why would heating the gas in an air balloon make the balloon rise?

Answer 1

Hot air rises. Hot air is ligter than cold air, so when you make the air in the baloon warmer than the sounding air, it becomes lighter and rises.

Answer 2

The preceding answers are correct. However, you may still be wondering why less dense air (or fewer molecules inside the balloon) cause it to rise. Here's an answer to that question:

The cooler air surrounding the balloon is more dense. Therefore, the earth's gravity is pulling harder on each cubic foot of the surrounding air than it is on each cubic foot of the balloon's contents.

As the surrounding air is pulled downward, it pushes the balloon upward.

A similar thing happens when you place a rock in a bucket of water. Gravity pulls harder on each cubic inch of rock than it does on each cubic inch of water. So the rock goes down, taking the place of an equal volume of water. As a result, the surface of the water rises, and the water that has been pushed up above the original surface level is like the balloon that rises from the surface of the earth.

If we're talking about a hot air balloon with a basket and passengers underneath, obviously not all parts of it are lighter than the surrounding air. However, as the heavier air pushes the hotter, lighter air upward, it does so with a force that is great enough to raise the basket and passengers along with the balloon.

If you want to raise the rock that we dropped into the bucket of water, you could attach something lighter than water to it (a piece of wood or a balloon, for example). As the water is pulled down by gravity and the wood or the balloon is forced upward, it pulls the rock up with it, even though the rock is heavier than water. Similarly, the basket and passengers that are heavier than air can rise through the air, lifted by the lighter-than-air balloon.

Obviously, a hot air balloon would not rise in a vacuum. The cooler, denser air around the balloon is needed to support the weight of the balloon.

I hope that this additional information helps you to make sense of hot air balloons.

Answer 3

When the air in the balloon got heated up, the air molecules in the balloon vibrates faster and expands. They take up more volume and the flow over escapes from the bottom of the balloon. Thus with less molecules now in the balloon, the density of the air in the balloon is now lighter then that outside the balloon, thus the balloon floats.

Answer 4

Heat causes atoms to move and expand, making warm air less dense. Since this warm air is lighter than the cooler air around it, the ballon rises.

Answer 5

When the gas is hot its weight get lower hence the whole baloon flies due to its lighter than air weight.

Answer 6

Density decreases.