Tuesday, April 12, 2005

Stupid Movie Physics

Did you ever watch a movie and say "No way can that happen!" Well, if you are talking about special effects, you are probably right.
Insultingly Stupid Movie Physics tells you what can and cannot happen in movies.
For example: Ever notice how cars in movies always burst into flames the instant they collide with anything? Our favorite is when a car falling from a high place explodes the instant before it hits the ground. It's as though its gas tank gets panicky and detonates at the mere thought of striking Earth. Fortunately, the physics are not so cooperative.
Gasoline has a very narrow flammable range of about 1.4 to 7.6 % gasoline vapor in air 2. In other words, the vapor-air mixture must be exactly as specified or the gas will not burn, let alone explode! Note that we say vapor. Liquid gasoline must change into vapor before it can burn (although this is no huge problem since it easily vaporizes).
For a car to explode during impact the tank must catastrophically rupture and spew a fine mist of gasoline over a large area so it can vaporize and mix with air in exactly the right proportions. The mixture must then find a source of ignition. Automobile gas tanks are built to withstand a considerable impact force and are usually located in a protected area between the beams of a car's frame. Common ignition sources in the car's engine are generally at the other end of the vehicle.
As portrayed in movies, gasoline tanks are fragile and gasoline so volatile that the vaporizing and mixing process occurs in milliseconds. It always results in an explosive mixture which always finds a source of ignition. Thank heavens it's not so easy or people would regularly be blowing themselves up while refueling at the pump.


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