Eraser Movie Analysis

The rail gun in the movie "Eraser" creates some of the most epic scenes in the movie. The amount of power that the gun has in the movie seems to be limitless as the projectiles launched out of the gun were able to pass through pretty much any medium in their path. Well, besides a human body for some reason. Although the gun is very amusing to watch in the movie, it seems that the filmmakers forgot to encorporate momentum laws and recoil to the operation of the gun itself.

Here's a quick video to show what recoil should look like:

To be fair, not all guns have a recoil like the sniper rifle in the video. The amount of recoil depends on the mass and velocity of the bullet being shot, or in the case of the rail gun, the armature. The amount of recoil one should experience due to a gun is based on a fundamental concept in physics called conservation of momentumThe total momentum of a system should always remain constant. The momentum of a single object is simply its mass multiplied by it's velocity. When analyzing the momentum in a system, the total momentum of the system is just the momentum of each object added together. Conservation of momentum can be written as follows:

Lets take a look at a scene from the movie where we can analyze the momentum of the rail gun as it is being shot.

The velocity of the bullet before the rail gun was fired was 0m/s because it was stationary in the gun. Furthermore, the velocity of the gun was 0m/s because it is also at rest. That being said, the momentum of the system initially can be written as:

That is, the total momentum of the system before the rail gun was fired was 0kgm/s.

It is said in the movie that the rail gun shoots projectiles at a velocity close to the speed of light(about 3x10^8m/s). With an armature being about 3kg, the momentum of the bullet itself after being fired can be written as:

So our current equation for conservation of momentum can be written as:

Here lies the problem. In order for this to be physically possible, the gun must have a very large velocity in the opposite direction of the bullet(the negative direction). This would cause the right side of the equation to equal 0kgm/s which would set the initial momentum and the final momentum to be the same. Thus, momentum would be conserved. Lets have a look at the scene again.

The gun simply goes nowhere, which causes the momentum to not be conserved. In reality, the velocity of this recoil would kill any human holding the gun. With the law of conservation of momentum being one of the most crucial and fundamental laws in physics, this is a huge mistake in the movie. 

Overall Rating: RP


  1. In the future, try to actually embed your videos into your blog, rather than just providing a link. Otherwise, nicely done.


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