和温度。
Vector quantities have both magnitude and direction, such as velocity, displacement, force, and acceleration. The direction is essential when adding vectors.
矢量既有大小又有方向，如速度、位移、力和加速度。进行矢量相加时，方向至关重要。
For example, if two forces of 5 N act on an object in opposite directions, the resultant force is 0 N; if they act in the same direction, it is 10 N.
例如，如果两个 5 N 的力沿相反方向作用在物体上，合力为 0 N；如果沿同一方向，合力为 10 N。
In WJEC, you must be able to distinguish scalars from vectors and calculate resultant forces in one dimension.
在 WJEC 考试中，你必须能够区分标量和矢量，并计算一维情况下的合力。
2. Distance, Displacement, Speed and Velocity | 距离、位移、速率与速度
Distance is a scalar measuring how far an object moves along a path. Displacement is a vector measuring the straight-line distance from start to finish in a given direction.
距离是标量，测量物体沿路径移动的总路程。位移是矢量，测量从起点到终点的直线距离并带有方向。
Speed (v) is the rate of change of distance: average speed = total distance / total time. Velocity is the rate of change of displacement; it is a vector.
速率（v）是距离变化的快慢：平均速率 = 总路程 / 总时间。速度是位移变化的快慢；它是一个矢量。
Constant speed in a circle means changing velocity because direction changes.
做匀速圆周运动的物体速率不变，但速度变化，因为方向在改变。
You will interpret distance–time graphs: gradient = speed. A flat line means the object is stationary.
你需要分析距离–时间图像：斜率 = 速率。水平线表示物体静止。
3. Acceleration | 加速度
Acceleration is the rate of change of velocity. It is a vector. If an object slows down, the acceleration is in the opposite direction to its motion (deceleration).
加速度是速度变化的快慢，是矢量。如果物体减速，加速度方向与运动方向相反（负加速度）。
The equation for average acceleration:
平均加速度的公式：
a = (v − u) / t
where v = final velocity (m/s), u = initial velocity (m/s), t = time (s).
其中 v = 末速度 (m/s)，u = 初速度 (m/s)，t = 时间 (s)。
In velocity–time graphs, gradient = acceleration and area under graph = displacement. Make sure you can calculate both.
在速度–时间图像中，斜率 = 加速度，图像下的面积 = 位移。务必能够计算这两者。
4. Newton’s First Law of Motion | 牛顿第一运动定律
An object remains at rest or in uniform motion in a straight line unless acted upon by an external resultant force.
一切物体在没有受到外力合力作用时，总保持静止或匀速直线运动状态。
If forces on an object are balanced, it will not accelerate: it stays still or continues at constant velocity. This is inertia.
如果物体受到的力相互平衡，则不会加速：保持静止或匀速运动。这就是惯性。
In WJEC exam questions, you might need to explain why a passenger lurches forward when a bus brakes suddenly (inertia).
在 WJEC 考题中，可能需要解释为什么公共汽车急刹车时乘客会向前倾（惯性）。
5. Newton’s Second Law of Motion | 牛顿第二运动定律
The acceleration of an object is directly proportional to the resultant force and inversely proportional to its mass.
物体的加速度与所受合力成正比，与物体的质量成反比。
F = m × a
where F = resultant force (N), m = mass (kg), a = acceleration (m/s²).
其中 F = 合力 (N)，m = 质量 (kg)，a = 加速度 (m/s²)。
This law lets you calculate any one quantity if the other two are known. Remember, this is vector: force and acceleration are in the same direction.
这个定律让你在已知两个量的情况下计算第三个量。记住这是矢量关系：力与加速度方向相同。
Practical: investigate the relationship between force and acceleration using a trolley and pulley system.
实验：使用小车与滑轮系统探究力与加速度的关系。
6. Newton’s Third Law of Motion | 牛顿第三运动定律
Whenever two objects interact, they exert equal and opposite forces on each other.
两个物体相互作用时，它们彼此施加大小相等、方向相反的力。
These forces are of the same type and act on different objects, so they never cancel out on a single free-body diagram.
这两个力同种类型，作用在不同物体上，因此不会在单一受力图中相互抵消。
Everyday examples: a swimmer pushes water backwards, the water pushes the swimmer forwards; a rocket pushes exhaust gases down, the gases push the rocket up.
日常例子：游泳者向后推水，水向前推游泳者；火箭向下喷气，气体向上推火箭。
Understand that these are ‘action–reaction’ pairs, not balanced forces on a single object.
理解它们是“作用力与反作用力”对，而非作用在同一物体上的平衡力。
7. Momentum and Collisions | 动量与碰撞
Momentum is a vector quantity, defined by:
动量是矢量，定义为：
p = m × v
where p = momentum (kg m/s), m = mass (kg), v = velocity (m/s).
其中 p = 动量 (kg m/s)，m = 质量 (kg)，v = 速度 (m/s)。
Conservation of momentum: in a closed system, total momentum before a collision equals total momentum after, provided no external forces act.
动量守恒：在没有外力作用的封闭系统中，碰撞前的总动量等于碰撞后的总动量。
Use this principle to calculate velocities after collisions or explosions (in one dimension). Example: two snooker balls collide and move apart.
利用这一原理计算碰撞或爆炸后物体的速度（一维情况）。例如：两个台球碰撞后分开。
8. Forces and Braking – Stopping Distances | 力与制动——停车距离
The stopping distance of a vehicle is the sum of the thinking distance and the braking distance.
车辆的停车距离 = 思考距离 + 制动距离。
Thinking distance is how far the car travels during the driver’s reaction time (affected by alcohol, drugs, tiredness).
思考距离是驾驶员反应时间内车辆行驶的距离（受酒精、药物、疲劳影响）。
Braking distance is how far the car travels under the braking force (affected by speed, road conditions, brakes, tyres).
制动距离是在制动力作用下车辆行驶的距离（受速度、路面状况、刹车、轮胎影响）。
For a given braking force, doubling the speed quadruples the braking distance because kinetic energy = ½mv².
给定制动力时，速度变为原来的两倍，制动距离变为原来的四倍，因为动能 = ½mv²。
You need to explain these factors and interpret stopping-distance graphs in WJEC papers.
在 WJEC 试卷中，你需要解释这些因素并分析停车距离图像。
9. Hooke’s Law and Springs | 胡克定律与弹簧
A spring stretches when a force is applied. For small extensions, the extension is directly proportional to the force.
施加力时弹簧会伸长。在小形变范围内，伸长量与力成正比。
F = k × x
where F = force (N), k = spring constant (N/m), x = extension (m). This is Hooke’s Law.
其中 F = 力 (N)，k = 弹簧劲度系数 (N/m)，x = 伸长量 (m)。这就是胡克定律。
The limit of proportionality is the point beyond which the extension is no longer proportional to the force. After this, the spring deforms permanently.
比例极限是超出后伸长量不再与力成正比的点。此后，弹簧将发生永久形变。
Practical: stretching a spring and plotting a force–extension graph; the gradient gives k. WJEC may ask you to describe this experiment.
实验：拉伸弹簧并绘制力–伸长量图像；斜率表示 k。WJEC 可能要求你描述这个实验。
10. Weight and Gravitational Field Strength | 重量与引力场强度
Weight is the force due to gravity acting on a mass. It is not the same as mass (which is measured in kg).
重量是作用在物体质量上的重力，与质量（以 kg 为单位）不同。
W = m × g
where W = weight (N), m = mass (kg), g = gravitational field strength (N/kg). On Earth, g ≈ 9.8 N/kg or 10 N/kg in exam calculations.
其中 W = 重量 (N)，m = 质量 (kg)，g = 引力场强度 (N/kg)。在地球上，g ≈ 9.8 N/kg，考试计算中通常取 10 N/kg。
The weight of an object depends on g, so it varies on different planets. Mass remains constant everywhere.
物体的重量取决于 g，因此在不同的星球上会变化。质量在任何地方都不变。
Make sure you can use the relationship W = mg in problems involving falling objects or tension in cables.
确保你能在涉及落体或缆绳张力的题目中运用 W = mg。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导