请注意此处的加速度，是一个常数，不随时间变化而变化，不过速度是变化的。
So notice that the acceleration is constant in time, is not changing but the velocity is changing.
随着认识的不断深入，人们逐渐发现波动不随时间变化的假设是不尽合理的。
As the realization about the volatility, people know that the assumption of the volatility as a constant is improper.
一个点上的水流状态，如压力、流速和流量不随时间变化而变化，称为恒定流。
The current status of one point, such as pressure, flow rate and flow don t change with time, intitules as the steady flow.
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