📚 Newton's Third Law of Motion | 牛顿第三运动定律
Newton's third law of motion is one of the most misunderstood topics in A-Level Physics. It states that forces always occur in pairs acting on different bodies. This article explains the law, develops the skill of identifying action-reaction pairs, and distinguishes them from balanced forces. Mastering this law is essential for topics such as momentum, collisions, rocket propulsion and connected-body problems.
牛顿第三运动定律是 A-Level 物理中最容易被误解的主题之一。它指出力总是成对出现，并作用在不同物体上。本文将解释该定律，培养识别作用力与反作用力对的技能，并将其与平衡力区分开。掌握该定律对于动量、碰撞、火箭推进和连接体等问题至关重要。
1. Statement of the Law | 定律表述
Newton's third law states: if body A exerts a force on body B, then body B simultaneously exerts a force on body A. These two forces are equal in magnitude, opposite in direction and act along the same straight line.
牛顿第三定律指出：如果物体 A 对物体 B 施加一个力，那么物体 B 同时会对物体 A 施加一个力。这两个力大小相等、方向相反，并且作用在同一条直线上。
F(A on B) = −F(B on A)
The negative sign shows that the force on B due to A points in the opposite direction to the force on A due to B. The two forces are always of the same type, for example both gravitational or both electrostatic.
负号表示 A 对 B 的力与 B 对 A 的力方向相反。这两个力总是属于同一类型，例如都是万有引力或都是静电力。
2. Action-Reaction Pair Characteristics | 作用力与反作用力的特征
A true third-law pair has four essential features: the forces are equal in magnitude, opposite in direction, act on different bodies, and are of the same nature.
真正的第三定律力对具有四个基本特征：两个力大小相等、方向相反、作用在不同物体上，并且具有相同的性质。
Because the two forces act on different bodies, they never cancel each other in a free-body diagram of a single object. They are simultaneous: the action and reaction are produced together and disappear together.
由于这两个力作用在不同物体上，它们在单个物体的受力图中永远不会相互抵消。这两个力也是同时的：作用力和反作用力同时产生、同时消失。
3. Third-Law Pairs vs Balanced Forces | 第三定律力对与平衡力的区别
Students often confuse a third-law pair with a pair of balanced forces. Balanced forces act on the same body, can be of different types, and may have different origins. A third-law pair acts on two different bodies and is always of the same type.
学生经常将第三定律力对与一对平衡力混淆。平衡力作用在同一个物体上，可以属于不同类型，并且可能具有不同的来源。第三定律力对作用在两个不同物体上，并且总是同一类型。
For example, a book resting on a table experiences an upward normal force from the table and a downward gravitational force from the Earth. These two forces act on the same book, so they are balanced forces, not a third-law pair.
例如，放在桌子上的书受到桌子向上的法向力和地球向下的引力。这两个力作用在同一本书上，因此它们是平衡力，而不是第三定律力对。
The third-law pairs in this example are: the book pushing down on the table and the table pushing up on the book; and the Earth pulling down on the book and the book pulling up on the Earth.
该例子中的第三定律力对是：书向下推桌子，桌子向上推书；以及地球向下拉书，书向上拉地球。
| Feature 特征 | Third-law pair 第三定律力对 | Balanced forces 平衡力 |
|---|---|---|
| Object acted on 作用对象 | Two different bodies 两个不同物体 | Same body 同一个物体 |
| Force type 力的类型 | Always same type 总是同一类型 | Can be different types 可以不同 |
| Effect 效果 | Do not cancel 不抵消 | Can cancel, giving equilibrium 可抵消，产生平衡 |
4. Identifying Pairs in Everyday Situations | 识别日常情境中的力对
To identify a third-law pair, use the statement "A exerts a force on B, so B exerts a force on A." If you swap the names of the objects, the direction of the force reverses.
要识别第三定律力对，可以使用 "A 对 B 施加力，因此 B 对 A 施加力" 的表述。如果你交换物体名称，力的方向就会反转。
For a ball hitting a wall: the ball exerts a force on the wall, and the wall exerts an equal and opposite force on the ball. The force on the ball causes its rebound.
对于球撞击墙壁：球对墙施加一个力，墙对球施加一个大小相等、方向相反的力。作用在球上的力使球反弹。
For a person pushing a trolley: the person pushes the trolley forward; the trolley pushes the person backward. If the person is standing on frictionless ice, pushing the trolley makes the person move backward.
对于人推手推车：人向前推手推车；手推车向后推人。如果人站在无摩擦的冰面上，推手推车会使人向后移动。
5. Rocket Propulsion | 火箭推进
Rocket propulsion is a classic application of Newton's third law. The rocket engine expels hot exhaust gases backward at high speed. The gases exert a forward force on the rocket, and the rocket exerts a backward force on the gases.
火箭推进是牛顿第三定律的经典应用。火箭发动机向后高速喷出高温废气。气体对火箭施加向前的力，火箭对气体施加向后的力。
The forward force on the rocket is the reaction to the backward force on the exhaust gases. Rockets work even in the vacuum of space because they do not need to push against air.
作用在火箭上的向前力是对作用在废气上的向后力的反作用力。火箭甚至在真空中也能工作，因为它们不需要推动空气。
F = Δp / Δt
The rate of change of momentum of the expelled gas gives the thrust. This is closely linked to the principle of conservation of momentum.
排出气体的动量变化率给出推力。这与动量守恒原理密切相关。
6. Walking and Swimming | 行走与游泳
When you walk, your foot pushes backward on the ground. The ground pushes forward on your foot with an equal and opposite force. This forward force from the ground accelerates you forward.
当你走路时，你的脚向后推地面。地面以大小相等、方向相反的力向前推你的脚。这个来自地面的向前力使你向前加速。
If the ground is slippery, the backward force cannot be large enough, so the forward reaction force is also small and walking becomes difficult.
如果地面很滑，向后的力不够大，因此向前的反作用力也很小，行走就会变得困难。
Swimming is similar: a swimmer pushes water backward with the arms and legs, and the water pushes the swimmer forward.
游泳与此类似：游泳者用手臂和腿向后推水，水则向前推游泳者。
7. Collisions and Impulse | 碰撞与冲量
In a collision between two objects A and B, the force that A exerts on B is equal and opposite to the force that B exerts on A at every instant of the collision.
在两个物体 A 和 B 的碰撞中，A 对 B 施加的力与 B 对 A 施加的力在碰撞的每一瞬间都大小相等、方向相反。
Therefore the impulse experienced by A is equal in magnitude and opposite in direction to the impulse experienced by B. This is why the total momentum of the two-body system is conserved if no external force acts.
因此，A 受到的冲量与 B 受到的冲量大小相等、方向相反。这就是为什么在没有外力作用时，两体系统的总动量守恒。
Δp(A) = −Δp(B)
The time of contact is the same for both forces, so equal and opposite forces produce equal and opposite changes in momentum.
两个力的接触时间相同，因此大小相等、方向相反的力会产生大小相等、方向相反的动量变化。
8. Connected Bodies and Tension | 连接体与张力
When two blocks are connected by a taut string, the string pulls block A forward and block B backward with the same magnitude of tension. These tension forces are not a third-law pair because both act through the string and can be considered internal to the string-block system.
当两个物块由一根拉紧的绳子连接时，绳子以同样大小的张力向前拉物块 A、向后拉物块 B。这两个张力不是
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