9 Laws of Physics That Don't Apply in Hollywood

In general, Hollywood filmmakers follow the laws of physics because they have no other choice. It’s just when they cheat with special effects that we seem to forget how the world really works.

1. Those Exploding Cars

No car explosions, please - found at LookyLuc [Flickr]

When you’re watching an action flick, all it takes is a crash, or maybe a stream of leaky gasoline that acts like a fuse, and suddenly, bang! You see a terrific explosion that’s complete and violent. But gasoline doesn’t explode unless mixed with about 93% air. Gas-induced car explosions were discovered on film relatively recently (you don’t see them in the old black-and-white movies), and now audiences just take them for granted. In general, there’s no need to rush out of a crashed car, risking injury, because you fear an imminent explosion – it’s probably not gonna happen.

2. Sound that Moves at the Speed of Light

Hollywood always gets this one wrong. On film, thunder doesn’t follow lightning (as in real life, because sound is slower); they occur simultaneously. Similarly, a distant volcano erupts, and the blast is heard immediately rather than five seconds later for each mile. Explosions on the battlefield go boom right away, no matter how far away spectators are. Even a small thing, like the crack of a baseball player’s bat, is simultaneous with ball contact, unlike at a real game.

3. Everything is Illuminated: The Myth of Radioactivity

Film would have you believe that radioactivity is contagious and makes you glow in the dark. Where did this idea come from? The Simpsons? Perhaps, but the truth is that the most common forms of radioactivity will make you radioactive only if the radioactive particles stick on you. Radioactivity is not contagious. If a person is exposed to the radioactive neutrons from a nuclear reactor, then he can become slightly radioactive, but he certainly won’t glow. And because radioactive things emit light only when they run into phosphor – like the coating on the inner surface of a TV tube – you don’t really need to worry.

4. Shotgun Blasts and Kung Fu Kicks Make Targets Fly across the Room

With the string of new kung fu films out (they run the gamut from The Matrix to Charlie’s Angels), you just can’t escape the small matter of bad physics. Yeah, the action scenes look great and all, but in reality momentum is conserved, such that every action has an equal and opposite reaction. So, when you see a gal kick someone across the room, technically, the kicker (or holder of a gun) must fly across the room in the opposite direction – unless she has a back against the wall.

5. Legends of the Fall

We aren’t surprised when the cartoon character Wile. E. Coyote runs off a cliff and is suspended there momentarily before he falls. But in the movies, buses and cars shouldn’t be able to jump across gaps in bridges, even if they go heavy on the accelerator. The fact is, a vehicle will fall even if it’s moving at a high speed. During the 1989 San Francisco earthquake, a driver saw a gap in the bridge too late, and probably inspired by the movies, accelerated to try to make it across. Unfortunately, the laws of physics were not suspended, and he fell into the hole and crashed on the other side. Movies with special effects should come with a warning: “Laws of physics are violated in this movie. Don’t try these stunts at home.”

6. The Sounds of Science

All across the silver screen, you’ll catch people screaming as their car flies in slow motion across the gap in the bridge. The problem, though, is that their voices don’t change. In reality, if you slow down motion by a factor of two, the frequency of all sounds should drop by an octave. Women will sound like men, and men will sound like Henry Kissinger. Sound is an oscillation of the air. Middle C, for example, is 256 vibrations per second. If time is slowed down, there are fewer cycles per second, and the resulting sound is lower in pitch.

7. Shell Shock! Exploding Artillery Shells that Blow Straight Up

In movies, shells tend to kill only the person standing directly over them. It seems like a waste of artillery, since – if you believe the movies – each shell can’t kill more than a single rifle bullet can. But in real life, artillery shells blow out in all directions, killing people all over. Movie directors like to have their actors running through a field of such shells, but they don’t want their actors killed, so they arrange for underground explosions in holes that blow straight up, missing anyone who’s more than 5 feet away.

8. The Sparking Bullet

Sparking bullets are relatively recent invention in movie special effects. The gimmick provides a way of letting the audience know that the bullet just barely missed its target. In real life, sparks do occur when you scrape steel or other hard metals on hard surfaces (such as brick) because little pieces of brittle materials are heated to glow and fly off. The problem here is that bullets are generally made of lead because it’s dense and soft, and you don’t want the bullets scarring the steel of the gun barrel. Ever notice that no sparks fly from the front of the gun? That’s because you’re seeing lead bullets.

9. Sound Travels in Space

This is the granddaddy of all scientific complaints about space movies. For instance, in space the hero shouldn’t be able to shout out instructions to the other astronauts from a spot several yards away. The movie Aliens corrected this misimpression with its tagline: “In space, nobody can hear you scream.” And it’s true. Sound is the vibration of air, and it’s sensed when the air makes your eardrums vibrate. But try to forget this rule as soon as possible; it’ll wreck a good many movies for you.

From mental_floss' book Condensed Knowledge: A deliciously Irreverent Guide to Feeling Smart Again, published in Neatorama with permission. [Update 3/6/07: Originally written by UC Berkeley physics professor Richard Muller]

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