The tipping point for so many people is the more constant concern of putting food on the table and providing for a family.

VOA: special.2011.05.21

You put a chunk of wood on a table and you try to move it at constant speed.

你把一大块木头放在桌子上,试着让它匀速运动

耶鲁公开课 - 基础物理课程节选