You cannot have a spring pulled only at one side, because then it will then accelerate with infinite acceleration in that direction.

你不可能对弹簧只施加一边的拉力,因为那样的话弹簧就会以无穷大的加速度加速,就在这个方向上

耶鲁公开课 - 基础物理课程节选

and then this man like walking passed me, pulled me up out of the manhole.

这时有一个人走过，把我拉了出来。

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Dynamics then tells you why the object goes up, why the object goes down and why is it pulled down and so on.

而动力学则会告诉你为什么物体向上运动,为什么向下运动,为什么停下来,等等

耶鲁公开课 - 基础物理课程节选