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Buoyancy is the tendency to float or rise up in a fluid. Buoyancy is a force in our environment that is useful in many ways. It helps us transport both goods and people through water and air with the technology of aircraft and ships. It also increases our options to travel across the world. Some objects float in water and others sink, because of their density. If an object is denser than water, it will sink. An object less dense than water will float. For example, a rock will sink in water because the particles in it are packed tighter than those of water. Of course, this does not mean the rock is heavier than the whole body of water it is in. It means that with the same amount of volume from each substance, the rock has more mass than the water. A stick will float because it is less dense than water. Another explanation for buoyancy is that objects take up space in a fluid; ... they push some of the fluid away, or displace the fluid. As they push the fluid away, it pushes back, causing it to float. 
The reason the whale can raise itself above the water, besides the fact that it is pushing itself up with its tail and fins, is because the whale can increase the size of some of its body cavities, letting the air inside them expand; this decreases its overall density to a value less than the density of water. This will allow him to rise up to the surface the water, gaining momentum and speed, so when he hits the surface, he comes above it. He then quickly comes down because he is denser than air, so there is very little buoyant force. Similarly, he can compress these cavities, decreasing the volume of air in his body, and increasing his overall density; this makes him sink. In fish, the organ that does this is called the swim bladder. Other Information on Buoyancy: Archimedes' principle states that the buoyant force of an object is equal to the weight of the fluid displaced by the object. Buoyancy is measured in Newtons (N) Since there is no formula for buoyancy, you can figure out whether something will float or not by it's density compared to the density of water. Density equals mass divided by volume. |