IGCSE CIE Physics: Newton's Laws Key Points | IGCSE CIE 物理：牛顿定律考点精讲

📚 IGCSE CIE Physics: Newton’s Laws Key Points | IGCSE CIE 物理：牛顿定律考点精讲

Newton’s laws of motion form the backbone of classical mechanics and are absolutely essential for IGCSE CIE Physics. Whether you are analysing the motion of a car, explaining why a rocket launches, or solving numerical problems on force and acceleration, these three laws underpin every calculation and concept. This revision guide breaks down each law, explores key applications, warns about common exam pitfalls, and provides clear, exam-focused explanations to help you master the topic.

牛顿运动定律是经典力学的核心，也是IGCSE CIE物理必考的重点内容。无论你是在分析汽车的运动、解释火箭为何能升空，还是解答关于力和加速度的计算题，这三条定律始终是基础。本考点精讲将逐条拆解定律，深入讲解关键应用，提醒常见易错点，提供清晰且紧扣考试的讲解，助你彻底掌握本专题。

1. Newton’s First Law of Motion (Inertia) | 牛顿第一定律（惯性）

Newton’s first law states that an object will remain at rest or continue moving at a constant velocity in a straight line unless acted upon by a resultant (net) external force. This property of an object to resist changes in its state of motion is called inertia. The greater the mass of an object, the greater its inertia, and the harder it is to change its velocity.

牛顿第一定律指出，除非受到合外力的作用，物体将保持静止或沿直线做匀速运动。物体抵抗运动状态改变的性质称为惯性。物体的质量越大，惯性越大，越难改变其速度。

In everyday contexts, you experience inertia when a car brakes suddenly and your body lurches forward. Your body tries to maintain its forward motion while the car decelerates. Similarly, a tablecloth trick relies on inertia: pulling the cloth quickly leaves the objects on top almost stationary because their inertia resists the sudden motion.

日常生活中，当汽车急刹车时身体会前倾，这就是惯性的体现——你的身体试图保持原本向前的运动，而汽车在减速。同理，快速抽走桌布的魔术也利用了惯性：迅速抽走桌布时，桌上的物体由于惯性几乎保持原地不动。

It is crucial to understand that a constant velocity means both constant speed and constant direction; any change in speed or direction requires a resultant force. In the absence of a resultant force, an object’s velocity does not change at all.

必须理解匀速指的是速度大小和方向均不变；任何速度大小或方向的改变都需要合外力。没有合外力时，物体的速度完全不会改变。

2. Newton’s Second Law and F = ma | 牛顿第二定律与 F = ma

Newton’s second law quantifies the relationship between force, mass and acceleration. It states that the acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass. The direction of the acceleration is the same as the direction of the resultant force.

牛顿第二定律定量描述了力、质量和加速度之间的关系：物体的加速度与它所受到的合外力成正比，与它的质量成反比，加速度的方向与合外力的方向相同。

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). This equation is one of the most used in IGCSE Physics, so you must be able to rearrange it to find any variable: a = F / m, m = F / a.

公式中，F 是合外力，单位牛顿 (N)；m 是质量，单位千克 (kg)；a 是加速度，单位米每二次方秒 (m/s²)。这个公式是IGCSE物理中最常用的计算式之一，你必须能熟练变形求解任意未知量：a = F / m，m = F / a。

An important exam point is that F represents the resultant (net) force, not just any applied force. If multiple forces act, you must first determine the vector sum. For example, a car engine provides a driving force of 2000 N, but friction opposes it with 500 N. The resultant force is 1500 N in the direction of motion, and this is the force you use to calculate acceleration.

一个重要的考点是，F 代表合外力，而非某个施加的力。如果多个力同时作用，必须先求出矢量和。例如，汽车发动机提供 2000 N 的驱动力，但摩擦力反向为 500 N。此时的合外力是运动方向上的 1500 N，这个力才用来计算加速度。

3. Newton’s Third Law: Action and Reaction | 牛顿第三定律：作用力与反作用力

Newton’s third law states that when one object exerts a force on a second object, the second object exerts a force of equal magnitude but opposite direction on the first object. These forces are often called action and reaction. They act on different objects, never on the same object, and are of the same type (e.g. both gravitational, both contact forces).

牛顿第三定律指出，当一个物体对另一个物体施加力时，第二个物体同时会对第一个物体施加一个大小相等、方向相反的力。这对力常被称为作用力与反作用力。它们作用在不同的物体上，绝不是作用在同一个物体上，而且力的性质相同（例如都是引力或都是接触力）。

A classic example: a book resting on a table. The book exerts a downward contact force on the table due to its weight. The table exerts an equal and upward contact force on the book. These two forces are an action-reaction pair. Note that the book’s weight and the upward contact force on the book are not an action-reaction pair because they act on the same object (the book) and are of different types. The weight’s reaction force is the gravitational pull of the book on the Earth.

经典例子：桌上放着一本书。书对桌面施加向下的接触力，桌子对书施加等大向上的接触力，这是一对作用力与反作用力。注意，书的重力与桌面对书的支持力并非一对作用力与反作用力，因为它们都作用在书（同一个物体）上，且类型不同。重力的反作用力是书对地球的引力。

Rocket propulsion perfectly illustrates the third law: hot gases are expelled downwards with great force, and the rocket experiences an equal and opposite upward force that propels it. Similarly, when you swim, you push water backwards, and the water pushes you forwards.

火箭推进是第三定律的绝佳例子：高温气体被高速向下喷出，火箭则受到一个等大反向的向上力而升空。同样，游泳时你向后推水，水就向前推你。

4. The Unit of Force: The Newton | 力的单位：牛顿

From F = ma, one newton is defined as the force required to give a mass of 1 kg an acceleration of 1 m/s². Therefore, 1 N = 1 kg m/s². This definition often appears in multiple-choice questions, so it is worth memorising.

根据 F = ma，1 牛顿被定义为使 1 kg 的物体产生 1 m/s² 加速度所需的力。因此 1 N = 1 kg m/s²。这个定义常在选择题中直接考查，需要记住。

When solving numerical problems, always ensure mass is in kg and acceleration in m/s² to obtain a force in newtons. If mass is given in grams, convert to kilograms by dividing by 1000. If acceleration is given in cm/s², convert to m/s² by dividing by 100.

解计算题时，必须保证质量用千克 (kg)，加速度用 m/s²，得到的力才是牛顿。如果题目给出质量单位是克，先除以 1000 化为千克；若加速度以 cm/s² 给出，先除以 100 化为 m/s²。

The newton is a vector unit because force is a vector quantity. You need to specify both magnitude and direction when describing a force. A force of 5 N to the right is different from 5 N to the left, even if the magnitude is the same.

牛顿是矢量单位，因为力是矢量。描述一个力时既要说大小，也要指明方向。向右的 5 N 和向左的 5 N 即使大小相同，效果也不同。

5. Mass vs. Weight | 质量与重量的区别

Mass is a measure of the amount of matter in an object and is a scalar quantity measured in kilograms (kg). It does not change with location. Weight, however, is the gravitational force acting on an object’s mass. It is a vector quantity measured in newtons (N) and depends on the local gravitational field strength, g.

质量是物体所含物质的多少，是标量，单位是千克 (kg)，不会随位置改变。重量则是作用在物体质量上的引力，是矢量，单位是牛顿 (N)，取决于当地引力场强度 g。

W = m × g

On Earth, g ≈ 9.8 m/s², so an object of mass 1 kg has a weight of 9.8 N. On the Moon, where g is about 1.6 m/s², the same 1 kg mass would weigh only 1.6 N, but its mass remains 1 kg. Many students confuse mass and weight; be careful to use the correct terminology and units in exams.

在地球表面，g 约等于 9.8 m/s²，因此质量为 1 kg 的物体重量约为 9.8 N。在月球上 g 约 1.6 m/s²，同样是 1 kg 质量，重量仅 1.6 N，但质量仍然是 1 kg。很多同学会把质量和重量搞混，考试中务必使用正确的术语和单位。

When you weigh yourself on a bathroom scale, the scale actually measures the contact force (normal reaction), which is calibrated to read your mass in kg assuming g = 9.8 m/s². In physics, you should appreciate that you are measuring a force that is proportional to mass, not mass directly.

你站在体重秤上时，秤实际测量的是支持力（法向接触力），再根据 g=9.8 m/s² 的假设标定成以千克为单位的读数。物理上应明白，你测量的是一个与质量成正比的力，而非质量本身。

6. Free-Body Diagrams | 受力分析图

A free-body diagram is a simplified sketch showing all the forces acting on a single object. Each force is represented by an arrow pointing in the direction of the force, with its length roughly proportional to the magnitude. Being able to draw and interpret free-body diagrams is essential for solving problems involving Newton’s laws.

受力分析图是一种简化示意图，只画出作用在某一个物体上的所有力。每个力用指向作用方向的箭头表示，长度大致与力的大小成比例。能正确画出并分析受力图对运用牛顿定律解题至关重要。

For example, a car moving at constant velocity has the driving force balanced by air resistance and friction; the upward contact force from the road balances the car’s weight. The free-body diagram would show four forces: weight downwards, normal reaction upwards, driving force to the right, resistive forces to the left. Since the velocity is constant, the resultant force is zero, meaning the left and right forces are equal and the up and down forces are equal.

例如，一辆匀速行驶的汽车，驱动力与空气阻力和摩擦力平衡；路面向上的支持力与重力平衡。受力图应表示四个力：向下的重力、向上的支持力、向右的驱动力、向左的阻力。因为速度恒定，合外力为零，所以左右力相等，上下力也相等。

In IGCSE exams, you may be asked to identify the resultant force from a free-body diagram or to calculate unknown forces when the object is in equilibrium. Always start by drawing a clear diagram and labeling each force.

IGCSE 考试可能要求你从受力图中找出合外力，或在物体平衡时计算未知力。务必先画出清晰的受力分析图，并标出每一个力。

7. Resultant Force and Equilibrium | 合外力与平衡

The resultant force (or net force) is the single force that has the same effect as all the individual forces acting on an object combined. When the resultant force is zero, the forces are balanced, and the object is in equilibrium. According to Newton’s first law, an object in equilibrium either remains at rest or moves with constant velocity in a straight line.

合外力（或净力）是能等效替代所有作用在物体上的力的单一力。当合外力为零时，各力平衡，物体处于平衡状态。根据牛顿第一定律，处于平衡的物体要么保持静止，要么沿直线做匀速运动。

If the resultant force is not zero, the object accelerates in the direction of the resultant force. The acceleration is calculated using F = ma, where F is this resultant. An object can be accelerating while moving at high speed, or it can be decelerating (negative acceleration) if the resultant force opposes motion.

如果合外力不为零，物体就会朝合外力的方向加速。加速度由 F = ma 计算，这里的 F 就是合外力。物体既可以高速运动同时加速，如果合外力与运动方向相反，物体也会做减速运动（负加速度）。

A common trick in exams is to give you multiple forces and ask for the acceleration. You must add forces vectorially: forces in the same direction add up, forces in opposite directions subtract. Only use the net force in the equation F = ma.

考试中常见的设题方式是给出多个力，让你求加速度。你必须进行力的矢量合成：同向相加，反向相减。代入 F = ma 的只能是净力。

8. Friction and Air Resistance | 摩擦力和空气阻力

Friction is a contact force that opposes the relative motion between two surfaces in contact. It arises from microscopic irregularities and adhesion. Air resistance (drag) is a type of friction that acts on objects moving through air. Both friction and air resistance always act in the opposite direction to motion or attempted motion.

摩擦力是一种接触力，阻碍两个接触表面间的相对运动，来源于微观凹凸和附着作用。空气阻力（曳力）是物体在空气中运动时受到的摩擦类阻力。摩擦力和空气阻力的方向总是与运动方向或运动趋势相反。

In many mechanics problems, friction and air resistance are the reasons why a constant driving force does not lead to unbounded acceleration. As speed increases, air resistance increases until it balances the driving force; then the resultant force becomes zero and the object moves at a constant terminal velocity. This is why a skydiver eventually stops accelerating and falls at a steady speed.

在许多力学问题中，摩擦和空气阻力解释了为什么恒定的驱动力不会导致无穷的加速。随着速度增大，空气阻力也不断增加，直到与驱动力平衡；此时合外力变为零，物体达到终极速度而匀速运动。跳伞者最终不再加速、匀速下落就是这个道理。

On IGCSE papers, you might be asked to explain how reducing friction (e.g. using lubricants or streamlining) affects motion. Lubrication reduces friction between solid surfaces; streamlining reduces air resistance by shaping objects smoothly, which lowers the drag force and increases efficiency.

IGCSE 试卷中可能要求解释减少摩擦力（如使用润滑剂或流线型设计）如何影响运动。润滑能减小固体间的摩擦；流线型设计通过光滑的外形减小空气阻力，即降低曳力，提升效率。

9. Experiment: Investigating Newton’s Second Law | 实验：探究牛顿第二定律

A typical IGCSE experiment investigates the relationship between force, mass and acceleration using a dynamics trolley, a pulley and hanging masses. First, to verify F ∝ a with constant mass, you vary the accelerating force (by changing the hanging mass) and measure the resulting acceleration, keeping the total mass of the system constant. You can plot a graph of force against acceleration, which should yield a straight line through the origin.

典型的 IGCSE 实验使用动力学小车、滑轮和悬吊砝码来探究力、质量与加速度的关系。首先，控制质量不变，改变加速力（通过改变悬挂砝码的质量）并测量加速度，验证 F ∝ a。保持系统总质量不变，绘制力-加速度图像，应得到一条过原点的直线。

Secondly, to verify a ∝ 1/m with constant force, you keep the accelerating force constant while varying the mass of the trolley (adding extra masses on top). Acceleration is measured, and plotting acceleration against 1/mass gives a straight line. In all cases, you must use a data logger or stopwatch and light gates to measure acceleration accurately, and repeat measurements to improve reliability.

然后，控制力不变，改变小车质量（在小车上增加砝码），测量加速度，验证 a ∝ 1/m。绘制加速度-（1/质量）图像会得到直线。实验过程中需使用数据记录仪或秒表和光电门精确测量加速度，并多次测量以提高可靠性。

A key detail: the force that actually accelerates the trolley is the tension in the string, not simply the weight of the hanging mass. However, if the hanging mass is very small compared to the trolley mass, the tension is approximately equal to the weight of the hanging mass. Exam questions often ask about the need for friction compensation (tilting the track slightly) so that the only horizontal force is the tension.

关键细节：真正使小车加速的力是绳子的张力，而非仅仅悬挂砝码的重量。不过当悬挂质量远小于小车质量时，张力近似等于悬挂砝码的重量。考试常问为什么要补偿摩擦力（将轨道稍微垫高），这样水平方向的合力就只剩下张力。

10. Applications in Everyday Life | 日常生活中的应用

Newton’s laws explain countless everyday phenomena. Seatbelts and airbags in cars are designed around Newton’s first law: they provide the unbalanced force needed to stop a passenger’s body in a collision, preventing the body from continuing at high speed into the dashboard. Similarly, head restraints prevent whiplash by providing a backward force on the head during a rear-end collision.

牛顿定律可以解释无数日常现象。汽车安全带和安全气囊就是基于牛顿第一定律设计的：它们提供碰撞时乘客身体减速所需的非平衡力，防止身体因惯性高速撞向仪表盘。同理，头枕在追尾碰撞中对头部施加向后的力，以预防挥鞭伤。

In sports, a football being kicked illustrates all three laws: the ball remains at rest until kicked (first), the acceleration depends on the kicking force and ball’s mass (second), and the kicker’s foot feels a force equal to the one applied to the ball (third). A sprinter pushing backwards against the starting blocks uses the third law to gain forward acceleration.

体育运动中，踢足球的过程就同时体现了三条定律：球静止直到被踢（第一定律），球的加速度取决于踢力和球的质量（第二定律），踢球人的脚也感受到与作用在球上等大的力（第三定律）。短跑选手向后蹬起跑器同样是利用第三定律来获得向前加速。

Understanding these applications helps you answer long-answer questions where you need to describe the physics behind real-world situations. Always identify the forces, refer to the appropriate law, and state clearly the outcome of the forces.

理解这些应用有助于回答需要描述现实情景物理原理的简答题。解题时务必先分析受力，引用相应的定律，并清晰说明力的作用效果。

11. Common Exam Pitfalls | 常见考点及易错点

Many students lose marks by confusing mass and weight. Remember: mass is in kg, weight is a force in N. Do not write ‘weight = 50 kg’ on an answer line; instead write ‘weight = 500 N’ (assuming g = 10 N/kg or 9.8 N/kg). Also, always use the correct formula W = m g with consistent units.

很多同学因混淆质量和重量而丢分。牢记质量单位是 kg，重量是力，用 N。不要在填空处写“重量=50 kg”，而应写成“重量=500 N”（假设 g 取 10 N/kg 或 9.8 N/kg）。始终用 W = m g 并保持单位统一。

Another common mistake is forgetting that F = ma uses the resultant force, not simply an applied force. When multiple forces are given, always calculate the net force first. Also, when an object moves at constant velocity, resultant force is zero, meaning driving force equals resistive forces; do not assume there is no driving force just because acceleration is zero.

另一个常见错误是忘记 F = ma 中的 F 是合外力，而不是某个施加的力。如果题中给了多个力，必须先求净力。此外，当物体做匀速运动时，合外力为零，意味着驱动力等于阻力；不要因为加速度为零就误以为没有驱动力。

For the third law, avoid stating that action and reaction forces act on the same object or cancel each other out. They act on different objects, so they never cancel. Also, be precise with direction: ‘the Earth pulls the book down’ and ‘the book pulls the Earth up’ are an action-reaction pair.

第三定律的常见陷阱是误认为作用力和反作用力作用在同一个物体上，或者相互抵消。它们作用在不同物体上，所以不会抵消。方向表述也要准确，例如“地球向下拉书”和“书向上拉地球”即为一对作用与反作用力。

In free-body diagrams, draw arrows to represent forces, not velocity or acceleration. Label forces clearly, and ensure arrow lengths reflect relative magnitudes when forces are balanced or unbalanced. Practise drawing such diagrams because they are often the first mark in a structured question.

画受力分析图时，箭头表示力，而非速度或加速度。清楚标注各力，当力平衡或不平衡时，箭头长度应反映相对大小。多练习画受力图，因为这类图往往是结构化问题的第一个得分点。

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IGCSE CIE Physics: Newton’s Laws Key Points | IGCSE CIE 物理：牛顿定律 考点精讲