Forces and Free-Body Diagrams · 力与受力图
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| force/fɔːs/ | 力 | lì |
| free-body diagram/friː ˈbɒdi ˈdaɪəɡræm/ | 受力图 | shòu lì tú |
| newton/ˈnjuːtn/ | 牛顿 | niú dùn |
| weight/weɪt/ | 重量 | zhòng liàng |
| normal force/ˈnɔːml fɔːs/ | 法向力 | fǎ xiàng lì |
| tension/ˈtenʃn/ | 张力 | zhāng lì |
| friction/ˈfrɪkʃn/ | 摩擦力 | mó cā lì |
| net force/net fɔːs/ | 合力 | hé lì |
Every push and pull, on one clean diagram
- A book on a table is doing nothing — yet several forces act on it at once.
- Gravity pulls down; the table pushes up; a hand might shove it sideways.
- To reason clearly, physicists strip away everything but the forces 力 on that one object.
- The result is a free-body diagram 受力图 — the single most useful sketch in mechanics.
每一个推与拉,画在一张干净的图上
- 桌上的一本书什么也没做——却同时有好几个力作用在它身上。
- 重力向下拉;桌子向上推;一只手也许把它往旁边推。
- 为了清楚地推理,物理学家把一切都剥去,只留下作用在那一个物体上的力。
- 结果就是受力图——力学中最有用的一张草图。
What a force is
- A force is a push or a pull — a vector, measured in newtons 牛顿 ($\text{N}$).
- It has a size and a direction, so forces add like vectors, not plain numbers.
- A force always comes from an interaction between two objects.
- $1\ \text{N}$ is roughly the weight of a small apple.
什么是力
- 力是一个推或一个拉——一个矢量,单位是牛顿($\text{N}$)。
- 它有大小又有方向,所以力像矢量那样相加,而不是普通的数。
- 力总是来自两个物体之间的相互作用。
- $1\ \text{N}$ 大约是一个小苹果的重量。
What is the SI unit of force? · 力的国际单位是什么？
Force is measured in newtons ($\text{N}$). Joules measure energy, watts power, pascals pressure. · 力的单位是 牛顿 ($\text{N}$)。焦耳测量能量，瓦特测量功率，帕斯卡测量压力。
The usual suspects
- Weight 重量 $mg$ — gravity's pull, always straight down.
- Normal force 法向力 $N$ — a surface pushing perpendicular to itself.
- Tension 张力 $T$ — a pull along a rope or string.
- Friction 摩擦力 $f$ — a surface resisting sliding, parallel to the surface.
常见的几种力
- 重力 $mg$——引力的拉,永远竖直向下。
- 法向力 $N$——表面垂直于自身方向的推。
- 张力 $T$——沿绳或线的拉。
- 摩擦力 $f$——表面阻止滑动的力,平行于表面。
Which force is always perpendicular to the surface an object rests on? · 哪个力始终垂直于物体所放置的表面？
The normal · 正态分布 force is defined as the surface's push perpendicular to itself. Friction runs parallel to the surface. · 法向力定义为表面对自身的垂直接触推力。摩擦力沿表面平行方向。
Drawing a free-body diagram
- Shrink the object to a dot and draw every force as an arrow from that dot.
- Include only forces acting on the object — never forces it exerts on others.
- Point each arrow the right way; longer arrow = larger force.
- Then the net force 合力 is the vector sum of them all.
画受力图
- 把物体缩成一个点,把每个力画成从那个点出发的箭头。
- 只画作用在物体身上的力——绝不画它对别的物体施加的力。
- 每个箭头指向正确方向;箭头越长 = 力越大。
- 然后合力就是它们全部的矢量和。
Build a free-body diagram · 构建受力图
Change the applied force, friction and mass, and watch the force arrows and the net force respond. · 改变施加的力、摩擦力和质量，观察力箭头和合力如何响应。
A free-body diagram should include the forces the object exerts on other objects. · 受力图应包含物体对其他物体施加的力。
A free-body diagram shows only the forces acting on the chosen object, never the forces it exerts on others. · 受力图仅显示作用于选定物体的力，从不显示它施加给其他物体的力。
A block is pushed along a rough table. Select all · 所有 forces that belong on its free-body diagram. · 一个木块在粗糙桌面上被推动。选择所有属于其受力图的力。
All four forces act on the block. The force the block exerts on the table acts on the table, so it is left out. · 四个力都作用在木块上。但木块对桌面的力作用在桌子上，因此被排除。
Adding the forces up
- Add forces component by component, just like any vectors.
- If they balance, the net force is zero and the object keeps constant velocity.
- If they don't, the leftover net force sets the acceleration (next lessons).
- Always ask: what is the net force, and which way does it point?
把力加起来
- 像任何矢量一样,按分量逐一相加。
- 如果它们平衡,合力为零,物体保持匀速。
- 如果不平衡,剩下的合力决定加速度(下几节)。
- 永远要问:合力是多少,它指向哪个方向?
A crate feels $10\ \text{N}$ right and $4\ \text{N}$ left. What is the net horizontal force, in $\text{N}$? · 一个箱子受到$10\ \text{N}$向右和$4\ \text{N}$向左的力。水平合力是多少，单位为$\text{N}$？
Take right as positive: $10 - 4 = 6\ \text{N}$ to the right. · 取向右为正：$10 - 4 = 6\ \text{N}$向右。
The net force on an object is the ____ sum of all the forces acting on it. · 物体上的合力是所有作用于其上力的____和。
Forces have direction, so they combine as a vector sum, not a plain arithmetic sum. · 力有方向，因此它们作为矢量和叠加，而非简单的算术和。
The normal force is not always equal to the weight. On a slope, or when something presses down or lifts up on the object, $N$ changes. Never assume $N = mg$ — work it out from the forces perpendicular to the surface.
法向力不总是等于重力。在斜坡上,或当有东西压下、提起物体时,$N$ 会改变。绝不要假设 $N = mg$——要从垂直于表面的那些力算出它。
A crate feels $F = 10\ \text{N}$ to the right and friction $f = 4\ \text{N}$ to the left.
- Horizontal net force $= 10 - 4 = 6\ \text{N}$, to the right.
- Vertically, $N$ and $mg$ balance, so the crate stays on the floor.
一个箱子受到向右的 $F = 10\ \text{N}$ 和向左的摩擦力 $f = 4\ \text{N}$。
- 水平合力 $= 10 - 4 = 6\ \text{N}$,向右。
- 竖直方向上,$N$ 与 $mg$ 平衡,所以箱子留在地面上。
A force (in newtons) is a push or pull from an interaction. A free-body diagram shows every force acting on one object as arrows from a dot. The net force is their vector sum — and the normal force is not always $mg$.
力(单位牛顿)是相互作用产生的推或拉。受力图把作用在一个物体上的每个力画成从一点出发的箭头。合力是它们的矢量和——而法向力不总是 $mg$。