A few years back, Myth Busters did a segment on the railcar that implodes in a YouTube video. The video has about a million and a half views. Here’s the Myth Busters episode:
You may wonder how this happens and why it doesn’t happen as trains go along the track routinely, without imploding.
The key is the hose you see coming from the railcar. This hose is pumping air out of the railcar, which has been sealed airtight. As soon as the pressure of the air inside the railcar pressing up against the steel walls is low enough, the railcar implodes as though it’s a cardboard box.
The key here is the difference in air pressure. Railcars don’t normally implode, because the air pressure inside is about equal to the air pressure outside. That is, there’s about as much pressure from air molecules inside the railcar pushing out on the steel as there is from air molecules outside the railcar pushing in. The forces from pressure are balanced, and there’s no tendency for the steel walls to implode or to explode.
But as the Myth Busters discovered, railcars are built pretty tough. They couldn’t force an implosion on a well-maintained railcar, no matter how much air they vacuumed out to create a great pressure differential. They had to dent the steel wall to, in a way, prime the pump, in order to see an implosion upon reducing the pressure inside the railcar to about 23 inches of mercury (normal atmospheric pressure is about 30 inches of mercury).
Using the ideal gas law and knowing that the maximum volume inside a railcar in the US is about 130,000 liters, how many molecules of air had to be vacuumed out of the railcar before the Myth Busters were able to get it to implode? See Next Generation Science Standard HS-PS3-2 for more information.
