Why does a hot air balloon begin to rise when heated air is trapped inside the balloon?

Answer 1

Less air molecules per square foot.

Let's say the hot air is 10% less dense than the air outside the balloon. Since air does have weight, the whole balloon as a single unit will be lighter than the surrounding same unit above the balloon. So, - it will rise.

If you had a child's balloon and stuck it in the freezer and pulled it out later, - it would be 10% smaller than when it went in, - since the molecules can not escape. If you have a hot-air balloon and heated it up, then 10% of the molecules would get flushed out of the bottom opening, making the whole balloon 10% lighter than the same (balloon) unit above the balloon.

Answer 2

Hot air is lighter than cold air and as such rises allowing the cold air to fall and take its place. When the hot air is "confined" to the ballloon, the force of it wanting to rise is exerted on the balloon making it want to rise. When this force becomes greater than the force gravity exerts on the balloon pulling it down (Weight), the balloon will rise.

Answer 3

cool air is stable and the molecules are close together causing the air to be dense.

when you heat air real hot it starts moving around rapidly causing the molecules to bump together creating space between the molecules causing it to be less dense. even though you have the same amount of air in the ballon this causes the air to act like a bubble in water and it rises up above the cooler more dense air

Answer 4

Because the hotter air is lighter than the cooler air around it.
You will also notice this in a 2 story home, the upper floor is usually warmer.

Answer 5

The warm air has less density (lighter) than the surrounding air so it floats like a cork in water.