I know it's an old movie (and it was an even older book before that), but I want to look at the physics of the special submarine drive in The Hunt for Red October. In the story, the Russians build a so-called "caterpillar drive" using hydro-magneto power instead of the traditional propeller. This new drive is way quieter than the traditional type—so quiet that it could sneak up on the United States and blow it up. Spoiler alert: It doesn't.

Here is the cool part: This magnetohydrodynamic drive, which turns water into a sort of rotor, is a real thing. (Although technically in the book version this drive is something other than magnetohydrodynamic. Quibbles.) In fact, it's pretty simple to build. All you really need is a battery, a magnet, and some wires. Oh, also this will have to operate in salt water, so you might need some salt. Here is the basic setup.

How does this work? Well, when you put a positive and negative plate in salt water, it produces an electric field. With salt in water, you get both positive and negative ions—both of these are influenced by the electric field. In the setup shown above, the negative ions would move to the right and the positive ones move to the left. But the ion motion by itself does not produce any propulsion. For that, you also need a magnetic field.

In the diagram, I have a magnet with the north side pointing down. This produces a magnetic field that also mostly points down (as indicated by the red arrow). Now for the awesome physics part. If you have an electric charge moving in a magnetic field, there is a force on that charge—the magnitude of this force depends on the strength of the magnetic field, the value of the electric charge, and the velocity of the charge. This magnetic force can be expressed as the following equation: