来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  Under the combined effect of these two,
  在这两个合力的作用下
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  I can say m · g · sin(θ) - T = m · a,
  可以写出 m·g·sin(θ) - T = m·a
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  where a is assumed to be positive going downhill.
  假设沿斜面向下 a 为正
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  That's one equation.
  这是一个等式
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  Then, I come to this fellow here, this block;
  接着 我们看看这个物体
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  well, it's got m · g acting down and tension acting up.
  它受到重力 m·g 和向上的张力
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  I'm sorry guys, I made one mistake here.
  同学们抱歉 我在这犯了个错误
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  I think you know what it is.
  我想你们知道错在哪里
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  These should be M,
  这些应该是 M
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  that should be M, sorry for that, and M .
  这个应该是 M 抱歉 还有这个 M
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  For m, the equation is T – m ·g.
  对于 m 方程是 T – m ·g
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  Now, let's be careful with what I do next.
  我们得小心地写下一步