Why do hot air balloons float?
The hot air will weigh less than cool air and hence the density of hot air is lower than that of cool air. The heat provides some energy to the air that is heated. The added energy will make the air molecules to move rapidly and collide with one another very often and with excess force. This forced spread of air molecules make the air to expand still more. So, when a given volume of air is heated, it occupies more volume as the air expands due to the fast movement of the molecules in it. This expansion of air makes the air to become less dense than the cool air.
When the balloon is filled with hot air, the nature of hot air to move high up due to its less weight makes the balloon also to go high up. The balloons filled with hot air will move up in the sky and float. It is evident in general that when the balloon with hot air is seen flying high up, there will be a person who keeps on generating fire inside the balloon. This has to be done continuously in the balloon as the heat in the balloon will be constantly getting exchanged with the surroundings and gets cooler. The cooled balloon might sink and cannot fly anymore. So, it is necessary to generate heat in the balloon as and when the air inside gets cooled. As the fire adds up thermal energy to the air particles in the balloon, the air inside will expand and become less dense and hence allows the balloon to float.
According to the principle of Archimedes, the object floats in the air if it has hot air that displaces the same volume of cool air. As the hot air balloon displaces the cool air, it rises high up and floats in the air. This principle is also applicable in the case of boats and ice.
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