总动量为，两者的矢量和，也就是这个东西。
The net momentum is the vectorial sum of these two, which is this.
这是系统总动量。
当收起双臂时，分布空间减少，于是其速度一定会加快以弥补差异，从而保持总动量不变。
When she draws her arms inwards, that distribution is reduced, so her speed must pick up to counteract this difference and keep her total momentum constant.
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