Using the known volume, air density, and gravitational field values, the resulting buoyancy is 11.8 Newtons or 2.7 pounds.
Now, let’s replace that air block with another block of the same shape and size. But this time, suppose it is 1 cubic meter of water with a density of ρwater = 1,000 kg/m23.
Since it has the same volume as the floating air, the block will have exactly the same buoyancy.If its volume is 1 m, then it doesn’t matter what you put in that space3, It will have a buoyancy of 11.8 Newtons. But for this side of water, it is not enough to make it float. The gravitational force pulling it down will be much greater-it is 9,800 Newtons. The Water Cube is about to fall.
In order to make the buoyancy greater than gravity, you need to fill the space with a substance with a density lower than air. There are two common ways to make it work in real life. One is to use a thin rubber container filled with low-density gas. (Think of helium balloons.) The other is to use low-quality containers to hold hot air. The density of hot air is lower than that of cold air and will rise above it. (Think of hot air balloons.)
So if you want the cloud to float, its density must be lower than the density of air. But if the cloud contains air at the same time, how can the density decrease? with water?
This is because the cloud does not really float.
Why is the size of the water important?
Suppose a cloud is composed of air and a bunch of very small water droplets. The size of the droplet is important. You may be surprised to find that even though they are all made of water and have the same shape, small water drops behave differently from large water drops. In order to understand the difference between them, we need to look at air resistance.
Let’s start with a quick demonstration. Extend your arms in front of you, and open your hands. Now swing your arms back and forth and let your hands move quickly in the air. How do you feel? It may be small, but there should be an interaction between your hand and the air, a backward thrust we call air resistance or air resistance. (If you stick your hand out the window of a moving car, you will definitely notice it.)
We can use the following equation to simulate the air resistance on a moving object: