DENJIRO: Today, let me show you a phenomenon caused by bubbles. Here, we have a pipe filled with water and a ball floating in it. What will happen when we have bubbles come out from the bottom?
Why did bubbles make the ball sink? When bubbles form, the water level rises. That's because the volume increases from the bubbles. Usually, the buoyant force acting on an object under water equals the mass of the water that is displaced. This is called Archimedes' principle. But when the water contains bubbles, the mass of water that is displaced by the ball decreases, causing the buoyant force to decrease. As a result, the ball sinks.
Now, let's try something with this water tank. There is a duck floating in it. Let's create some bubbles. It's not sinking this time. Why is that? Let's measure the buoyant force.
According to this spring scale, this ball weighs about 180 g. When we put the ball under water, the scale shows 80 g. This means the buoyant force on the ball is 100 g. Now, let's see what happens to the buoyant force when we turn on the bubbles. The weight shown on the scale isn't changing. Since bubbles move and disperse, the duck is only displacing water in this case. Therefore, the buoyant force doesn't change, and the duck does not sink.
Now, let's place the ball into a pipe before turning on the bubbles. Since the buoyant force decreased, the weight on the scale increased by about 20 g. Since the tube gets filled with bubbles, the duck is displacing both the water and bubbles. This will cause the buoyancy to decrease, and therefore the duck to sink.
I hope energy will bring you all happiness. Our magic word is "Happy Energy!"