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If you connect the body to a spring, which is neither compressed nor extended, there's no force acting on it.
如果将一个弹簧连在这个物体上,弹簧既没有被压缩也没有被拉伸,也不会有力作用在它上面
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Every time you are finding out a different force that's acting on a body with different origin.
每次你都会发现不同来源的力,作用在物体上
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When a body is going around a circle, they say that's some centrifugal force acting.
如果一个物体做圆周运动,他们就说有离心力作用于这个物体
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If you leave the body alone, maybe there's no force acting on it.
如果你把物体丢在一边,可能就没有外力作用在它上面
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Then you say, "Oh, that's because there's a force acting on it."
然后你就说,"那是因为有力作用在上面"
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If you pull the spring, there is a force acting on it.
如果拉这个弹簧,就有力作用在物体上了
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Newton finds out there's a force of gravity acting on everything.
牛顿发现所有的物体都受到引力作用
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Maybe you should complete the force acting on this one.
也许该由你们来完成它的受力分析
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Then, you have to ask, "What other force is acting?"
然后你要问,"还有哪些作用力呢"
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First one says, if you knew the force acting on any body, without going into what caused the force, then you may set that force equal the mass times acceleration of the body.
第一部分是,如果你知道作用在任一物体上的力,无论这个力的来源是什么,都可以令力等于质量乘以物体的加速度
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A typical problem that you may have done in your first pass at Newton's law, someone tells you a force of 36 Newtons is acting on a mass of whatever, 4 kilograms; what's the acceleration?
在你们第一次接触牛顿定律时,可能见过这么一道典型题目,告诉你,一个 36 牛的力,作用在一个物体上,4 千克吧,问加速度是多少
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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
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Then we may, from this equation, say a certain force is acting in a given situation by multiplying the m times the a.
然后根据这个方程,我们就能说,得到给定条件下的作用力,它等于 m 和 a 的乘积
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F = ma is universally true, independent of the nature of the force acting on a body.
= ma 是一条普适定律,与作用在物体上的力的属性无关
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The only force acting on this guy is this little f.
这个物体只受一个力的作用,也就是 f
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