The thrust of a hot air balloon isn't exactly a "thrust" in the same way as a jet engine or a rocket. It's more accurately described as buoyancy force.

Here's the breakdown:

* Hot air is less dense than cold air: Heating the air inside the balloon makes the air molecules move faster and spread out, resulting in lower density.

* Buoyancy: This less dense hot air inside the balloon is lighter than the surrounding cooler air, creating an upward force called buoyancy.

* Lift: This buoyancy force is what lifts the balloon and its basket into the air.

So, while it's not technically "thrust," the buoyant force generated by the heated air is the primary force responsible for the upward motion of a hot air balloon.