What is the acceleration in the y direction at time zero?
那y方向上,0时刻的加速度是多少？
Zero vector can have a lot of direction, but only one length.
零向量可以有很多方向，却只有一个长度。
Otherwise my velocity in this direction 0 could never become zero.
否则我的速度,不会变到。
Otherwise my velocity in this direction 0 could never become zero.
否则我的速度,不会变到。
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