Newton's First Law · 牛顿第一定律
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| inertia/ɪˈnɜːʃə/ | 惯性 | guàn xìng |
| mass/mæs/ | 质量 | zhì liàng |
| equilibrium/ˌiːkwɪˈlɪbrɪəm/ | 平衡 | píng héng |
Yank the tablecloth — the dishes stay put
- Whip a tablecloth out fast and the plates barely move.
- They "want" to stay where they are — that reluctance to change motion is inertia 惯性.
- Newton's first law: an object keeps its velocity unless a net force acts on it.
- At rest it stays at rest; moving, it keeps moving in a straight line at constant speed.
猛抽桌布——盘子却纹丝不动
- 快速把桌布一抽,盘子几乎不动。
- 它们"想"留在原地——这种不愿改变运动状态的性质就是惯性。
- 牛顿第一定律:除非有合力作用,物体保持它的速度不变。
- 静止时保持静止;运动时以恒定速率沿直线继续运动。
Inertia and mass
- Inertia is an object's resistance to any change in its motion.
- Mass 质量 is the measure of inertia — more mass, harder to speed up, slow down, or turn.
- A loaded truck is far harder to stop than a bicycle at the same speed.
- Inertia is why you lurch forward when a car brakes: your body keeps its velocity.
惯性与质量
- 惯性是物体抵抗其运动状态发生任何改变的性质。
- 质量是惯性的量度——质量越大,越难加速、减速或转向。
- 同样速度下,满载的卡车比自行车难停得多。
- 惯性就是汽车刹车时你向前猛冲的原因:你的身体保持它的速度。
An object's resistance to a change in its motion is called . · 物体抵抗运动状态改变的性质称为。
Inertia is that resistance, and mass · 质量 is its measure. · 惯性就是这种阻力，而质量是其度量。
At the same speed, which is harder to stop — and therefore has more inertia? · 在相同速度下，哪一个更难停下——因而具有更多惯性？
More mass means more inertia. The heavy truck resists stopping far more than the bicycle. · 质量越大意味着惯性越大。重型卡车比自行车更难停下。
Balanced forces mean constant velocity
- If the forces on an object balance, the net force is zero.
- Zero net force ⇒ zero acceleration ⇒ constant velocity (an equilibrium 平衡).
- "At rest" is just the special case of constant velocity equal to zero.
- So no force is needed to keep moving — only to change the motion.
力平衡意味着匀速
- 如果物体所受的力平衡,合力为零。
- 合力为零 ⇒ 加速度为零 ⇒ 匀速(一种平衡状态)。
- "静止"只是速度等于零的匀速这一特例。
- 所以保持运动不需要力——只有改变运动才需要。
Balance the forces · 平衡力
Reduce the applied force and friction to zero and see the net force vanish — constant velocity. · 减小施加的力和摩擦力至零，观察合力消失——保持恒定速度。
An object has zero net force acting on it. What must be true of its motion? · 一个物体所受合力为零。关于其运动，什么必然成立？
Zero net force ⇒ zero acceleration ⇒ constant velocity. Rest is the special case where that velocity is zero. · 合力为零 ⇒ 加速度为零 ⇒ 恒定速度。静止是该速度为零的特殊情况。
A box feels $8\ \text{N}$ up, $8\ \text{N}$ down, $5\ \text{N}$ right and $5\ \text{N}$ left. What is the net force, in $\text{N}$? · 一个箱子受到$8\ \text{N}$向上、$8\ \text{N}$向下、$5\ \text{N}$向右和$5\ \text{N}$向左的力。合力是多少，单位为$\text{N}$？
Each pair cancels ($8-8=0$, $5-5=0$), so the net force is $0\ \text{N}$ — the box is in equilibrium. · 每对力相互抵消（$8-8=0$, $5-5=0$），所以合力为$0\ \text{N}$——箱子处于平衡状态。
Select all · 所有 objects that are in equilibrium (zero net force). · 选择所有处于平衡状态（合力为零）的物体。
Rest and constant-velocity motion both mean zero net force. The falling ball is accelerating, so it is not in equilibrium. · 静止和恒定速度运动都意味着合力为零。下落的球正在加速，因此不处于平衡状态。
Coasting forever
- A spacecraft far from any star cuts its engines and drifts on, unchanged, for years.
- With no friction and no net force, nothing slows it — the first law in its purest form.
- On Earth, friction and air resistance are the "hidden" forces that eventually stop things.
- Remove them (ice, space) and motion simply continues.
永远地滑行
- 一艘远离任何恒星的飞船关掉引擎,就这样不变地漂流多年。
- 没有摩擦、没有合力,什么都不减慢它——第一定律最纯粹的形式。
- 在地球上,摩擦和空气阻力是最终让物体停下的"隐藏"力。
- 去掉它们(冰面、太空),运动就会一直继续。
A moving object does not need a continuous force to keep moving. The old intuition "things stop unless you keep pushing" is really about friction quietly opposing the motion. Take friction away and no push is needed at all.
运动的物体不需要持续的力来保持运动。"不推就停"的旧直觉,其实是摩擦在悄悄地反抗运动。去掉摩擦,就根本不需要任何推力。
A moving object needs a continuous forward force just to keep moving at constant velocity. · 运动的物体需要持续的前向力才能保持匀速运动。
No net force is needed to keep constant velocity. It only feels that way on Earth because friction opposes the motion. · 维持恒定速度不需要合力。只是在地球上由于摩擦力阻碍运动，才会有这种感觉。
A puck slides on frictionless ice with the only vertical forces being weight down and the normal force up.
- Those two balance, so the net force is zero.
- The puck therefore glides at constant velocity forever — no push required.
一个冰球在无摩擦的冰面上滑动,竖直方向上只有向下的重力和向上的法向力。
- 这两者平衡,所以合力为零。
- 因此冰球永远以匀速滑行——不需要任何推力。
Newton's first law: with zero net force, an object keeps a constant velocity (rest is the zero case). Inertia is resistance to that change, measured by mass. Motion continues on its own; a net force is needed only to change it.
牛顿第一定律:在合力为零时,物体保持匀速(静止是零的特例)。惯性是对这种改变的抵抗,由质量量度。运动会自行继续;只有改变它才需要合力。