“Eureka!”, the Greek mathematician Archimedes cried out as he walked naked through the streets. What had he just discovered in his bathtub? That an object under water is pushed upwards by a certain force.
Gravity pulls the submarine downwards, making it displace water. Because where the submarine is, there can’t be any water. There’s also an opposite force. It pushes the submarine upwards. This is known as Archimedes’ principle or the buoyant force. The buoyant force is equal to the weight of the water that the submarine displaces. If gravity is equal to the buoyant force, then the submarine floats.
If you pump the tanks full of air, the submarine becomes lighter and rises. In that case, the buoyant force is greater than the gravitational force. If you fill them with water, it becomes heavier and sinks because the gravitational force is greater than the buoyant force.
Buoyant force in fish
How do fish go up and down in the water?
They usually have a kind of balloon in their body, a swim bladder. They blow gas into the swim bladder to go up and let gas escape to go down. This is how they move up and down.
