Buoyancy exists in air as objects immersed in gases experience an upward buoyant force equal to the weight of the displaced gas, and balloons filled with lighter gases like helium will float in air. Hot air balloons use this principle by heating the air inside the balloon, decreasing its density so the balloon rises, and then releasing heat so outside cooler air enters and increases the density, causing the balloon to descend.

1. BUOYANCY IN AIR

2.  Does buoyancy exist in air?
=> Yes, it does.
 Object immersed in gas also experiences
a buoyant force equal to the weight of the
gas displaced.

3.  Air is composed of different gases,
mainly:
= nitrogen
= oxygen
= carbon
 Some gases like helium:
=> have lower densities like air
 Thus, balloons with helium gas float.

4. HOT AIR BALLOONS
 In hot air balloons, hot air is forced inside
the balloon.
 As air is heated, air expands and the
density decreases.
 The balloon goes up.
 As air in the balloon cools, it contracts,
letting outside cool air to enter.
 The density increases.
 The balloon now goes down.

5.  During World Wars I and II, hot air
balloons were used for:
=> transport
=> warfare
=> surveying
=> communication
 Today, hot air balloons are used for:
=> leisure
=> gathering data in the
atmosphere