DENJIRO: Today's topic is the force that allows an object to float, called buoyancy. Here is a container filled with water and a boat made of aluminum foil. Let's see what happens when we place it in the water. It doesn't float on top of the water, it actually floats above the water in the air! Isn't this strange?
The container actually had a gas that's heavier than air. The upward buoyant force that is exerted on a body immersed in a gas or fluid is equal to the weight of the gas or fluid that the body pushes away. This is called Archimedes' principle. In this experiment, the aluminum boat displaced some of the gas, resulting in buoyant force on the boat. That's why it floated.
Now, here is a bar of lead. It's heavy, so it'll obviously sink in water. However, when we put the same block of lead in this silver liquid... it floats! The liquid is mercury. The density of lead is about 0.8 times that of mercury. Since lead has a lower density, it floats in mercury.
Now, we have prepared a bar of pure gold. What will happen when we place this in mercury? It sank! The density of pure gold is about 1.4 times larger than mercury. The density of pure gold is larger than mercury, so it sank.
Now, let's try another experiment. Here's a block of wood. This will float in water. However, when we push it down underwater, it stays there and doesn't float up, despite Archimedes' principle. Why does this happen?
This can be explained by thinking of water pressure. The top and bottom of the wood are at a different depth. There's more water pressure at the bottom, and this results in a buoyant force. However, when we place the wood at the bottom of the container, there is no water pressure at the bottom of the block. Since there is no more buoyant force, the block does not float up.
I hope energy will bring you all happiness. Our magic word is "Happy Energy!"