If I am an inertial observer, another person moving relative to me at constant velocity will also be an inertial observer.
如果我是惯性观察者,那另一个相对于我做匀速运动的人,也是一个惯性观察者
But things which were at rest will move at a constant velocity, opposite of the velocity that you have relative to me.
这些静止的东西都在匀速运动,方向正好跟你相对我运动的速度方向相反
You put a chunk of wood on a table and you try to move it at constant speed.
你把一大块木头放在桌子上,试着让它匀速运动
The great discovery that Galileo and Newton made is that you don't need a force for a body to move at constant velocity.
伽利略和牛顿的伟大发现就是,物体不需要力来维持匀速运动
But adding a constant velocity to objects does not change the fact that those which were maintaining constant velocity still maintain a constant velocity.
但是加上一个恒定速度并不会改变下述事实,即,那些正在匀速运动的物体,仍然保持匀速运动
One way to say it is, every body will continue to remain in a state of rest or uniform motion in a straight line.
另外一种说法是,任何物体都会继续保持静止,或匀速直线运动的状态
There's infinite number of them, but they're all moving relative to each other at constant velocity.
有无数个惯性观察者,但是他们都在相对于彼此,做匀速运动
So the Earth is an inertial frame of reference, if you go in a train relative to the Earth at constant velocity, you're also inertial.
所以,地球是一个惯性参考系,如果你坐在相对于地球匀速运动的火车里,你也处于惯性状态
The summary is that constant velocity doesn't require anything.
概括起来就是,匀速运动并不需要任何外力
We are moving at constant velocity.
我们现在是让物体匀速运动
It looks like a tautology because you're never able to show me something that moves forever at a constant velocity, because every time you don't find such a thing, I give an excuse, namely, a force is acting.
这听上去像个死循环,因为你不可能,找到永远匀速运动下去的物体,因为每当你找不到这样的物体时,我就会给出一个理由,也就是,有力作用在上面
Whenever you see a particle moving in a circle, even if it's at a constant speed, it has an acceleration, v square over r directed towards the center.
只要看到质点做圆周运动,即使是匀速圆周运动,也存在一个加速度,大小为 v^2 / r,方向指向圆心
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