## Sunday, September 13, 2015

### Physics: Erased

Everybody knows of the classic railgun, and anybody who knows what they're talking about knows that Eraser is indisputably the best move ever created in the history of mankind. Here is an example why:

During one climatic scene of the movie, Arnold Schwarzenegger must use two railguns at once to fend off the baddies. This scene totally makes physical sense and here is why.
First, we begin with a picture to help visualize the scene:

Now that you can perfectly visualize the scene at hand, we can try to put some numbers behind it. Here is what we already know:

M,a (Mass of Arnold) = 100 kg (estimated with Google).
M,b (Mass of Bullet) = 0.01kg (estimated by rounding the weight of a heavy bullet.)
Vi,a (the initial velocity of Arnold) = 0 m/s (as he is not moving before he shoots.)
Vi,b (the initial velocity of the bullet) = 0 m/s (as it does not move before it is shot.)

M,v (Mass of Arnold's victim) = 80 kg (the approximate weight of the average man.)
M,b (still) = 0.01 kg
Vi,b = 3x10^7 m/s (as the movie says the bullet moves close to the speed of light)
Vi,v = 0 m/s (as he is not moving before he is shot.)

Now, to prove that this masterpiece of a movie is realistic, we must use that information to see if the law of conservation of momentum is kept. (Pf = Pi)
I'll start by finding the final velocity of the victim with the bullet lodged in him.

Vf = (M,b)(Vi,b) / (M,v + M,b)

Vf = (0.01kg + 3x10^7m/s) / (80kg + 0.01kg)

Vf = 37,000m/s
(This also tells us what Vf,b on Arnold's side is)
Already we've come across perfect physics, as the victim in the movie abso- wait. No he doesn't. This is right at all. That's ridiculous. This means that the baddie should have flown back at impossible speeds after the bullet lodged in him.

Now we'll do the same for Arnold. If the momentum is conserved, then the recoil should be pretty hefty.

Vf = -(M,b)(Vf,b) / (Ma)

Vf = -(0.01kg)(3x10^7m/s) / (100kg)

Vf= -3x10^4 m/s
Arnold doesn't seem to move at ALL when he fires not just one of these guns, and he fires two! If he doesn't fly back, his arms should at least be ripped from their sockets as if they were made of wet paper. Could it be that the greatest movie ever does not contain accurate move physics??

I rate Eraser -3x10^4 out of 10 on physics accuracy.

#### 1 comment:

1. Nice graphics! I like that touch. And the rating system. Rating systems should use more scientific notation! I wonder about your taste, though. Eraser is a classic bad action movie, but it shouldn't make anyone's list of good movies.