然而，现实世界充满了摩擦。
每次我使用方向盘时，都会摩擦掉一些微粒。
两国间的贸易摩擦是由贸易保护主义者引起的。
Trade frictions between the two countries had been caused by trade protectionists.
他们可能发尖锐的摩擦声。
这是测量静摩擦，系数的一种方法。
This is a way that you can measure the static friction coefficient.
绳子的这个方向的,摩擦力会翻转。
Those frictional forces for this rope to go in this direction will flip over.
最大摩擦力和m有关,和d无关。
The maximum friction has this little m in it, and that's independent of d.
那么，究竟什么是摩擦力呢？
我讲过，摩擦系数，与物体质量无关。
I told you that the friction coefficient is independent of the mass of the object.
那摩擦力是多少，什么方向？
What will the friction be, how large, and in what direction?
若这棒上没有摩擦力，那么。
由此我得到，摩擦力的大小。
So I know what the frictional force is given by this result.
若我们利用摩擦力，就能得到稳定状态。
However, if we make use of the friction, then we can have a stable situation.
压力于两个板块间积聚而受摩擦力抑制。
Between the two plates, stresses build up that are held in check by friction.
为什么摩擦力不起作用？
摩擦力可以转移动能。
摩擦力一定抵消动能。
对比起木地板来说，冰面的摩擦力要小得多。
Compared to, say, a wooden floor, ice has much less friction.
假设现在没有摩擦力。
轮胎与地面间的摩擦系数是多少呢？
How much is the coefficient of friction between the tire and ground?
如果没有摩擦力，如果没有其他力，只有重力。
If there is no friction, if there are no other forces, only gravity.
摩擦力指的是阻碍运动的一种力。
据说在受到很大的摩擦力之后。
我们发现了摩擦力。
要是没有摩擦力，我们就不能行走。
所以这个是摩擦力。
室内的湿度会随时变化,这对摩擦系数影响很大。
The humidity could change in the room and that could change the friction coefficient.
它未曾变过,如果没有空气阻力,或任何摩擦力。
That never changes if there is no air drag or air friction of any kind.
我们需要摩擦力。
现在没有摩擦力。
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