Newton’s Three Laws of Motion Explained | 牛顿三大运动定律解析

📚 Newton’s Three Laws of Motion Explained | 牛顿三大运动定律解析

Newton’s three laws of motion form the foundation of classical mechanics. They describe how forces affect the motion of objects and allow us to predict the motion of everything from a falling apple to a rocket in space.

牛顿三大运动定律是经典力学的基础。它们描述了力如何影响物体的运动，使我们能够预测从落下的苹果到太空中的火箭等多种物体的运动状态。

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1. Newton’s First Law of Motion | 牛顿第一运动定律

Newton’s first law is often called the law of inertia. It states: ‘An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction, unless acted upon by a net external force.’

牛顿第一定律通常被称为惯性定律。它指出：一切物体在没有受到外力作用时，总保持静止状态或匀速直线运动状态。

This law means that if a body is moving at constant velocity, the forces acting on it must balance to zero. A book sliding on a rough table slows down because friction acts; without friction it would keep sliding forever.

该定律表明，如果物体做匀速运动，那么它所受合外力必然为零。书本在粗糙桌面上滑动会减速，是因为受到摩擦力；若无摩擦，它将永远滑动下去。

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2. Inertia and Mass | 惯性与质量

Inertia is the resistance of an object to any change in its velocity, including starting from rest or changing direction. Mass is the quantitative measure of inertia.

惯性是物体抵抗速度变化的属性，包括从静止开始运动或改变方向。质量是惯性大小的量度。

A heavy lorry has more inertia than a bicycle. It is much harder to start moving or stop once it is moving, even if both have the same velocity.

重型卡车比自行车具有更大的惯性。即使两者速度相同，要启动或停止卡车也要困难得多。

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3. Newton’s Second Law of Motion | 牛顿第二运动定律

Newton’s second law relates net force, mass, and acceleration. The net external force on an object is equal to the mass of the object multiplied by its acceleration.

牛顿第二定律把合外力、质量和加速度联系起来：物体的合外力等于质量乘以加速度。

F_net = m × a

The direction of the acceleration is always the same as the direction of the net force. If the net force is zero, the acceleration is zero and velocity remains constant.

加速度的方向总是与合外力的方向相同。如果合外力为零，则加速度为零，速度保持不变。

This law can also be written in momentum form: F_net = Δp / Δt, where p = m × v. This form is more general and is valid even when mass changes.

牛顿第二定律还可写成动量形式：F_net = Δp / Δt，其中 p = m × v。该形式更具普遍性，质量变化时同样适用。

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4. Force, Mass and Acceleration | 力、质量与加速度

The relationship F = ma means that for a fixed force, a smaller mass produces a larger acceleration; for a fixed mass, a larger force produces a larger acceleration.

F = ma 表明：当力一定时，质量越小则加速度越大；当质量一定时，力越大则加速度越大。

For example, if a 2 kg object receives a net force of 10 N, then a = 10 ÷ 2 = 5 m/s². If the mass is increased to 5 kg with the same force, the acceleration becomes 2 m/s².

例如，一个 2 kg 的物体受到 10 N 的合外力，则 a = 10 ÷ 2 = 5 m/s²。如果质量增大到 5 kg，同样大小的力产生的加速度变为 2 m/s²。

a = F_net / m

In practice, if you push a shopping trolley and then push it when full, you experience how a larger mass leads to a smaller acceleration for the same push.

实际中，推空购物车与推装满货物的购物车相比，同样的推力产生的加速度会小得多。

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5. Units of Force | 力的单位

In SI units, force is measured in newtons (N). One newton is the force required to accelerate a mass of 1 kilogram at a rate of 1 metre per second squared.

在国际单位制中，力的单位是牛顿（N）。1 牛顿是使质量为 1 千克的物体产生 1 米每平方秒加速度所需的力。

1 N = 1 kg·m/s²

Quantity	Symbol	SI Unit
Force	F	N (kg·m/s²)
Mass	m	kg
Acceleration	a	m/s²

Gravity near Earth’s surface gives every object an acceleration of about 9.8 m/s², so an object’s weight is W = m × g.

地球表面附近的重力使物体产生约 9.8 m/s² 的加速度，因此物体重力为 W = m × g。

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6. Newton’s Third Law of Motion | 牛顿第三运动定律

Newton’s third law states: ‘To every action there is always an opposed and equal reaction.’ In modern terms, if object A exerts a force on object B, then object B exerts an equal and opposite force on object A.

牛顿第三定律指出：两个物体之间的作用力与反作用力总是大小相等、方向相反，且作用在不同物体上。

When you push against a wall, the wall pushes back with exactly the same magnitude of force. When a rocket expels hot gas downward, the gas pushes the rocket upward.

当你推墙时，墙也在以同样大小的力推你。当火箭向下喷射高温气体时，气体给火箭一个向上的推力。

It is important to note that the action and reaction forces cannot cancel each other because they act on different bodies.

需要特别注意：作用力与反作用力不能相互抵消，因为它们作用在不同物体上。

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7. Common Misconceptions | 常见误解

Students often misinterpret the three laws. One common misconception is that a force must be continuously applied to keep an object moving. In fact, only a net force is needed to change motion, not to maintain constant velocity.

学生常对三大定律产生误解。一种常见错误是认为物体运动必须持续用力维持。实际上，只有改变运动状态才需要合外力，匀速运动不需要。

Another misconception is that action and reaction forces cancel. They do not cancel because they act on different objects. A third is that the second law only applies to constant mass; the momentum form covers variable mass cases.

另一个误解是作用力与反作用力相互抵消。它们作用在不同物体上，因此不能抵消。还有人认为第二定律只适用于质量不变的情况；其实动量形式可以处理质量变化的情形。

To avoid confusion, always identify the object of interest and compare only the forces acting on that object.

为避免混淆，应始终明确研究对象，并只比较作用在该物体上的力。

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8. Applying Newton’s Laws to Problem Solving | 应用牛顿定律解题

When solving problems with Newton’s laws, follow a systematic method: draw a free-body diagram, label all forces, choose a coordinate system, resolve forces into components, and apply F_net = ma.

用牛顿定律解题时，应遵循系统步骤：画出受力分析图，标出所有力，选取坐标系，分解力，然后应用 F_net = ma。

Example: A 3 kg block on a frictionless surface is pulled by a 12 N horizontal force. The acceleration is a = F/m = 12/3 = 4 m/s².

例：光滑水平面上，一个 3 kg 的物块受到 12 N 的水平拉力。其加速度为 a = F/m = 12/3 = 4 m/s²。

a = 12 N ÷ 3 kg = 4 m/s²

If there is friction, the net force is the applied force minus friction. Always calculate the resultant force first.

若有摩擦力，则合外力为拉力减去摩擦力。务必先求合力，再算加速度。

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9. Newton’s Laws in Everyday Life | 日常生活中的牛顿定律

Newton’s laws explain many daily phenomena. Tension in a rope, normal forces from surfaces, and friction all obey these laws. A car’s seatbelt prevents you from continuing forward when it brakes suddenly due to inertia.

牛顿定律解释了许多日常现象。绳中的张力、桌面的支持力和摩擦力都遵循这些规律。刹车时安全带防止你因惯性继续向前冲。

Walking relies on the third law: your foot pushes backward on the ground, and the ground pushes forward on you. A swimmer pushes water backward and water pushes the swimmer forward.

行走依赖于第三定律：脚向后蹬地，地向前推人。游泳者向后推水，水向前推游泳者。

Ball sports involve the second law: kicking a football with a larger force gives it a larger acceleration, and a heavier ball accelerates less for the same kick.

球类运动涉及第二定律：踢足球时力越大，加速度越大；同样一脚踢出，较重的球加速度较小。

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10. Summary | 总结

Newton’s laws summarised: the first law defines inertia, the second law quantifies how force affects acceleration, and the third law describes the mutual nature of forces.

牛顿三定律总结：第一定律定义惯性，第二定律定量描述了力如何影响加速度，第三定律描述力的相互性。

They are extremely successful in everyday and engineering contexts, but they break down for speeds close to the speed of light and for very small objects. In those regimes, Einstein’s relativity and quantum mechanics are used.

它们在日常生活和工程中极其成功，但在接近光速和微观尺度下不再适用。此时需要使用爱因斯坦的相对论和量子力学。

Law	Statement	Key Idea
First	No net force → constant velocity	Inertia
Second	F_net = m × a	Quantitative relation
Third	Equal and opposite forces on different bodies	Action and reaction

Mastering these laws is essential for solving mechanics problems in physics examinations, including A-level and IGCSE.

掌握三大定律是解决 A-level 和 IGCSE 等物理考试中力学问题的关键。

Published by TutorHao | Physics Revision Series | aleveler.com

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