A New York City bus driver has been hailed as a hero after catching a 7-year-old girl in midair when she plunged three stories from the window of a housing complex.

Stephen St. Bernard, 52, was walking home Monday afternoon (July 16) when he spotted the girl — who police later said has autism — standing on top of an air-conditioning unit three stories up. "She just stood up there teetering, teetering," Bernard later told reporters. He recalled thinking, "Please let me catch her, please let me catch her ... Let me catch the little baby."

And when she finally fell, he did just that. Bystanders captured the amazing incident on video.

But how was Bernard's life-saving catch possible? We asked Louis Bloomfield, a physicist at the University of Virginia, to analyze the incident.

"The girl fell about 25 feet, which took about 1.25 seconds. The man stopped her fall in about 3 or 4 feet, which took about 0.1 second, depending on the stopping distance and how he supported her. So, she accumulated downward momentum over about 1.25 seconds and gave that momentum to the man (and ground) in about 0.1 seconds," Bloomfield told Life's Little Mysteries.

For Bernard to bring the girl's body to a stop in one-twelfth of the time she spent accelerating toward him — that's 0.1 seconds of stopping time, compared to 1.25 seconds of falling time — he had to exert an upward force 12 times greater than her (downward) weight. (Pushing upward is how you slow a falling body to a stop.) That means, "If she weighs 50 pounds, the man and ground must push up with an average of 12 times that force, or 600 pounds." [How Powerful is Willpower?]

"In a pinch, a man could exert an upward force of 600 pounds briefly, but it would likely hurt and could cause injury," Bloomfield said. "And the girl could tolerate that much upward force briefly, if distributed properly."

Indeed, Bernard sustained a torn tendon in his shoulder, and the 7-year-old was treated for minor injuries. But the catch may well have saved her life. "Had the man not caught her, she'd have stopped much faster on the ground and the forces involved in stopping her would have been more severe — say, 1,200 pounds or even 2,000 pounds," Bloomfield said. "Big injury."

Follow Life's Little Mysteries on Twitter @llmysteries. We're also on Facebook & Google+.