on the topic of Student Reading
Not so very long ago, space was not the final frontier.
Although many early attempts at flight involved gliders or wings that mimicked birds, one of the first successful methods of early aviation was much simpler. Rather than creating lift using wings and a source of propulsion, clever aviators discovered that heated air trapped inside of a thin, lightweight container would rise.
In 1783, two brothers—Joseph-Michel and Jacques-Etienne Montgolfier—used this knowledge to successfully pilot a hot air balloon without any safety harnesses in front of an amazed crowd in Paris, France.
Although they aren’t our primary means of air travel today, we still see hot air balloons in use. You’ve probably seen pictures of hot air balloons floating gently over some picturesque scenery, or if you’re very lucky, perhaps you’ve flown in one yourself.
But why, exactly, do hot air balloons fly? Why, oh why, do they fly?
(That was a haiku).
Hot Air Rises?
At first glance, anything “floating” in the air seems a bit ridiculous. How can that possibly happen? How can a big heavy balloon with a basket full of people float in the air?
Because we cannot see the particles that make up the air around us, it can be tempting to regard air as a sort of nothingness, but be careful: although we can’t see it, air is matter. And, as such, it has a mass and takes up space.
We know from our studies in Middle School Chemistry that air is made up of atoms and molecules which are attracted to each other and in motion. We also know that that when we add heat to a substance, its particles move faster and are spread a bit farther apart.
A hot air balloon works by applying these simple principles to air.
First, a fan is used to blow air into the balloon, but not enough to fill it up completely. Then, a burner is used to heat up the air inside of the balloon. When this happens, the air molecules in the balloon speed up and move farther apart. This causes the heated air to occupy a larger volume than it did before it was heated.
Because the heated air now occupies a greater volume, but still has the same mass, we know that its density has decreased. How do we know this? Remember that density is a measurement that compares the mass of a substance to the amount of volume that it occupies.
As we discussed in a previous post on density, you can express density mathematically using the following equation:
This equation tells us that the density of a substance is equal to its mass divided by its volume.
So when you heat air, the volume gets bigger and the equation tells us that when you divide by a bigger V, the density gets smaller. Why, then, do hot air balloons “float” or rise in the air? Hot air balloons float because the volume of the balloon is so large that the entire balloon, including the basket and the people is less dense than the surrounding air.
Recall once again that air is matter, so it has mass and takes up space. And just like a ship floating in the ocean, a hot air balloon displaces air, in the same way that the ship displaces water.
The hot air balloon floats because it weighs less than an equal volume of the air it is displacing. Or, stated more simply, the balloon is less dense than the air around it. And just like a piece of wood placed in water, this less dense matter floats in matter that is more dense than itself.
Submarines: Stealthy Density Changer
Submarines are another example of an unusual craft that relies on density changes to float and sink. Instead of changing their volume, however, submarines control their density by adjusting their mass.
Because density compares mass and volume, density can be adjusted by making a change to either of these properties.
Submarines change their mass by taking in seawater or releasing it. Subs store water (or air) in a special chamber called the ballast tank. Before a submarine is deployed, it floats normally in the water just like any other boat. But when the submarine replaces the air in its ballast tank with water, it increases its total mass, which increases its density, and the submarine sinks. Why does this happen? Recall once again the density equation:
If we don’t change the volume, but we increase the mass, then we have a larger number for density. And a larger number for density means the object is more dense.
For example, imagine that the mass of the submarine was 1,000 grams (obviously it’s much more than this in real life) and the volume of the submarine was 1,000 cm3. Remember that cubic centimeters (or cm3) are the units commonly used for volume of solids.
Applying the density equation, we would see that the density was 1.0 g/cm3 because (1000/1000) = 1. Now imagine that when the submarine takes on some water in its ballast tank and the mass of the submarine is now 2,000 grams. The density would now be 2.0 g/cm3, meaning that the submarine with water inside is more dense.
By gaining or discarding mass, the submarine alters its density. When the submarine increases its mass by storing extra seawater, its density increases, and the sub sinks. When the submarine decreases its mass by releasing seawater, its density decreases, and the submarine floats upward.
Submarines can float at different water depths by precisely controlling the amount of water and air they store in their ballast tanks.
A recent HBO miniseries about John Adams (the second president of the United States) includes a scene in which John Adams, Abigail Adams, and Thomas Jefferson observe the Montgolfier brothers famous 1783 balloon launch in Paris.
You can view the clip below.