In the original Pirates of the Caribbean, Captain Jack Sparrow and Will Turner escape Port Royal by strolling out to an anchored ship. They do this by walking on the seafloor, using an upside-down rowboat to hold air so they can breathe. It’s an awesome scene, but is it, er, possible? Could you do this yourself?
Of course, the great thing about movies is that they don’t need to be realistic (and the Pirates franchise makes the most of that exemption). I’m only asking about this because it’s a fun way to do some physics and understand how the world works. So grab your goggles and let’s plunge in!
What Floats Your Boat?
It may not seem like it, but objects in the water still have gravity acting on them, same as on land. Then why do you feel weightless underwater? Well, let’s back up for a moment. Imagine you have a block of steel and a block of styrofoam, both measuring 1 cubic foot in volume. Each one has a mass, which equals its volume (V) times its density (ρ). Then the gravitational force (Fg) on each is simply mass times the gravitational field (g):
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