Buoyancy or upthrust is an upward force exerted by a fluid that opposes the weight of an immersed object; it is what makes an object float, sink, or remain neutrally buoyant in the water (or other fluids).
The symbol for the magnitude of buoyancy is \(B\) or \(F_B\). As a vector, it must be stated with both magnitude and direction. Buoyancy acts upward for the kind of situations encountered in everyday experience. As with other forces, the SI unit of buoyancy is the newton [N].
Buoyancy arises from the fact that fluid pressure increases with depth and from the fact that the increased pressure is exerted in all directions (Pascal’s principle) so that there is an unbalanced upward force on the bottom of a submerged object.
When an object floats, the upward buoyant force exerted by the water is greater than the downward force of the weight of the object. You can also understand this concept with numbers. If an object’s density is less than water’s density (1 g/cm3), it will float. When an object sinks, the weight of the object is greater than the upward buoyant force exerted by the water and its density is greater than 1 g/cm3.
When an object is neutrally buoyant, meaning it neither sinks nor floats, then the weight of the object is equal to the upward buoyant force exerted by the water. When neutrally buoyant in water, the object also has the same density as water. Neutral buoyancy is a very important principle in the sea perch competition. Just like a real submarine, you will want your sea perch to be able to submerge below the surface of the water without just sinking to the floor.