Stupid movie physics


The season of the movie blockbuster is upon us, and that means it’s time once again to ask the question: what planet do the people in movies live on? Judging by the physics displayed, it’s not this one.

In our world, for example, you cannot outrun the fireball of an exploding bomb down the hallway of a building and leap through a handily placed window just before it engulfs you.

And that’s just one example. For others, I turned to the Intuitor Insultingly Stupid Movie Physics Web site, where a list of the most common examples of stupid movie physics is maintained.

First: sparking bullets. In movies, bullets which strike other objects, especially steel objects, create sparks or flashes of light. In real life, bullets don’t spark. They’re mostly made of copper-clad lead, and copper and lead hammers are used in industries involving flammable fumes precisely because they don’t spark.

And where does all that ammo come from, anyway? Oh, I know, people in movies have been not running out of bullets since the days of Tom Mix. But that still doesn’t address the weight problem. Just three minutes of sustained fire from the Mac 10 machine gun, a popular choice with movie heroes, would expend 3000 bullets (100 full magazines), each weighing about 15 grams–a total of 45 kilograms of lead. So where are the sidekicks with wheel barrows to carry it all?

Next on the list: exploding cars. Cars in movies blow up the moment they strike anything. Sometimes, they leap off of cliffs and explode either the moment they hit the ground, or (my favorite) while they’re still in mid-air. But gasoline is only flammable if it’s mixed with air in a ratio between 0.8 and six percent gasoline vapor to air. For a car to explode in a collision, gasoline must escape its tank and be instantly sprayed in a fine mist over a large area in exactly the right proportion to the air, at the same instant being exposed to a source of ignition. If real cars were as explosion-happy as movie cars, they’d be detonating like popcorn.

Windows are another example of stupid movie physics. Broken glass is so sharp it takes almost no force for it to cut skin Yet in movies, people are always smashing through plate glass windows and escaping without a scratch. In real life, that would very likely be suicidal. All stationary objects have inertia–it takes force to move them. If you run through a window, the shards of glass created by its breaking will resist being moved by your body–and that momentary resistance is plenty to allow them to slice through clothing and skin.

Speaking of glass, ever notice how many times shooting victims in movies get knocked backward into a window or some other large glass object? Glass in the movie world apparently has some strange attractive force. But more to the point, even a shotgun blast won’t hurl a victim backward through the air. Calculations on the Stupid Movie Physics Web site show that a shotgun blast will move the victim backward, under ideal conditions, at about 0.193 metres per second–only about one-tenth of normal walking speed. The victim is likely to be spun around or to stumble back, but not to be hurled through the air.

And speaking of hurling through the air, people in movies routinely survive the most amazing falls. But falling just one metre imparts the same kinetic energy as being shot by a .45 caliber bullet–plenty of people of broken bones falling out of bed. Falling six metres, which action heroes seem to be able to do with impunity, is equivalent to being hit by six bullets at once. Sure, falling doesn’t penetrate vital organs, but still–something’s gotta give.

Finally, two of my pet peeves: outer space explosions and visible laser beams. Outer space explosions do not make noise! No air, no noise. And laser beams do not make visible streaks through the air. (And spraying an aerosol into the beam, as happens in some slightly smarter movies, isn’t a fix for this problem: it would almost certainly activate the alarm.

Not only that, real-world security systems seldom use expensive, complicated lasers. They use cheap, hard-to-fool passive infrared sensors that pick up on movement and body heat.

I know, I know, “they’re just movies.” But you can tell just as exciting a story using realistic physics as you can using stupid movie physics.

I just wish someone would try.

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