Newton's Third Law · 牛顿第三定律
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| Newton's third law/ˈnjuːtnz θɜːd lɔː/ | 牛顿第三定律 | niú dùn dì sān dìng lǜ |
| action-reaction pair/ˈækʃn rɪˈækʃn peə/ | 作用反作用对 | zuò yòng fǎn zuò yòng duì |
| cancel/ˈkænsl/ | 抵消 | dǐ xiāo |
You cannot push without being pushed back
- Push hard on a wall and your hand feels the wall pushing back -- just as hard.
- Fire a rifle and it kicks your shoulder; a rocket throws gas down and is thrown up.
- Forces never come alone -- they come in pairs.
- This simple idea powers walking, swimming, and every rocket ever launched.
你不推别人,就不会被推回来
- 用力推墙,你的手会感到墙在推回来——一样用力。
- 开枪,枪会撞你的肩;火箭把气体向下喷,自己就被向上抛。
- 力从不单独出现——它们成对出现。
- 这个简单的道理支撑着走路、游泳,以及有史以来每一次火箭发射。
Newton's third law
- Newton's third law 牛顿第三定律: if object A pushes on object B, then B pushes on A.
- The two forces are equal in magnitude and opposite in direction.
- Together they make an action-reaction pair 作用反作用对.
牛顿第三定律
- 牛顿第三定律:如果物体 A 推物体 B,那么 B 也推 A。
- 这两个力大小相等、方向相反。
- 它们合在一起构成一对作用反作用对。
A truck ($3000\ \text{kg}$) hits a car ($1000\ \text{kg}$). During the crash, the force the truck exerts on the car compared with the force the car exerts on the truck is... · 卡车（$3000\ \text{kg}$）撞击汽车（$1000\ \text{kg}$）。碰撞过程中，卡车对汽车的力与汽车对卡车的力相比是...
Third-law forces are always equal and opposite, regardless of mass. (The car accelerates more because it has less mass, not more force.) · 第三定律的力总是相等且相反，与质量无关。（汽车加速更多是因为质量更小，而非受力更大。）
The pair acts on different objects
- The two forces of a pair act on different bodies -- one on A, one on B.
- So they can never cancel 抵消 each other.
- Cancellation would need both forces on the same object.
这一对作用在不同物体上
- 一对力中的两个力作用在不同的物体上——一个在 A,一个在 B。
- 所以它们永远不能抵消彼此。
- 要抵消,就需要两个力都作用在同一个物体上。
The two forces of an action-reaction pair cancel each other out. · 作用-反作用对中的两个力互相抵消。
They act on different objects, so they never cancel. Cancellation needs both forces on the same object. · 它们作用在不同物体上，因此从不抵消。抵消需要两个力作用在同一物体上。
The two forces of a third-law pair always act on ____ objects. · 第三定律对的两个力总是作用在____物体上。
One force on A, the other on B -- different objects, which is why they cannot cancel. · 一个力在A上，另一个在B上——不同物体，这就是它们不能抵消的原因。
It is how we move
- Walk forward: your foot pushes back on the ground, and the ground pushes you forward.
- Swim: you push water back, the water pushes you ahead.
- A rocket: engines hurl gas down, the gas hurls the rocket up.
我们就是这样运动的
- 向前走:你的脚向后蹬地,地面就向前推你。
- 游泳:你把水向后推,水就把你向前推。
- 火箭:发动机把气体向下喷,气体就把火箭向上推。
Third-law pairs · 第三定律配对
Newton's third law pairs act on different objects. Sort each statement. · 牛顿第三定律的作用力对作用在不同物体上。对每个陈述进行排序。
When you walk forward, what actually pushes you forward? · 当你向前走时，真正推动你前进的是什么？
You push the ground backward; by the third law the ground pushes you forward -- that reaction is what moves you. · 你向后推地面；根据第三定律，地面推你向前——这个反作用力就是你移动的原因。
A rocket in empty space can still accelerate, with nothing to push against. · 在太空中，火箭仍能加速，无需推任何东西。
It pushes its own exhaust gas backward; by the third law the gas pushes the rocket forward -- no outside surface needed. · 它向后推自己的废气；根据第三定律，废气推火箭向前——不需要外部表面。
Not the same as balanced forces
- A book on a table has gravity down and the normal force up -- these balance on one object.
- That is not a third-law pair (both act on the book).
- The book's third-law partner to gravity is the book pulling Earth up.
与平衡力不是一回事
- 桌上的书受向下的重力和向上的法向力——它们在同一个物体上平衡。
- 那不是第三定律的一对力(两者都作用在书上)。
- 书对重力的第三定律搭档,是书向上拉地球。
Select all · 所有 genuine action-reaction (third-law) pairs. · 选出所有真正的作力-反作用力（第三定律）对。
The first two are pairs on different objects. The third is two forces on the same · 相同 book -- balanced forces, not a third-law pair. · 前两个是作用在不同物体上的对。第三个是作用在同一本书上的两个力——平衡力，而非第三定律对。
You push a wall with $50\ \text{N}$.
- By the third law, the wall pushes your hand with exactly $50\ \text{N}$ the other way.
- One force acts on the wall, the other on your hand -- different objects, so they do not cancel.
你用 $50\ \text{N}$ 推墙。
- 根据第三定律,墙以正好 $50\ \text{N}$ 反方向推你的手。
- 一个力作用在墙上,另一个作用在你手上——不同物体,所以它们不抵消。
If a horse pulls a cart and the cart pulls back equally, why does the cart move? Because those two forces act on different objects. The cart accelerates from the forces on the cart alone (the horse's pull minus friction) -- the reaction pull acts on the horse.
如果马拉车、车也同样地反拉马,车为什么还会动?因为这两个力作用在不同的物体上。车的加速来自作用在车本身上的力(马的拉力减去摩擦力)——那个反作用拉力作用在马身上。
Newton's third law: forces come in equal-and-opposite pairs, an action-reaction pair. The two act on different objects, so they never cancel. This is different from two balanced forces on a single object -- that is why a horse-and-cart still accelerates.
牛顿第三定律:力成对出现,大小相等、方向相反,即一对作用反作用对。这两个力作用在不同的物体上,所以永远不抵消。这与作用在同一个物体上的两个平衡力不同——这就是为什么马拉车仍然会加速。