Fly like a bird in the vast sky has always been a dream of mankind since ancient times. Hot air balloons were the earliest flying machines. On the afternoon of November 21, 1783, after repeated experiments, the Montgolfier brothers of France finally carried out the world's first manned hot air balloon flight at Muette Fort in Paris. The flight lasted 25 minutes and covered half of Paris. Today, hot air ballooning, as a sport, is deeply loved by the public, and grand hot air balloon festivals are held around the world. When people cheer and rejoice as colorful giant hot air balloons take off into the sky, some may ask: Why can hot air balloons rise into the air?
Hot air balloons rise into the air by taking advantage of air buoyancy. The buoyancy of air on a hot air balloon is equal to the weight of the air displaced by the balloon. As long as the average density of the hot air balloon (including the weight it carries) is less than that of air, it can float. Before taking off, the hot air balloon needs to be heated. After the air in the balloon is heated, its volume expands, and its density becomes lower and the density difference becomes larger, allowing it to carry more weight. If we don't use hot air, but fill the balloon or airship with helium or hydrogen, which have lower densities, we can also obtain sufficient lift and fly in the sky.
The utilization of buoyancy has a long history. Archimedes, a Greek scientist, first discovered the cause of buoyancy to help King Hieron II of Syracuse solve the problem of the purity of the gold in the crown. Later, this was called the "Archimedes Principle", which states that the buoyancy of an object in a liquid or gas is equal to the weight of the liquid or gas displaced by the object. The Archimedes Principle has many applications, such as calculating the density of objects and the maximum load capacity of ships. Life jackets are filled with air, and because air is much less dense than water, they can help people who fall into the water float on the surface and wait for rescue. The hot air balloons used in China for entertainment or auspicious purposes also utilize the buoyancy of hot air. According to legend, Zhuge Liang was once besieged by Sima Yi. He made paper lanterns with oil cotton balls at the bottom and lit the cotton balls. The lanterns rose into the sky to send out distress signals, and he was eventually able to escape. The legend of "Cao Chong Weighing the Elephant" is also an excellent example of cleverly using the law of buoyancy.