Buoyancy
From MyMCAT
Introduction
Have you ever wondered why a boat doesn't sink? or why objects underwater are a lot easier to lift than when you try to lift the object out of the water? These effects are a result of buoyancy.
Archimedes' principle & Buoyancy
Bouyancy is caused by pressure differences around the surface of an object (Specifically the difference between the top and bottom). If one imagines fluid pushing on an object from all sides, and then one recognizes that the bottom side has a strong push (high gauge pressure), then it should be evident that in some way, the overall force is pushing up. The strength of the upward force, which we label the buoyancy force, in fact is equal to the magnitude of the weight of fluid that was displaced by the submerged object. This force enables objects to float or at least to seem lighter.
In general, the buoyancy force is equal to,
however pay special attention to what each of these terms are. The density, is that of the fluid which is being displaced, NOT the density of the object in the fluid, and similarly, the volume is that of the fluid displaced, not of the object. (This is important when an object isn't completely submerged, because in this case the volume of the object is greater than the volume of fluid it displaces.)
Another way of stating this is that "any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."
Buoyancy and Density
If the weight of an object is less than the weight of the displaced fluid when fully submerged, then the object has an average density that is less than the fluid and has a buoyancy that is greater than its own weight. If the fluid has a surface, such as water in a lake or the sea, the object will float at a level where it displaces the same weight of fluid as the weight of the object. If the object is immersed in the fluid, such as a submerged submarine or air in a balloon, it will tend to rise. If the object has exactly the same density as the fluid, then its buoyancy equals its weight. It will remain submerged in the fluid, but it will neither sink nor float. An object with a higher average density than the fluid has less buoyancy than weight and it will sink. A ship floats because although it is made of steel, which is much denser than water, it encloses a volume of air which is lighter than water, and the resulting shape has an average density less than that of the water.
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