That's another way of saying maintaining velocity if it's not acted upon by a force.

这就是不受外力作用时,速度保持不变的另一种说法

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Because we find in that plane, when objects are left at what you think is at rest with no external agency acting on them, they all slide backwards towards the rear end of the plane.

因为我们发现,在飞机上,当物体掉在地上,你认为它不受外力作用,应该保持静止时,它却朝着飞机的尾部滚去

耶鲁公开课 - 基础物理课程节选

The Law of Inertia says that, "If a body has no forces acting on it, then it will remain at rest if it was at rest to begin with, or if it had a velocity to begin with, it will maintain that velocity."

惯性定律就是,"如果一个物体不受外力作用,那么静止的物体依然保持静止,如果物体有个初速度,那么就保持此速度不变"

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What makes the law surprising is that if I only gave you half the law, namely every body will remain at rest if it's not acted upon by a force, you will say, "That's fine. I accept that, because here's something, you leave it there, it doesn't move.

这条定律妙就妙在,如果只看定律的前半段,即,如果物体不受外力作用,将保持静止状态,你肯定会觉得,"这没什么,我能理解,因为你把一个物体放在那里,自己肯定不会动

耶鲁公开课 - 基础物理课程节选