The anger of the many educated but unemployed young people has been a driving force in the Arab Spring movement.

VOA: special.2011.05.27

I draw a graph here of the amount by which I pull the spring versus the force it exerts.

我在这里画个图,纵轴是弹簧的拉伸量,横轴是力的大小

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Newton is not going to come and tell you what force the spring will exert when it's pulled by some amount.

牛顿不会过来告诉你,弹簧被拉伸一定量时会产生多大的力

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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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First, we got to make sure the spring exerts a fixed force every time.

首先,要保证,弹簧每次产生的力是恒定的

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If every time I get the same acceleration, I'm convinced this is a reliable spring that is somehow producing the same force under the same condition.

如果每次都能得到相同的加速度,这个弹簧的可靠性就能令人信服,即它在相同的条件下能够产生相等的力

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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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This is the force exerted by the spring on the mass.

这是弹簧对物体的作用力

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We don't know what the force of the spring is.

我们不知道弹簧的力多大

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A spring will, on the other hand, exert a force.

但是一个弹簧可以提供一个力

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You cannot say to me, "Take a force, due to a spring, and see what force it applies and divide by the acceleration and get the mass, " because we haven't defined force, either.

你不能说,"给弹簧施加一个力,看看它能提供多大的力,然后除以加速度,得到质量",因为我们还没给力下一个定义

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But if I'm accelerating, the force exerted by the spring and therefore, the needle the weighing machine reads is m.

但是如果我在加速运动,那么弹簧的弹力,也就是称重机上指针的读数就是 m

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Therefore, it'll compress by an unknown x, which is equal to mg divided by the force constant of the spring.

因此它的压缩量为 x,也就等于 mg 除以弹簧的弹性系数

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It says, if you pull it to the right, so that x is positive, the spring will exert a force which is in the negative direction; that's why you have a minus sign.

就是说,如果你向右拉,x 取正值,弹簧产生的力方向为负,这就是这里用负号的原因

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We did the comparison with the elephant or something; a is the second derivative of x and for this problem, when F is due to a spring, we know the force is that by studying the spring.

我们也已经把它与大象或其它东西作过比较,a 是 x 的二阶导数,在这个问题中,F 是由弹簧产生的,我们在讨论弹簧问题时已经知道了力的大小

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