Forces & Movement | 力与运动

📚 Forces & Movement | 力与运动

In IGCSE Physics, understanding forces and movement is fundamental to explaining how objects interact and change their state of motion. The Edexcel IGCSE Physics topic ‘Forces & Movement’ explores scalars and vectors, Newton’s three laws of motion, friction, momentum, and impulse. Mastering these concepts allows you to predict and describe everything from a falling apple to a car crash, and they form the backbone of mechanics. This article covers key definitions, equations, experimental ideas, and real-world applications to support your exam preparation.

在IGCSE物理中，理解力与运动是解释物体如何相互作用和改变运动状态的基础。Edexcel IGCSE物理的”力与运动”主题探讨了标量和矢量、牛顿三大运动定律、摩擦力、动量和冲量。掌握这些概念可以让你预测和描述从下落的苹果到汽车碰撞的种种现象，它们构成了力学的支柱。本文涵盖关键定义、方程、实验思路和实际应用，为你的备考提供支持。

1. Scalars and Vectors | 标量和矢量

A scalar quantity has only magnitude (size) and no direction. Common examples include distance, speed, mass, time, and energy. When you add scalars, you use ordinary arithmetic – for example, a total distance of 5 m plus 3 m gives 8 m.

标量只有大小（量值）而没有方向。常见的例子包括距离、速率、质量、时间和能量。标量相加使用普通算术——例如，5 m的距离加上3 m得到8 m。

A vector quantity has both magnitude and a specific direction. Displacement, velocity, acceleration, force, and momentum are all vectors. Vectors are represented by arrows: the length shows the magnitude, and the arrow points in the direction of the quantity. When adding vectors, you must account for direction – for instance, by placing them tip to tail or resolving into perpendicular components.

矢量既有大小也有特定的方向。位移、速度、加速度、力和动量都是矢量。矢量用箭头表示：长度表示大小，箭头指向该量的方向。矢量相加时必须考虑方向——例如，采用箭头接尾法或分解为垂直分量。

It is vital to distinguish between distance (a scalar) and displacement (a vector). Distance travelled is the total length of the path, whereas displacement is the straight-line distance in a straight line from start to finish, with its direction stated.

区分距离（标量）和位移（矢量）至关重要。经过的距离是路径的总长度，而位移是从起点到终点的直线距离，并说明其方向。

• Scalars: distance, speed, mass
• Vectors: displacement, velocity, weight

• 标量：距离、速率、质量
• 矢量：位移、速度、重量

2. Types of Forces | 力的类型

Forces can be classified as contact or non-contact. Contact forces act when two objects touch, and include tension in a rope, friction on a surface, normal reaction force, air resistance (drag), and upthrust. Non-contact forces act over a distance without physical contact, such as gravitational force, electrostatic force, and magnetic force.

力可分为接触力和非接触力。接触力在两个物体接触时作用，包括绳子的张力、表面摩擦力、法向反作用力、空气阻力（阻力）和浮力。非接触力无需物理接触就能作用，如引力、静电力和磁力。

Weight is a non-contact force caused by gravity. It is a vector that always points vertically downwards towards the centre of the Earth (or other massive body). The size of an object’s weight is given by the equation:

W = m × g

where W is weight in newtons (N), m is mass in kilograms (kg), and g is gravitational field strength. On Earth, g ≈ 9.8 N/kg, but for IGCSE we often use 10 N/kg for simplicity.

重量是由引力引起的非接触力。它是一个矢量，始终竖直向下指向地心（或其他大质量天体的中心）。物体重量的大小由方程给出：W = m × g，其中W是重量，单位为牛顿（N）；m是质量，单位为千克（kg）；g是引力场强度。地球表面g≈9.8 N/kg，但IGCSE中常取10 N/kg以简化计算。

3. Free-Body Diagrams and Resultant Force | 自由体图和合力

A free-body diagram is a sketch that shows all the forces acting on a single object. The object is drawn as a dot or a box, and the forces are represented by arrows pointing away from it. Label each force clearly. This tool helps you see whether forces are balanced or unbalanced.

自由体图是表示作用在单个物体上所有力的简图。物体画成一个点或方框，力用从物体发出的箭头表示。清楚标注每个力。这个工具帮助你判断力是平衡的还是非平衡的。

The resultant force (also called the net force) is the single force that has the same effect as all the original forces combined. It is found by vector addition: forces acting in the same direction add up, while forces in opposite directions subtract. If two forces of 10 N right and 4 N left act on a box, the resultant force is 6 N to the right.

合力（也称净力）是与所有原始力共同作用效果相同的单一力。它通过矢量加法求得：同向力相加，反向力相减。如果一个箱子上作用10 N向右和4 N向左的力，则合力为6 N向右。

If the resultant force is zero, the forces are balanced. According to Newton’s First Law, the object will remain at rest or continue moving at constant velocity in a straight line. If the resultant force is not zero, the object accelerates in the direction of the resultant force.

如果合力为零，则各力平衡。根据牛顿第一定律，物体将保持静止或继续沿直线匀速运动。如果合力不为零，物体将沿合力方向加速。

4. Newton’s First Law | 牛顿第一定律

Newton’s First Law states: An object will remain at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force. This property of matter is called inertia – the tendency to resist any change in its state of motion.

牛顿第一定律指出：除非受到外部非平衡力的作用，物体将保持静止或匀速直线运动状态。物质的这种性质叫做惯性——抵抗运动状态变化的倾向。

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