### Question

I'm an actor playing the role of a balloonist and I want to be able to explain in scientific terms why helium and hydrogen make balloons rise.

Archimedes is your man for the answer to this question. He is the Greek fellow who figured out that the weight of a fluid displaced by an object in that fluid is equal to the buoyant force placed on that object by the fluid. In other words there is a force, called the buoyant force, that pushes in the direction opposite gravity. the magnitude of that force is exactly equal to the weight of the fluid the object displaces. So, you and I displace about 100 Liters of air. Since we weigh a lot more that 100 liters of air the force of gravity is stronger that the buoyant force of air. But, for a helium balloon this is not the case. The amount of air this balloon displaces weighs more than the balloon and so up it goes. The buoyant force of the air is greater than the force of gravity.

Why is the weight of a helium balloon less than the weight of the air it displaces? Well, because helium is a very light gas. Air is mostly nitrogen which has an atomic weight of 14 while helium has an atomic weight of only 4. Hydrogen, which is not used for balloon because it is too dangerous, is even less massive with an atomic weight of only 1.

Now, if you are a balloonist chances are you are not using these gases in your balloon. A balloon used by balloonists today works on hot air. The principle is the same for helium balloons but on a much larger scale. For a hot air balloon to float in air it, too, must displace an amount of air that weighs more than the balloon. Yes, that is a LOT of air! This can be done because hot air is less dense than cold air, thus less of it takes up more space than an equal amount of cold air. So, the hotter the air the less dense it is and the more space a little if it takes up so that on balance the amount of hot air in the balloon is a whole lot less than what it would take cooler air to fill the same space. Thus the total weight of the balloon, the air inside, the gondola, the fuel tanks (needed to heat the air inside the balloon), and the passengers is less than the weight of the air the balloon displaces.

I hope this helps you in your play. Break a leg!
Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas

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### Science Quote

'Knowledge can be communicated, but not wisdom. One can find it, live it, be fortified by it, do wonders through it, but one cannnot communicate and teach it.'

Hermann Hesse
(1877-1962)