Newton's First Law · 牛顿第一定律
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| Newton's first law/ˈnjuːtnz fɜːst lɔː/ | 牛顿第一定律 | niú dùn dì yí dìng lǜ |
| net force/net fɔːs/ | 合力 | hé lì |
| inertia/ɪˈnɜːʃə/ | 惯性 | guàn xìng |
| translational equilibrium/trænˈsleɪʃənl ˌiːkwɪˈlɪbrɪəm/ | 平动平衡 | píng dòng píng héng |
| inertial reference frame/ɪˈnɜːʃl ˈrefrəns freɪm/ | 惯性参考系 | guàn xìng cān kǎo xì |
A puck that never stops
- Slide a hockey puck across smooth ice and it glides on and on, barely slowing.
- Make the ice perfectly frictionless and it would glide forever.
- Nothing keeps it going -- it just refuses to stop on its own.
- That refusal is one of the deepest ideas in physics.
永不停下的冰球
- 让冰球在光滑的冰面上滑行,它会一直滑下去,几乎不减速。
- 如果冰面完全没有摩擦,它就会永远滑下去。
- 没有任何东西驱使它前进——它只是不肯自己停下来。
- 这种"不肯停"正是物理学中最深刻的思想之一。
Newton's first law
- Newton's first law 牛顿第一定律: an object keeps constant velocity unless a net force acts on it.
- At rest? It stays at rest. Moving? It keeps the same speed and direction.
- "No net force" does not mean "no motion" -- it means "no change in motion."
牛顿第一定律
- 牛顿第一定律:除非有合力作用,物体保持速度不变。
- 静止的?它保持静止。运动的?它保持相同的速率和方向。
- "没有合力"不等于"没有运动"——它意味着"运动不改变"。
An object moving at constant velocity must have a nonzero net force pushing it. · 以恒定速度运动的物体必须受到非零合力的推动。
Constant velocity means zero net force. A force is needed only to change · 变化 velocity, not to maintain it. · 恒定速度意味着合力为零。力仅用于改变速度，而非维持它。
Inertia
- The property that makes objects resist changes in motion is inertia 惯性.
- More mass means more inertia -- a loaded truck is far harder to start or stop than a bike.
- Inertia is why the first law holds.
惯性
- 使物体抵抗运动变化的性质叫惯性。
- 质量越大,惯性越大——满载的卡车比自行车难得多才能启动或停下。
- 惯性正是第一定律成立的原因。
Which object has the most inertia? · 哪个物体惯性最大？
Inertia is measured by mass -- the $2000\ \text{kg}$ car resists changes in motion the most. · 惯性由质量衡量——$2000\ \text{kg}$ 汽车最抵抗运动状态的改变。
Translational equilibrium
- When the net force 合力 is zero, an object is in translational equilibrium 平动平衡.
- That covers both an object at rest and one moving at constant velocity.
- To solve these problems, set $\sum F_x = 0$ and $\sum F_y = 0$.
平动平衡
- 当合力为零时,物体处于平动平衡。
- 这既包括静止的物体,也包括匀速运动的物体。
- 要解这类问题,令 $\sum F_x = 0$ 且 $\sum F_y = 0$。
Balanced or unbalanced? · 平衡还是不平衡？
By Newton's first law, decide whether the forces on each object are balanced. · 根据牛顿第一定律，判断每个物体上的力是否平衡。
An object with zero net force is said to be in translational ____. · 合外力为零的物体被称为处于平动____状态。
Zero net force → translational equilibrium, which covers rest and constant velocity. · 合外力为零 → 平动平衡，包括静止 和 匀速运动。
Select all · 所有 situations in translational equilibrium (zero net force). · 选择所有处于平动平衡（合外力为零）的情况。
Rest and constant velocity both mean zero net force. A falling ball accelerates, so its net force is not zero. · 静止和匀速运动都意味着合外力为零。下落的球在加速，因此其合外力不为零。
A crate sits still while pulled right with $40\ \text{N}$ and left with $40\ \text{N}$. What is the net horizontal force (in N)? · 一个木箱静止不动，右侧受到 $40\ \text{N}$ 的拉力，左侧受到 $40\ \text{N}$ 的拉力。水平方向的合外力是多少（单位：N）？
$40 - 40 = 0\ \text{N}$ -- balanced forces, so it stays in equilibrium (at rest). · $40 - 40 = 0\ \text{N}$ —— 力是平衡的，所以它保持平衡（静止）。
Inertial reference frames
- The first law only holds in an inertial reference frame 惯性参考系 -- one that is not accelerating.
- In a braking bus you lurch forward with no force pushing you -- because the bus is a non-inertial frame.
- Physics is written from the point of view of inertial frames.
惯性参考系
- 第一定律只在惯性参考系中成立——即不加速的参考系。
- 在刹车的公交车上,你会向前冲,却没有力推你——因为公交车是非惯性系。
- 物理定律是从惯性系的角度写下的。
When a bus brakes hard and you lurch forward, a force pushed you forward. · 当公交车急刹车而你向前倾时，有一个力推着你向前。
No forward force acts -- your body's inertia keeps it moving while the bus (a non-inertial frame) slows beneath you. · 没有向前的力作用——是你的身体惯性使其保持运动，而公交车（非惯性系）在你身下减速。
A lamp hangs still from two angled cords.
- It is in translational equilibrium, so the net force is zero.
- The cords' horizontal pulls cancel, and their vertical pulls together hold up the weight -- two equations, $\sum F_x = 0$ and $\sum F_y = 0$, solve for the tensions.
一盏灯静静地挂在两根成角度的绳上。
- 它处于平动平衡,所以合力为零。
- 两绳的水平拉力抵消,竖直拉力共同托起重量——两个方程 $\sum F_x = 0$ 和 $\sum F_y = 0$ 就能解出张力。
Constant velocity does not require a forward force. A car cruising at a steady $100\ \text{km/h}$ has zero net force -- its engine thrust exactly balances air drag and friction. Force is needed only to change velocity, not to maintain it.
匀速运动不需要向前的力。以稳定的 $100\ \text{km/h}$ 巡航的汽车合力为零——发动机的推力恰好平衡空气阻力和摩擦力。只有改变速度才需要力,维持速度不需要。
Newton's first law: velocity stays constant unless a net force acts -- objects resist change through their inertia. Zero net force means translational equilibrium (at rest or constant velocity), solved with $\sum F_x = 0$, $\sum F_y = 0$. It holds only in a non-accelerating inertial reference frame.
牛顿第一定律:除非有合力作用,速度保持不变——物体通过惯性抵抗变化。合力为零意味着平动平衡(静止或匀速),用 $\sum F_x = 0$、$\sum F_y = 0$ 求解。它只在不加速的惯性参考系中成立。