When a man (Matt Damon) is rescued from the ocean it turns out he has amnesia. A mysterious Swiss deposit box is his only tie to his past. But as he sets out to find his identity, he discovers he has a talent for beating the fool out of anyone who even accidentally messes with him. His quest leads to a series of attempts on his life, car crashes, etc. before the final dramatic showdown.

Our favorite scene occurs when a would-be assassin makes an attempt on Damon's life. He uses a European assault rifle equipped with a telescopic sight and silencer. Unfortunately, the bullet from such a rifle will be supersonic and create a miniature sonic boom as it heads toward its target. While the silencer would help, supersonic bullets still make a lot of noise. However, this is not the only flaw. The assassin slides the silencer over the barrel of his rifle so that the business end of the silencer corresponds to the muzzle of the rifle. In other words, the gasses exiting the muzzle don't even pass through the silencer.

Silencers work by allowing gasses in the barrel to travel outward in a perpendicular direction from the the bullet's path. The gasses are contained in a specially designed chamber which allows them to swirl around and lose most of their kinetic energy before exiting through the end of the silencer. This prevents the shock wave which is normally emitted when a firearm is discharged.

The movie silencer could still work if the end of the assassin's rifle was drilled with numerous properly-spaced and -sized holes. These would allow gasses to enter the silencer. However, there were no holes. There was no way for gasses to enter the silencer and no way for it to work.

At one point Damon deliberately shoots a propane tank with a shotgun and causes it to instantly explode. He appears to be using buck or bird shot and fires from a significant distance, making it highly unlikely that the shotgun pellets would even be able to penetrate the tank. Even if they did, as noted elsewhere, ordinary lead projectiles are a rather poor source of ignition. However, if the tank had exploded it would probably have been far more dramatic than was portrayed in the movie.

When Damon needs a quick escape from the top floor of a building he simply pushes a dead body over the side of a stair well, jumps on top of it, and rides it to the floor. Yes, it would be less damaging to hit the floor on top of a body rather than fall directly on concrete, but would still almost certainly result in serious injuries. Our physics rating fell with Damon and the body.

The Bourne Identity has its moments and we must admit if we were going to have amnesia it would be pretty nice to find out we had a secret Swiss deposit box stuffed with cash. Being a secret agent with deadly martial skills might also be interesting. However, the kind of physics amnesia the moviemakers seemed to have is another matter.