Physics flies off the rails in 'Unstoppable'

It’s a gripping premise: “1 million tons of steel. 100,000 lives. 100 miles to impact.”

The new movie “Unstoppable,” features a runaway train, loaded with toxic chemicals, speeding towards a town with a bend in the tracks. It’s actually loosely based on a 2001 incident when a renegade locomotive was halted just short of Kenton, Ohio.

But is the science in “Unstoppable” accurate? Could the train’s impact decimate a town?

Emory physicist Sidney Perkowitz applies the basic formula of kinetic energy – one-half the mass times the speed squared – to compare the impact of the “Unstoppable” train to nuclear bombs and an asteroid.

A hydrogen bomb carries energy equivalent to 20 million tons of TNT. A train weighing one million tons, traveling at 50 miles per hour, as the movie’s publicity describes it, would carry the equivalent of 60 tons of TNT. “That would be a major explosion,” Perkowitz says.

But can a train really weigh 1 million tons?

“I think whoever wrote the publicity forgot the difference between tons and pounds,” Perkowitz says. If the train had a weight of 1 million pounds, then it would only have the energy of 60 pounds of TNT.

“That’s certainly enough to take out a building,” he says. “It would turn an SUV into molecules and it would be a terrible crash, but not enough to take out a whole city block.”

Interesting science, but not likely to stop movie goers in their tracks. “It sounds like a fun film,” Perkowitz says. “I would sit back and munch some popcorn and let the train run over me, and not worry too much about the math.”

Check out this video of Perkowitz talking about the physics in the movie “Iron Man,” and how it has some basis for reality in the U.S. military.

Sound too far-fetched? Watch this news video from CNN, below, about a half-man, half-robot contraption developed by the defense contractor Raytheon: