Do bullets throw their targets backwards?

In this quiet corner of the world there are thankfully few opportunities to verify this theory first hand. But we’ve all seen the movie hero popping off the baddies one by one, each thrown violently backwards by the impact of the bullets — preferably straight through a convenient plate-glass window. It looks great, but it’s dead wrong.

When it comes to entertainment, accuracy will always take a back seat. The more explosions, space battles and city-crushing mutants the better, and hang the science.

There’ll be none of that here. Sit quietly, boys and girls, it’s time to learn about momentum. In plain language, the heavier something is or the faster it is moving the more momentum it has. Total momentum is always conserved when two objects (such as a bullet and, oh, let’s say a Nazi) collide. So if we know how fast the bullet is going before it strikes and how heavy everything is we can work out how fast the Nazi will be moving after the impact.

A tiny object can pack a real punch if it is going fast enough, but it needs to be going faster than a speeding bullet to provide the kick we’re used to seeing. A typical bullet fired from a rifle carries enough momentum to bring an 80kg person to only about a tenth of walking speed. More powerful weapons don’t improve much on that.

But just as real snow looks fake on screen, we’d be disconcerted to see the bad guys taking their lumps without all the gymnastics. That is reserved for the super villain of the piece to highlight his alien nature. It looks less creepily unreal even though technically it is more accurate.

People aren’t hurled backwards when they are shot, but then it is just entertainment. It can be fun to pick at the holes in Hollywood’s latest offerings — and movie science is so patchy it’s like shooting fish in a barrel — but be careful. If everyone else is there only for the thrills, your trenchant observations will just be shot down.

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