A senior State Department official said the United States is in contact with other participants in the six-party process on how to get North Korea back to the negotiating table.

VOA: standard.2009.04.15

Whenever there is a force, it can be traced back to a tangible material cause, which is all the time a force of contact, with the exception of gravity.

如果有作用力,那它一定能找到一个明确的实体施力物体,也就是无论何时物体受的都是接触力,引力除外

耶鲁公开课 - 基础物理课程节选

It expands. You change your constraints on your system, you heat it up some more, then you take the heat source away, and you put it back in contact with the atmosphere. And you cool it a little bit, change the constraints, cool it a little bit more, and heat, and you've got a closed cycle engine.

让它与大气接触冷却,改变约束，使它继续冷却，然后又再次加热,这就是循环热机，我们以前也研究过，更复杂的机器。

麻省理工公开课 - 热力学与动力学课程节选