Because of the resistive forces, because of the drag by the liquid, the object is going to get a velocity in this direction, so there's going to be a component of the resistive force opposing it.
因为阻力，因为液体中的阻力，物体的速度，是朝这个方向，所以会有，一个相反的阻力。
The drag force acts in the opposite direction to the path of the ball.
阻力方向与球的运行方向相反。
Drag onto the page. Drag round rotation handle to change arrow direction; yellow diamond to resize line ends.
拖到绘图页上。拖动圆形旋转手柄可更改箭头方向；拖动黄色菱形可调整线端的大小。
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