📚 Newton’s Laws: Key Points for IB AQA Physics | IB AQA 物理：牛顿定律 考点精讲
Sir Isaac Newton’s three laws of motion form the backbone of classical mechanics and are essential for the IB Physics curriculum (AQA-style examinations). Mastering these laws enables you to analyse forces, predict motion, and solve problems involving equilibrium, acceleration, and interaction. This article breaks down each law, common applications, free-body diagrams, and exam strategies tailored for IB AQA Physics.
艾萨克·牛顿的三大运动定律是经典力学的支柱，也是IB物理（AQA考试风格）的核心内容。掌握这些定律能够让你分析力、预测运动，并解决涉及平衡、加速和相互作用的问题。本文逐一解析每条定律、常见应用、受力分析图以及针对IB AQA物理的考试策略。
1. Newton’s First Law & Inertia | 牛顿第一定律与惯性
Newton’s first law states that an object at rest stays at rest, and an object in motion continues in uniform motion in a straight line, unless acted upon by a net external force. This property of matter is called inertia – the tendency to resist changes in velocity. Inertia depends solely on mass; the greater the mass, the greater the inertia.
牛顿第一定律指出，一切物体在不受合外力作用时，总保持静止状态或匀速直线运动状态。这种属性叫做惯性——物体抵抗速度变化的趋势。惯性只取决于质量；质量越大，惯性越大。
For the IB AQA exam, you must be able to identify when an object has zero net force (equilibrium) and thus zero acceleration, even if it is moving at constant speed. Common examples include a book resting on a table (weight and normal force balance) and a car cruising at constant velocity (engine force balances air resistance and friction).
在IB AQA考试中，你必须能够识别出物体所受合外力为零（平衡状态）因而加速度为零的情况，即使物体正以恒定速度运动。常见例子包括：静止在桌面上的书（重力与法向力平衡）、匀速行驶的汽车（发动机驱动力与空气阻力和摩擦力平衡）。
2. Newton’s Second Law (F = ma) | 牛顿第二定律 (F = ma)
The second law quantifies the relationship between net force, mass, and acceleration: Fnet = m a. The net force is the vector sum of all forces acting on an object. Acceleration is directly proportional to net force and inversely proportional to mass, and it always points in the same direction as the net force.
第二定律量化了合外力、质量和加速度之间的关系：F合 = m a。合外力是作用在物体上所有力的矢量和。加速度与合外力成正比，与质量成反比，并且始终与合外力方向相同。
When solving problems, always resolve forces into perpendicular components (typically horizontal and vertical) and apply Fnet = m a separately in each direction. Remember that the units must be consistent: mass in kg, acceleration in m s−2, and force in newtons (N).
解题时，务必将力分解为相互垂直的分量（通常为水平和竖直方向），并分别对每个方向应用 F合 = m a。注意单位须保持一致：质量用千克（kg），加速度用米每二次方秒（m s−2），力用牛顿（N）。
3. Newton’s Third Law | 牛顿第三定律 (作用力与反作用力)
Newton’s third law: If body A exerts a force on body B, then body B exerts an equal but opposite force on body A: FA on B = − FB on A. These forces are always of the same type (e.g., both gravitational, both normal) and act on different bodies. They never cancel out because they act on different objects.
牛顿第三定律：若物体A对物体B施加一个力，则物体B同时对物体A施加一个大小相等、方向相反的力：FA对B = − FB对A。这两个力总是同种性质（比如都是引力、都是法向力）且作用在不同物体上。它们永远不会抵消，因为它们作用在不同物体上。
A classic exam pitfall is confusing the normal force with the weight as an action–reaction pair. The weight (Earth pulling on object) pairs with the object pulling on Earth. The normal force (table pushing up on object) pairs with the object pushing down on the table. Always identify the two interacting objects.
考试中一个经典误区是将法向力与重力误认为一对作用力与反作用力。重力（地球吸引物体）的反作用力是物体吸引地球。法向力（桌子向上推物体）的反作用力是物体向下压桌子。一定要明确指出相互作用的两个物体。
4. Free-Body Diagrams | 受力分析图
A free-body diagram (FBD) is a simplified sketch showing all forces acting on a single object, drawn as arrows originating from the object’s center. It is the essential first step in solving Newton’s laws problems. Label each force clearly: weight (mg), normal (N or R), tension (T), friction (f), applied forces (F).
受力分析图（Free-body diagram, FBD）是一种简图，表示作用在单个物体上的所有力，用从物体中心出发的箭头表示。这是解决牛顿定律问题的关键第一步。清晰标出每个力：重力（mg）、法向力（N或R）、张力（T）、摩擦力（f）、施加的力（F）。
Follow these steps: 1) choose the object of interest; 2) draw it as a dot or box; 3) draw all force vectors with correct direction and approximate relative magnitude; 4) establish a coordinate system (tilted axes for inclined planes); 5) resolve forces into components if needed.
遵循以下步骤：1) 选定研究对象；2) 将其画作点或方块；3) 画出所有力矢量，方向正确且相对长度大致反映大小；4) 建立坐标系（斜面问题时倾斜坐标轴）；5) 必要时将力分解为分量。
5. Tension, Normal Force & Friction | 张力、法向力与摩擦力
Tension is the pulling force transmitted along a rope, string, or cable when it is pulled tight by forces acting from opposite ends. For a light, inextensible string, tension is the same at both ends. Normal force is the contact force perpendicular to the surfaces pushing two objects apart; it adjusts to balance other perpendicular forces but does not always equal mg.
张力是沿绳、线或索传递的拉力，当两端受到相反方向的力拉紧时产生。对于轻质且不可伸长的绳，两端张力大小相等。法向力是垂直于接触面将两物体推开的接触力；它会自动调整以平衡其他垂直方向的力，但不一定等于 mg。
Friction opposes relative motion or attempted motion between surfaces. Static friction (fs ≤ μs N) prevents sliding; kinetic friction (fk = μk N) acts during sliding. The coefficients μs and μk are dimensionless and usually μs > μk.
摩擦力阻碍接触面之间的相对运动或相对运动趋势。静摩擦力（fs ≤ μs N）阻止滑动；动摩擦力（fk = μk N）在滑动时起作用。摩擦系数 μs 和 μk 无量纲，通常 μs > μk。
6. Objects on Inclined Planes | 斜面上的物体
For an object on a smooth inclined plane, the weight mg is resolved into components parallel and perpendicular to the slope: mg sin θ down the slope, and mg cos θ perpendicular to the slope. If the plane is frictionless, the net force along the slope is mg sin θ, giving acceleration a = g sin θ.
对于光滑斜面上的物体，重力 mg 分解为平行于斜面和垂直于斜面的分量：沿斜面向下的 mg sin θ，垂直于斜面的 mg cos θ。如果平面无摩擦，沿斜面的合外力为 mg sin θ，因此加速度 a = g sin θ。
With friction, the kinetic friction force fk = μk mg cos θ acts up the slope (opposing motion). The net force becomes mg sin θ − μk mg cos θ, and acceleration a = g (sin θ − μk cos θ). For an object to slide down with constant velocity, sin θ = μk cos θ, so tan θ = μk.
有摩擦时，动摩擦力 fk = μk mg cos θ 沿斜面向上（与运动方向相反）。合外力变为 mg sin θ − μk mg cos θ，加速度 a = g (sin θ − μk cos θ)。若物体沿斜面匀速下滑，则 sin θ = μk cos θ，即 tan θ = μk。
7. Connected Masses (Pulley Systems) | 连接体问题（滑轮系统）
In Atwood machines or two-mass systems over a light frictionless pulley, the tension is uniform throughout the rope. Apply Newton’s second law to each mass separately, taking into account the direction of acceleration. For example, if m1 > m2, then m1 accelerates downward and m2 accelerates upward with the same magnitude a.
在阿特伍德机或者绕过光滑轻质滑轮的两体系统中，绳中张力处处相等。分别对每个物体应用牛顿第二定律，注意加速度的方向。例如，若 m1 > m2，则 m1 向下加速，m2 以相同大小的
Published by TutorHao | IB Physics 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