When Hollywood Goes Badly Wrong

We’re going to settle this whole “Guy flying backward when hit with bullets or other projectiles” thing once and for all.

In the movies and television, very often an overly-creative director instructs his special effects people to make it look like people who were shot with projectile objects fly backwards in space, as if the bullet or arrow hit them with so much force that it lifted them up and tossed them back like a ragdoll.

That couldn’t be further from what really happens.  Projectiles are small, light, fast objects designed to penetrate flesh.  The projectile is supposed to enter a body (human or animal) and cause internal damage so that the body stops not due to momentum, but due to structural damage to the bones and organs.  For example, a bow hunter tries to shoot a deer through its lungs and heart in one shot, meaning it won’t be able to breathe or pump blood, which will bring it down in a matter of moments as its brain runs out of oxygen.

When we fire our submachine gun “hoses” at bugs and other bad guys, we first aim for the balls/hips.  Our bullets smash into the bones of the upper legs and hips, shattering the platform that allows a creature to move.  If the undead don’t have moveable legs, they can’t run after us, dig?  This is why this whole “flying across the room” thing is so annoying to professionals like us.  See, if we only shoot them in the chest, the bullets don’t even slow them down.  Let me explain with some physics.

ridiculous graphic of man getting blown away by a tiny crossbow bolt

Clearly this dude is getting blasted by a massive force, emanating from a magical aluminum tube?!

Let’s take the case of the image above where this dude is getting hit with a crossbow bolt in the beginning of season 1, episode 1 of the show “Revolution.”  In it the bolt clearly hits him in the chest, because his head and legs snap forward.  His torso stops immediately in space, and falls to the ground at the speed of gravity.  This is an absolutely impossible scenario, because the dude simply has too much momentum for that tiny little crossbow bolt to handle.

The Victim:
The victim appears to be about six feet tall, as he is carrying a standard-looking pickaxe (36 inches long), and since he is running with it one-handed with apparent ease, let’s presume he weighs about 180lbs.  The average human can run about 11mph (some as fast as 23mph), but since the guy had only started running seconds before, let’s say he was running at 5mph when he was hit with the bolt.

The Shooter:
The shooter appears to be about equal size and weight, firing a basic crossbow from across the street.  Because we can’t determine the type of crossbow he’s using, we’re going to give him the incredibly powerful PSE Tac 15i crossbow.  Now, he is CLEARLY not using anything even remotely close to this powerful a crossbow, but we’re going to give it to him anyway so that all the medieval weaponry nuts can’t argue the point that the crossbow could have been more powerful.  The PSE Tac 15i fires a 425 grain bolt at about 402 feet per second.

In order for this man to have been stopped dead in his tracks, the momentum from the crossbow bolt wouldn’t just have to be equal to the momentum from the running man, it would also have had to expend all of its energy when it hit, meaning it can’t fly through him and keep going (like bullets tend to do).

Momentum:
To determine momentum, we have to multiply the mass of the objects in motion by the speed at which they are moving.  First we convert to the metric system, so the 180lb victim weighs 81.65 kilograms, and moving at 5mph that converts to 2.235 meters per second.  That gives the running man a momentum of 182.49 kg x m/s.  The 425 grain crossbow bolt weighs .0275 kilograms, and 402 feet per second converts to 122.5 meters per second.  That gives the bolt a momentum of 3.37 kg x m/s.

Once again, in order to stop the man dead in his tracks, the momentum values would have to be at least close to equal, as some energy would be lost in heat, deceleration, flying limbs, etc.  But the figures we get clearly aren’t even close.

Victim = 182.49 kg x m/s.
Crossbow bolt = 3.37 kg x m/s.

With 54 times the momentum of the crossbow bolt, the running man may have slowed the tiniest amount (roughly .092mph) upon impact, but he would have sent the bolt flying.  The only reason the bolt didn’t take off like a baseball off a bat is because it was sharp and buried itself into his chest.

US Air Force hyperspeed craft

This US Air Force hyperspeed craft flew at about Mach 20, and peeled apart from the heat and friction, but a crossbow bolt is fine at that speed?

In order for the bolt to stop the man dead in his tracks it would have had to have been moving much faster, been much heavier, or been Scotch-taped to the front of a truck.  By much faster I mean it would have needed to be flying at 6636 meters per second, or roughly 14,840mph — which is about 19.5 times the speed of sound!  That’s right, there should have been a loud crack as the bolt created a sonic boom, which clearly didn’t happen.

That means the bolt would have to have been heavier.  In fact, it would have had to have weighed 1.49 kilograms, or 3.29 pounds, which is 54 times heavier than the bolt.  Let’s presume the wooden handle to the pickaxe the victim was carrying was made of ash, the 36 inch handle would weigh about 1.45 pounds.  If the shooter fired a wooden stake from his crossbow it would have had to have been over six feet long if made of ash, and been as thick as that pickaxe handle!

Which it clearly wasn’t… nevermind that the weenie crossbow couldn’t even have launched that big of a piece of wood.

So no matter how you slice it, or how you try to bend reality, gravity still exists, so physics still apply.  Sure, people who love medieval times probably never heard of a guy named Isaac Newton, as he came up with a few laws of motion (the second one in 1713) that clearly dictate how projectile objects will behave when hitting other objects.  Since we can calculate precisely what it takes to make human beings go flying around (without taking into account people are basically bags of Campbells Soup, and tend to burst apart when struck violently), we can very clearly do the math showing that it’s impossible for a tiny bullet, bolt, or arrow to toss someone across a room or into the air.

So please, JJ Abrams and other Hollywood types, cut it out.  It makes your work look amateurish.

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