As the increase of running time, the resonant frequency of ultrasonic motor will decrease, which then leads to the decrease of speed.

随着超声波电机运行时间的增加，其谐振频率会下降，进而使电机速度降低。

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The assumptions of a 230 mph wind speed and a 1.2 psi rapid pressure decrease in about 1 second along with large and fast missiles is quite a force to design against.

FORBES: Tornados and Nukes -- How Big Can They Get?

The most persuasive evidence that Voyager 1 has crossed the termination shock is its measurement of a sudden increase in the strength of the magnetic field carried by the solar wind, combined with an inferred decrease in its speed.

BBC: Voyager impression (Nasa)