Final answer:
Submarines float on the water surface and dive down into water by controlling their density using ballast tanks, which adjust their buoyancy. This allows submarines to float or sink as desired.
Step-by-step explanation:
Submarines float on the water surface and dive down into water using a principle known as buoyancy, which is governed by Archimedes' principle. According to Archimedes' principle, an object will float in a fluid if the buoyant force acting on it is equal to or greater than the gravitational force pulling it down. Submarines achieve this by controlling their density using ballast tanks, which allow them to adjust their buoyancy.
When a submarine wants to float on the water surface, it fills its ballast tanks with air, which increases its overall density and makes it less buoyant. As a result, the gravitational force acting on the submarine becomes greater than the buoyant force, causing it to sink. On the other hand, when a submarine wants to dive down into the water, it pumps out water from its ballast tanks, replacing it with air. This reduces the submarine's density and increases its buoyancy, causing it to rise to the water surface.
Overall, the ability of a submarine to control its density and buoyancy allows it to float or sink as desired, enabling it to navigate underwater.