Fish & Buoyancy

Today let's revise some form 4 topics on Buoyant Force. In SPM, we learned that buoyant force depends on its volume of water displaced. The larger the volume displaced, the greater the buoyant force. Buoyant force also depends on density of the water, i.e greater water density produce greater buoyant force.
Learning physics will be interesting if u are able to blend its concept with nature & things around u. When i was a kid, i loved to keep fish as a pet and guppy was one of my favorites. Fish is very mobile in water. They are able to swim to the surface, stay in the middle of the water or sink to the bottom.

Ok how they did it.. I mean to float and sink?? Basically fish is able to do it through air filled bag called swim bladder. This bladder has nothing to do with its urine product. In fact swim bladder helps most fish to maintain its buoyancy. To swim upward a fish fills the bladder with oxygen through the gills. When the bladder is filled with oxygen, the fish has a greater volume and its bladder is expanded. This displace more water and so it experiences a greater buoyant force.

In order to sink,air is removed from the bladder that causes decrease in its volume. Thus its average weight will be greater greater than buoyant force which then pull it towards ocean floor.

To float at certain level, the bladder is filled with air until it displaces a volume of water to produce a buoyant force that equal to its weight. In this case, the weight & buoyant force are in equilibrium. Thus it stays at that level. The same concept is applied in submarine except submarine needs a ballast tank to increase or decrease its buoyancy by adding or removing sea water inside it.