All extrapolation operators face the problem of choosing the reference velocity due to continuation in depth.

此类波场外推算子沿深度层进行波场外推，都存在参考速度选择问题。

youdao

The velocity of the center of mass has not changed as seen from your frame of reference, because the velocity of center of mass is always the same, remember, because momentum is conserved.

质量重心的速度，并没有发生改变，你在参照系中可以看出，因为质量重心的速度，总是保持不变，要记住，由于动量守恒。

youdao

Now, a word about placeholder substitution: Velocity will print any unrecognized reference as plain text, as illustrated by the next two bold lines in Listing 6.

关于占位符替换的一点说明：Velocity把任何不能识别的引用作为普通文本打印，如清单6中下面突出显示的两行(蓝色和红色)所示。

youdao

So the Earth is an inertial frame of reference, if you go in a train relative to the Earth at constant velocity, you're also inertial.

所以,地球是一个惯性参考系,如果你坐在相对于地球匀速运动的火车里,你也处于惯性状态

耶鲁公开课 - 基础物理课程节选

Second formula tries to relate the final velocity of some time, t, to the initial velocity and the distance traveled with no reference to time.

第二个式子能把某段时间t的末速度,和初速度,经过的路程联系在一起,并且不引入时间

耶鲁公开课 - 基础物理课程节选