You hook one end of the spring to a wall and you pull it from rest by some amount and you attach the one kilogram mass to it.

你把弹簧的一端固定在墙上,然后拉伸一段距离,接着你把 1 千克物体固定在上面

耶鲁公开课 - 基础物理课程节选

Particles with rest mass greater than zero mean time travel is possible.

FORBES: Faster Than Light Neutrinos: Not So Fast

For example, computing and electronics embraced the Internet early and will therefore reach critical mass earlier than the rest.

ECONOMIST: Business and the Internet

The pollution has turned so bad that more than 100 Utah doctors called Wednesday on authorities to immediately lower highway speed limits, curb industrial activity and make mass transit free for the rest of winter.

NPR: Sickening Fog Settles Over Salt Lake City Area