An object that floats in a liquid is positively buoyant. This means that the amount of water displaced by the object weighs more than the object itself. For example, a boat that weighs 50 lbs (23 kg) but displaces 100 lbs (45 kg) of water will easily float. The boat displaces more water than its weight in part because of its size and shape; most of the interior of a boat is air, which is very light. This explains why massive ocean liners float: as long as the water displaced weighs more than the ships themselves, they will not sink.
Negative buoyancy is what causes objects to sink. It refers to an object whose weight is more than the weight of the liquid it displaces. For example, a pebble may weigh 25 grams, but if it only displaces 15 grams of water, it cannot float. If the 50 lbs (23 kg) boat was loaded down with 75 lbs (34 kg) of freight, it would no longer float because its weight (125 lbs or 56.69 kg) is heavier than the weight of the water it displaces (100 lbs or 45 kg).
It is also possible for an object to be neutrally buoyant. This means that the object’s weight and the amount of liquid it displaces are about the same. A neutrally buoyant object will hover in the liquid, neither sinking nor floating. A submarine can adjust it weight by adding or expelling water in special tanks called ballast tanks. By properly balancing its ballast, the sub can hover at various levels under the surface of the water without sinking.
Size and Shape
How much of an object’s surface touches the water has an effect on its buoyancy. A very large ship has a lot of surface area, which means that the ship’s weight is spread out over a lot of water, all of which is pushing up on the ship. If the same ship was in the water with the bow pointing down, it would start to sink because all of the weight is concentrated in one small area, and the water it is displacing weighs less than the weight of the ship.