And we want to get the world's attention focused where we think it needs to be, with these continuing efforts by al Qaida and others to get just enough nuclear material to cause terrible havoc, destruction and loss of life."

VOA: standard.2010.04.11

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