1. The problem statement, all variables and given/known data To research the physics of how a hot air balloon works. 2. Relevant equations 3. The attempt at a solution I just would like someone to check the main points I am going to include. The air inside the balloon is less dense because it is heated so that particles of air are further apart than the particles outside the balloon. The pressure inside is equal to the pressure outside otherwise the balloon would collapse or explode. To maintain the same pressure inside there are less particles but they have more kinetic energy which gives the same pressure as outside. The main point I am confused with are the forces acting on the balloon, particularly the buoyant force. I would have said that the buoyant force is constant since the volume of the balloon is constant. So the balloon rises purely because the weight force is reduced i.e. the weight of the air inside the balloon + balloon + passengers etc is less than the buoyant force. However the first article that comes up in google (http://www.real-world-physics-problems.com/hot-air-balloon-physics.html) talks about manipulating the buoyant force and I don't see how that is possible as the balloon's volume doesn't change? A word for word quote from this site: "If the balloon operator wishes to lower the hot air balloon, he can either stop firing the burner, which causes the hot air in the envelope to cool (decreasing the buoyant force), or he opens a small vent at the top of the balloon envelope (via a control line). This releases some of the hot air, which decreases the buoyant force, which also causes the balloon to descend." Is this correct? Thanks for any help offered!