Kinetic and Static Friction · 动能与静摩擦
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| static friction/ˈstætɪk ˈfrɪkʃn/ | 静摩擦 | jìng mó cā |
| kinetic friction/kɪˈnetɪk ˈfrɪkʃn/ | 动摩擦 | dòng mó cā |
| coefficient of static friction/ˌkəʊɪˈfɪʃənt ɒv ˈstætɪk ˈfrɪkʃn/ | 静摩擦系数 | jìng mó cā xì shù |
It won't budge... then suddenly it slides
- Push a heavy crate gently and nothing happens; push harder and still nothing; push harder still — it lurches into motion.
- Before it moves, static friction 静摩擦 grows to match your push exactly.
- Once it slides, kinetic friction 动摩擦 takes over — usually a little weaker.
- Both come from surfaces gripping each other, and both act parallel to the surface.
它纹丝不动……然后突然滑动
- 轻推一个沉重的板条箱,什么也没发生;推得更重,还是没动;再更用力——它猛地动了起来。
- 在它动起来之前,静摩擦力会增长到恰好与你的推力相等。
- 一旦它滑动,动摩擦力接手——通常稍微弱一点。
- 两者都源于表面互相咬合,并且都平行于表面作用。
Static friction adjusts itself
- Static friction opposes an object that is not yet moving.
- It grows to match the applied push, up to a maximum: $f_s \le \mu_s N$.
- Below that maximum the object stays put; the friction is only as big as it needs to be.
- $\mu_s$ is the coefficient of static friction 静摩擦系数, and $N$ is the normal force.
静摩擦力会自我调节
- 静摩擦力反抗尚未运动的物体。
- 它增长到与所施加的推力相等,直到一个最大值:$f_s \le \mu_s N$。
- 在这个最大值以下,物体保持不动;摩擦力只有它需要的那么大。
- $\mu_s$ 是静摩擦系数,$N$ 是法向力。
Which friction acts on an object that is being pushed but has not · 不 started to move? · 当一个被推动但尚未移动的物体受到哪种摩擦力？
Before sliding, static friction acts and grows to match the push, up to $\mu_s N$. · 在滑动之前，静摩擦力起作用并增长以匹配推力，直至达到$\mu_s N$最大静摩擦力。
Kinetic friction is roughly steady
- Once sliding starts, kinetic friction acts: $f_k = \mu_k N$.
- It stays roughly constant no matter how fast the object slides.
- Usually $\mu_k < \mu_s$ — it takes more force to start sliding than to keep sliding.
- That is why the crate "breaks free" and then feels easier to push.
动摩擦力大致恒定
- 一旦开始滑动,动摩擦力起作用:$f_k = \mu_k N$。
- 无论物体滑得多快,它都大致保持不变。
- 通常 $\mu_k < \mu_s$——开始滑动比保持滑动需要更大的力。
- 这就是板条箱先"挣脱",随后感觉更好推的原因。
Friction on a slope · 斜面上的摩擦力
Raise the angle or the friction coefficient and see when the block finally starts to slide. · 增大倾角或摩擦系数，观察滑块何时开始滑动。
A sliding box has a normal force $N = 40\ \text{N}$ and $\mu_k = 0.3$. What is the kinetic friction force, in $\text{N}$? · 一个滑动的箱子具有正压力$N = 40\ \text{N}$N和$\mu_k = 0.3$动摩擦系数μ_k。其动摩擦力是多少，单位是$\text{N}$N？
$f_k = \mu_k N = 0.3 \times 40 = 12\ \text{N}$.
For most surfaces, how do the coefficients compare? · 对于大多数表面，摩擦系数如何比较？
Usually $\mu_s > \mu_k$, so it takes more force to break free than to keep sliding. · 通常$\mu_s > \mu_k$μ_s > μ_k，因此挣脱静止所需的力大于维持滑动所需的力。
What friction depends on
- Friction depends on the normal force $N$ and the surfaces, through $f = \mu N$.
- Press harder (larger $N$) and friction grows; that is why heavy things are hard to slide.
- Surprisingly, it barely depends on the contact area — a brick slides the same on any face.
- It does not depend on speed (for kinetic friction, to a good approximation).
摩擦力取决于什么
- 摩擦力通过 $f = \mu N$ 取决于法向力 $N$ 和接触表面。
- 压得越重(更大的 $N$),摩擦越大;这就是重物难滑的原因。
- 出人意料的是,它几乎不取决于接触面积——砖块用任一面滑动都差不多。
- 它不取决于速度(对动摩擦力,近似而言)。
The friction force depends strongly on the area of contact between the surfaces. · 摩擦力强烈依赖于接触面之间的接触面积。
To a good approximation friction depends on $\mu$ and the normal force, not · 不 the contact area. · 近似而言，摩擦力取决于$\mu$摩擦系数和正压力，不取决于接触面积。
Friction force is the coefficient $\mu$ times the ____ force. · 摩擦力等于系数$\mu$μ乘以____力。
$f = \mu N$, where $N$ is the normal · 正态分布 force pressing the surfaces together. · $f = \mu N$μ，其中$N$N是压合表面的正压力。
Select all · 所有 changes that increase the kinetic friction force on a sliding box. · 选择所有能增加滑动物体上动摩擦力的变化。
Friction $= \mu_k N$: a larger normal force or rougher surfaces increase it. Contact area and speed barely matter. · 摩擦力$= \mu_k N$f_k=μ_k N：更大的正压力或更粗糙的表面会增加它。接触面积和速度几乎无影响。
Static friction is not always $\mu_s N$ — that is only its maximum. Below the slipping point, static friction equals whatever push it must balance ($f_s \le \mu_s N$). Also, friction barely depends on contact area, so a wider box is not "grippier".
静摩擦力不总是 $\mu_s N$——那只是它的最大值。在滑动临界点以下,静摩擦力等于它必须平衡的那个推力($f_s \le \mu_s N$)。此外,摩擦力几乎不取决于接触面积,所以更宽的箱子并不"更抓地"。
A box presses on the floor with a normal force $N = 40\ \text{N}$, and $\mu_k = 0.3$.
- While sliding, kinetic friction is $f_k = \mu_k N = 0.3 \times 40 = 12\ \text{N}$.
- This $12\ \text{N}$ opposes the motion, whatever the box's speed.
一个箱子以法向力 $N = 40\ \text{N}$ 压在地面上,$\mu_k = 0.3$。
- 滑动时,动摩擦力为 $f_k = \mu_k N = 0.3 \times 40 = 12\ \text{N}$。
- 无论箱子速度如何,这 $12\ \text{N}$ 都反抗运动。
Static friction ($f_s \le \mu_s N$) grows to prevent motion up to a maximum; kinetic friction ($f_k = \mu_k N$) opposes sliding and is roughly constant. Usually $\mu_s > \mu_k$. Friction $= \mu N$ — it depends on the normal force, not the contact area or speed.
静摩擦力($f_s \le \mu_s N$)增长以阻止运动,直到一个最大值;动摩擦力($f_k = \mu_k N$)反抗滑动且大致恒定。通常 $\mu_s > \mu_k$。摩擦力 $= \mu N$——它取决于法向力,而非接触面积或速度。