Could You Use a Rowboat to Walk on the Seafloor Like Jack Sparrow?

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 ( F g ) on each is simply mass times the gravitational field ( g ): But you already know about this, because F g is what normies call an object’s “weight,” and for a given volume, weight depends only on the density. Now, if you dropped these blocks in a lake, obviously the styrofoam would float and the steel would sink. So clearly it has something to do with density. What if you had a block of water with the same volume? If you could somehow hold this cube of water, it would feel pretty heavy, about 62.4 pounds. Now, if you place it carefully in a lake, will it sink or bob on the surface like styrofoam? Neither, right? It’s just going to sit there. Since it doesn’t move up or down, the total force on the block of water must be zero. That means there has to be a force counteracting gravity by pushing up with equal strength. We call this buoyancy , and for any object, the buoyancy force is equal to the weight of the water it displaces. So let’s think about this. The steel block displaces the same amount of water, so it has the same upward-pushing buoyancy force as the block of water. But because it’s denser and has more mass, down it goes. In general, an object will sink if the gravitational force exceeds the buoyancy force, and it will float if the buoyancy force exceeds the gravitational force. Another way of saying that is, an object will sink if it’s denser than water and it will float if it’s less dense. And right in the middle an object will neither sink nor rise to the surface-we call that neutral buoyancy. Humans are pretty close to neutral because our bodies are 60 percent water. That’s why you feel weightless underwater-the buoyancy...

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