在伽利略生活的年代，人们还不会计算作变速运动物体的即时速度。

In the Galileo's times, the people still could not calculate the forthwith speed of variable motion's object.

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归结起来，大约有四种主要类型的问题：第一类是研究运动的时候直接出现的，也就是求即时速度的问题。

To sum up, there are about four main types of problems: the first is to study the movement occurs when the direct, nor is the desire for instant speed.

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尤尔韦特松讲了一个故事：他和儿子曾经把一支火箭的速度提升至2.5马赫(即时速约1,900英里)，但尾翼随后脱离，造成了严重故障。

Jurvetson tells a story about the time he and his son pushed a rocket to Mach 2.5 — approximately 1,900 miles per hour — before a fin separated and caused a critical failure.

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尤尔韦特松讲了一个故事：他和儿子曾经把一支火箭的速度提升至2.5马赫(即时速约1,900英里)，但尾翼随后脱离，造成了严重故障。

Jurvetson tells a story about the time he and his son pushed a rocket to Mach 2.5 — approximately 1,900 miles per hour — before a fin separated and caused a critical failure.

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