Why can a hot air balloon fly?

Nowadays, as a sport, hot air balloon is deeply loved by the public, and grand hot air balloon festivals have been held all over the world. When people cheer for the colorful giant hot air balloon to take off, many people will wonder why the hot air balloon can fly. What is the flight principle of hot air balloon?

A hot air balloon takes off by the buoyancy of the air. The buoyancy of the air to the hot air balloon is equal to the gravity of the air displaced by the hot air balloon. As long as the average density of a hot air balloon (along with the weight it carries) is less than the density of air, it can float. Hot air balloons need to be heated before launching. After heating, the volume of air in the balloon expands. The lower the density, the greater the density difference, and the more heavy objects it can carry. If we fill balloons or airships with low-density helium or hydrogen instead of hot air, we can also get enough lift to fly in the air.

The use of buoyancy has existed since ancient times. When Archimedes, an ancient Greek scientist, helped King Siracuse II solve the problem of gold purity in the crown, he first discovered the cause of buoyancy, which was later called Archimedes principle, that is, the buoyancy obtained by an object in a liquid or gas is equal to the gravity of the liquid or gas displaced by the object.

Archimedes principle has many applications, such as calculating the density of objects and the maximum load of ships. Life jackets are full of air. Because the density of air is much lower than that of water, it can help people who fall into the water to float on the water and wait for rescue. As a Kongming lantern for entertainment or blessing, China also uses the buoyancy generated by hot air. Legend has it that Zhuge Liang was besieged by Sima Yi. He made a paper lantern with a cotton ball under it. After lighting the cloth ball, the paper lantern rose to the sky and sent out a distress signal, and finally escaped from danger. The legendary story of "Cao Chong as an elephant" is also an excellent example of skillfully applying the law of buoyancy.