Grades 3–5 · Machines
How can warm air lift a hot-air balloon?
A hot-air balloon can lift a basket without wings. The surrounding air presses on the balloon from all sides. Pressure is greater lower down, so the combined push includes an upward force called buoyancy.
In the diagramBuoyancy must support the whole balloonDisplaced outside air: 1,200 kgAir inside1000 kgEnvelope, basket and load: 100 kgBuoyancyWeightForce imbalance: upward; total mass 1100 kg
Buoyancy equals the weight of the outside air displaced by the balloon. Heating air at roughly the same pressure makes it less dense: there is less mass in the same volume. A hot-air balloon has an opening, so expanding air can leave rather than simply building pressure.
The upward force must support more than the air inside. The envelope, basket and carried load all add weight. In the illustration, displaced outside air has a mass of 1,200 kilograms. Inside air and the other parts together have a mass of 1,100 kilograms, giving an upward force imbalance.
Warm air alone therefore does not guarantee lift. The comparison includes the whole balloon system, and it explains force rather than an actual flight’s speed or height.
About this explanation
Starting knowledge: Compare mass amounts and recognize that air occupies space and can exert a push.
Sources
- NASA Goddard Space Flight Center — Ask an Astrophysicist: Scientific Principles Behind Hot Air Balloons
- OpenStax, Rice University — College Physics 11.7: Archimedes’ Principle