Buoyancy in FE8B

Hello everybody,

In FE8B Science, we have been learning about buoyancy just like Eric said in FE8A. The definition of buoyancy is the ability of a fluid to support an object that is in or on it. Buoyant force is the opposite force to gravity, so if there is more buoyancy than gravity on an object in a fluid, it floats. The opposite happens if there is more gravity than buoyancy (it sinks).

But if this is true, than why do small rocks sink in water while large ships float? This is where average density comes in. Average density is the total mass divided by the total volume. A rock has a large mass, as there is a lot of matter in a rock, but not as big a volume as it is not very big. This results in a large density. A ship doesn't have a very large mass as it has a hollow hull, but it has a very large volume so the average density of a ship is fairly small. The density of a ship is much less than that of water, so it floats. The density of a rock is more than that of water, so it sinks.

Another cool thing with buoyancy is how objects of larger area on the water float easier compared to those with the same mass but less area on the water. For example, when you just stand straight in water without moving your hands, you will sink. However, if you lie on your back without moving your hands, you float. You still have the same mass and gravity pulling you down as when you were straight up and down, but this time you float. This is because there are more water particles supporting you when you take up a larger area. More particles under you means that there is more buoyant force pushing you up than the gravity pulling you down, so you float. When you are straight up and down there aren't as many water particles underneath you. This results in less buoyant force than gravity which makes you sink.

That's all for now.
Until next Wednesday,
-K