What Is Buoyancy & How Does it Relate to Balloons?

A Lesson From Archimedes

• Archimedes, the ancient Greek scientist, had to solve a problem for King Hiero of Syracuse, to determine the purity of a gold crown. He did not know about density, but reasoned that the volume displacement of gold of the same weight as the crown should also be the same. It was not, and they concluded the crown was not pure, without having to destroy it to find out.
  
  

Archimedes and Buoyancy

• At least, that's the story related by the Roman Vitruvius over a century afterward. It would be hard to measure differences by water displacement, however, and Archimedes knew about buoyancy. So it is more likely that he solved the problem instead using this knowledge.
  
  He knew that the upward buoyant force on an object in water equals the weight of the displaced water. This is known as Archimedes' principle. In the crown affair, it could have been used as follows: Hang the crown on one side of a beam; Hang an equal mass of pure gold on the other side, so that the beam balances; Submerge both. If they displace the same volume, they will balance underwater as well. If the crown is larger, due to lighter metals, the volume will be larger and the buoyancy will be greater, causing the water to force the crown higher than the pure gold pieces.
  
  

Icebergs and Global Warming

• If buoyancy is such that a floating object displaces a volume of water equal to its own mass, then why worry about icebergs melting from global warming and contributing to rising sea levels? True, icebergs will melt into the volume of the water they displace, and therefore won't raise sea levels; however, the concern has not been with melting icebergs but instead with melting glaciers. They are on land, and therefore don't melt into a volume already displaced but instead contribute to ocean water levels when they drop or melt into the ocean.
  
  

Use of Helium in Balloons

• As objects can buoy in water, so can they float in air. If you want your balloon to float, likely you will fill it with helium. Why? Because the air around the balloon needs something less dense to buoy it up, and helium is less dense than air, which is mostly made of the heavier gases oxygen and nitrogen.
  
  A balloon that doesn't stretch, such as mylar, won't compress the helium, which would increase its density and the amount of helium needed to fill the balloon. Mylar balloons are therefore more buoyant than elastic balloons with the same mass and volume. If helium is sufficiently compressed, it could be denser than the surrounding air and the balloon would sink. Ordinary balloons tend to pop before this level of compression is reached though.
  
  

Balloons That Don't Float

• If you fill a balloon by blowing into it, it will not float. Why? After all, air is in the balloon and air is outside, and air certainly floats in air. Is it the weight of the balloon rubber? Partly, but it is also the high density of the air inside from being compressed by the elastic balloon material, which is trying to contract to its normal size.
  
  That the air in balloons significantly adds to the balloon weight can be demonstrated as follows: Hang two air-filled balloons of equal size on the ends of a ruler; Hang the ruler from a support above, placing the supporting string at a place on the ruler so that the two balloons balance and the ruler is more or less horizontal; Then prick a small hole into one balloon with a pin. As the air leaks out, the other balloon will drop, since it no longer has to balance the weight of compressed air inside the other balloon.