来自《耶鲁大学开放课程：基础物理 第8集 第8集》
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they will be leaving you at that speed v0.它们将会以速度 v0 远离你 -
A short time later-- What happens a short time later,一段很短的时间以后 -
the rocket has a mass M - δ火箭的质量变为 M - δ -
because it's lost some of its own body mass因为它失去了一些自身的质量 -
in the form of exhaust fumes.以向外排气的方式 -
The exhaust fumes, I'm just showing them as a blob here,这些排出的气体 我用这一团来表示 -
and the rocket's velocity now is not v, but v + Δv.现在火箭的速度不是 v 而是 v + Δv -
And what's the velocity of the fumes?这些气体的速度又是多少呢 -
Here's where you've got to be careful.这里你要小心了 -
If your velocity was v at that instant,如果在这一瞬间你的速度为 v -
the velocity of the rocket fume,气体的速度 -
with respect to the ground is v - v0;相对于地面来说是 v - v0