1. Buoyancy: This is the upward force exerted by the air surrounding the balloon. Hot air is less dense than cold air, so the balloon filled with hot air is lighter than the surrounding air, causing it to rise.
2. Gravity: This is the downward force pulling the balloon towards the Earth. The weight of the balloon, its basket, the passengers, and the hot air itself all contribute to this force.
These two forces are in opposition to each other. For the balloon to ascend, the buoyancy force needs to be greater than the force of gravity.
