This 2D side-view simulator reduces a hot-air balloon to the physics that actually lifts it: fire the burner to raise the envelope's internal air temperature, which lowers its density relative to the cooler ambient air outside, and Archimedes' principle does the rest — the buoyant force from the displaced ambient air is weighed live against the combined weight of the hot air, envelope, basket and payload. Adjust the burner heat, envelope volume and payload mass to see how each changes the net lift force, vertical speed and altitude, and vent the envelope to release hot air and descend.