"讨论磨擦后的整体效果和讨论一个点上的磨擦力是两回事"
Then please tell me where that up force come from after this 磨擦后的整体效果 and whether this up force is 磨擦力 or not.
Then please tell me where that up force come from after this 磨擦后的整体效果 and whether this up force is 磨擦力 or not.