All right, so far, about speeds and average velocities and accelerations.
目前为止，是关于速率，平均速度和加速度。
The velocities reverse directions, but the speeds remain the same.
,速度改变了方向，但它的大小不变。
For soft beam-stiff half space system, wave speeds in beams are also critical velocities.
对软梁-硬半空间系统，梁的剪切波速和压缩波速也是临界速度；
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