1. Buoyancy: This is the upward force exerted on the balloon by the surrounding air. The heated air inside the balloon is less dense than the cooler air outside, creating an upward force.

2. Gravity: This is the downward force exerted on the balloon by the Earth's gravity. It acts on the weight of the balloon, the basket, and any passengers.

The balloon rises when the buoyant force is greater than the force of gravity. The balloon descends when the force of gravity is greater than the buoyant force. Pilots control the balloon's ascent and descent by manipulating the amount of hot air inside the balloon.