Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

How bad would a person's injuries be if an elevator's cables snapped at the 100th floor so that the elevator fell to the bottom?

Category: Physics      Published: December 6, 2012

Elevators have automatic brakes so that they don't plummet if the cables snap. Public Domain Image, source: Library of Congress.

Elevator's cables almost never snap. And if a cable did snap, there are backup cables attached. Even if all the cables snapped, an elevator still would not fall to the bottom floor. Elevators have a passive, automatic, built-in braking system. The brakes require no electricity, no remote control, and no human operator. These brakes lock in place whenever the elevator starts to fall faster than an allowed amount. The brakes are passive in the sense that it takes no energy, computer, or external agent to turn them on. They still work if the power goes out or if no one is paying attention. This is possible because the brakes are simple mechanical devices. It is the initial falling motion of the elevator itself that engages the brakes. Way back in 1852 Elisha Otis invented this automatic braking system, as documented in the book Otis: Giving Rise to the Modern City by Jason Goodwin. His safety elevator has been in use ever since. If you were to be on an elevator where all the cables snapped, you would not feel much. The elevator would only fall a few feet and then stop. This is such a small amount that you wouldn't even think that you had been falling in the first place. You would think simply that a door had jammed or a motor had hiccupped. You may well ask: what would happen if all the cables snapped and the automatic brakes failed. Then, yes, the elevator would fall to the bottom floor. But in order to fail completely, the brakes have to be physically damaged by a significant accident, such as a colliding airplane. In that case, you have more to worry about than your elevator.

Topics: elevator, elevator brakes, elevator falling, elevator plunge, safety elevator