Scientific common sense around us 2

How can a submarine dive, float or even suspend in the water?

This mainly depends on the relationship between buoyancy and gravity. According to the "Archimedes Principle", any object in the liquid will be supported by buoyancy, the magnitude of which is equal to the weight of the liquid expelled by the object itself. When the weight of an object is greater than the buoyancy, it will sink; If it is less than the buoyancy, it will float upward. When it is equal to buoyancy, it will suspend in the liquid. The two forces are equal in magnitude but opposite in direction.

When the submarine is in water, both forces will act on the submarine. As mentioned above, the weight of the submarine itself is called gravity, and the weight of the seawater expelled by the submarine is called buoyancy. If you want to make the submarine dive, that’s easy, just make it weigh more than its buoyancy.

Then how to increase the weight of the submarine?
     All submarines are equipped with ballast tanks. As long as the empty ballast tanks are filled with water, the submarine will become heavier. At this moment, the weight of the submarine will be greater than the weight of the expelled water by it(i.e. greater than the buoyancy force), and the submarine will gradually dive. When the submarine floats normally, the water in the ballast water tank will be squeezed out by high-pressure air step by step, thus reducing the weight of the submarine underwater. When the weight of the submarine is less than the weight of the same volume of water (i.e. less than the buoyancy), the submarine floats until reaching the surface.
       In addition, the submarine in navigation can also be adjusted to a safe depth of 30m from the water surface(the safe depth is a limited depth to prevent collision with water surface vessels). If it continues to float to a depth of 10-30m, then it reaches a dangerous depth. The depth less than 10m is a submersible depth, and after reaching the submersible depth, it will be allowed to drain water and keep floating to the water surface. The principle of submarine diving and floating is similar to that of fish. There is a bladder filled with gas in the fish's belly. Its function is similar to the ballast tank in the submarine, and it is the main regulating organ for fish to float or sink in the water.



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