Hot Air Balloons: Hot air balloons are a classic demonstration of the principle of floatation. The heated air inside the balloon is less dense than the surrounding cold air, making the balloon positively buoyant and causing it to rise.

Hot-air balloons and blimps can float in the air thanks to buoyancy, an upward force that the air exerts on them. The balloon must weigh less than the air it displaces (the air that would occupy the same space if the balloon wasn’t there).

Helium-filled balloons float because the helium is lighter than the surrounding air. In other words, the weight of the air displaced by the balloon is greater than the weight of the balloon and the gas inside, so the balloon floats upward. Figure 1. Ascending helium balloons. (Wikipedia, 2009.)

Ships and Submarines: Designed to float by displacing water equal to their weight. Hot Air Balloons: Float because hot air inside the balloon is less dense than the surrounding cooler air. Hydrometers: Measure the density of liquids by floating at different levels based on fluid density.

Let's say that y­ou take a plastic 1-liter soda bottle, empty out the soft drink it contains, put the cap back on it (so you have a sealed bottle full of air), tie a string around it like you would a balloon, and dive down to the bottom of the deep end of a swimming pool with it.