摩擦力作用于运动着的物体，并使其停止。
一根手指的弹击足够使一个气球加速离去，当气球运动时，一点动力，甚至空气的摩擦力可以很快使它慢下来。
The flick of a finger is enough to make a balloon accelerate away. When moving, it has little momentum and even the friction of the air quickly slows it up.
物体运动，并向上加速度，因为摩擦力方向斜上，值为这个。
It is moving, so sure, it's going to be accelerated so the frictional force is in this direction, has this value.
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