Archimedes' principle has many applications:
1. Submarine: A submarine has a large ballast tank, which is used to control its position and depth from the surface of the sea. A submarine submerges by letting water into the ballast tank so that its weight becomes greater than the buoyant force. Conversely, it ascends by reducing water in the ballast tank; thus its weight is less than the buoyant force.
2. Hot-air balloon: The atmosphere is filled with air that exerts a buoyant force on an object. A hot air balloon rises and floats due to the buoyant force. It descends when the balloon's weight is higher than the buoyant force. It becomes stationary when the weight equals the buoyant force. The weight of the hot-air balloon can be controlled by varying the quantity of hot air in the balloon.