当两层气体有相对运动时，气体的分子之间不仅具有与运动方向一致的相对移动而造成的内摩擦，而且由于气体分子无秩序的热运动，两层气体分子之间可以互相扩散和交换。当流动速度较快的气层分子跑到流速较慢的一层里面去时，这些具有较大动能的气体分子，将使较慢的气层产生加速作用，反之，流动速度较慢的气层的分子跑进较快的气层里时，则对气层产生一种阻滞气层运动的作用，结果两层气体之间就产生了内摩擦。When two layers of gas move relative to each other, there is not only internal friction caused by the relative movement in the same direction as the motion, but also diffusion and exchange between two layers of gas molecules due to the disorderly thermal motion of gas molecules. When flow faster gas molecules ran into a slower velocity layer, which have large kinetic energy of gas molecules, will make the atmosphere with a slower produces acceleration, on the other hand, the flow speed is slower as he ran around the gas molecules in the atmosphere of the faster, the effect on gas to produce a block of gas movement, the result is created between two layers of gas internal friction.