Back-of-the-envelope Physics -

Ten minutes later, the girder was safely back on the ground. A single bolt had sheared halfway through.

"Okay," he whispered, smoothing the paper. "Let’s see if that cable is about to snap." Back-of-the-Envelope Physics

He didn’t have his laptop or a calculator. What he did have was a crumpled utility bill envelope and a leaking ballpoint pen. Ten minutes later, the girder was safely back on the ground