A Boeing 747 weighs around 750,000 pounds, so how does it manage to get off the ground? It’s all thanks to a force called lift and something you may be familiar with from other AstroCamp videos: Bernoulli’s Principle.
Lift is just the name for the force pushing an object up into the air, but Bernoulli’s Principle is a little more involved. It states that whenever a flowing substance like air increases in speed, the pressure inside the flowing portion decreases. This means that if the air flowing along one side of a surface is moving faster than on the other side, there’s higher pressure on one side, pushing the surface. We can see that with Lin’s sheet of paper; blowing on only the top side causes the now higher pressure below to push up on the paper, generating lift.
The same concept applies with the wings of a 747, though with one important difference: there’s airflow along both sides of the wing. However, airplane wings are designed to provide some air resistance along the underside, so the air moving along the top is still flowing faster, generating lift.
What sorcery is this!? Science it turns out! This is a great example of Bernoulli’s Principle! In short, this states that moving air has a lower pressure. Imagine trying to dig a hole in a pool of water: as soon as some of the water gets moved out of the way, the surrounding water rushes in to take its place. Air does the same thing: whenever it moves, the lower pressure draws air in around it!
The cup and straw demonstration shows this nicely, and it’s also a great experiment to try at home! By taking each apparatus and blowing into it, we can see there is a pretty huge difference.
Inside the cup, air is moving quickly. This causes a lower pressure inside the cup than outside, and air that tries to fill up the space suctions the balloon in place. Alternatively, the cup with holes in the sides allows the surrounding air to help out. Very similar to the Bernoulli Bag, nearby air joins in creating a larger column of air that can lift, and even suspend, the balloon.
We know this looks like a trick, because it’s hard to tell from watching that he is exhaling vigorously in both cases. If you don’t believe us, definitely try it yourself! Of course, it is possible to hold the balloon into the cup by inhaling, but it’s actually more difficult to suck up the balloon from a short distance away! The difference in pressure from blowing fast moving air into the cup is more effective at sucking up the balloon.
While Bernoulli’s Principle can be tricky to understand, it is very important! The fact that fast moving air has a lower pressure is one of the primary ways that airplanes are able to generate lift! The air going over the wing actually goes faster than the air below, which you can see in this very cool shot from a wind tunnel!
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